Instrumental Observations.
The minimum requirement of instrumental observations by a traveller is the reading twice daily of the barometer and of the dry and wet bulb thermometers, to ascertain the temperature and humidity of the air, also the reading once daily, in the morning, of the minimum thermometer which has been exposed all night, and on days in camp of the maximum thermometer also. It is very desirable to expose a rain gauge whenever it is practicable to do so. Unless special meteorological researches are to be carried out, nothing farther in the way of observations need be attempted. A very useful supplement to the necessary observations is the use of a self-recording barograph or thermograph; but these are delicate instruments, liable to get out of order unless very carefully handled, and it will not always be possible to make use of them.
The observer must understand what his instruments are intended to measure, how they act, and how they should be exposed, read, and the reading recorded. He must know enough about all these things to be able to dispense with unnecessary precautions only possible at fixed observatories, and, at the same time, to neglect nothing that is necessary to secure accuracy in the results.
Thermometer Corrections.—All thermometers, without exception, should have the degree marks engraved on the stem, or on a slip of enamel within the outer tube, and be supplied with a certificate from the National Physical Laboratory showing the error of the scale at different points. This certificate should be in duplicate, and a copy ought to be left in a safe place at home. After a long journey the thermometers which have been in use should be sent to have their errors re-determined. The corrections are not, however, to be applied by the observer unless he is working out his observations for some special purpose. No thermometer is passed at the National Physical Laboratory if its error approaches one degree, so that for all ordinary purposes of description a certificated thermometer may be looked on as correct. But when the readings are being critically discussed and compared with the observations of other people, the correction is of the greatest importance. It cannot be too strongly impressed upon an observer that, in reading meteorological instruments, he must read exactly what they mark, and record that figure in his observation-book on the spot. The corrections can be applied afterwards by the specialist who discusses the work. For subsequent reference it is necessary to note in the observation-book the registered N.P.L. number of the thermometer in use, and if a thermometer should get broken and another be used instead, the number of the new instrument must be noted at the date where it is first employed. Care should be taken to use the same thermometer for one purpose all the time if possible, and only an accident to the instrument should necessitate a change being made.
Thermometers are either direct-reading or self-registering. The former are used for obtaining the temperature at any given moment, the latter for ascertaining the highest or the lowest temperature in a certain interval of time. They are filled either with mercury, or a light fluid which freezes less readily, such as alcohol or creosote.
Thermometer Scales.—The particular system on which the thermometers are graduated is of no importance, but merely a matter of convenience. The Fahrenheit scale is used for meteorological purposes in English-speaking countries; but for all other scientific purposes the Centigrade scale is used everywhere. One can be translated into the other very simply by calculation[2]; but it is convenient for a traveller to have all his thermometers graduated in accordance with one scale only.
The graduation, as marked on the stem of the thermometer, is usually to single degrees, but anyone can learn to read to tenths of a degree by a little practice. Care must be taken to have the eye opposite the top of the mercury column. Suppose it to be between 50 and 51, the exact number of tenths above 50 is to be estimated thus: If the mercury is just visible above the degree mark it is 50°.1, if distinctly above the mark 50°.2, if nearly one-third of the way to the next mark 50°.3, if almost half-way 50°.4, exactly half-way 50°.5, a little more than half-way 50°.6, about two-thirds of the way 50°.7, if nearly up to the next mark 50°.8, and if just lower than the mark of 51° it is 50°.9. The eye soon becomes accustomed to estimating these distances.
Fig. 1.—Reading Thermometer Scale above and below Zero.
In using a thermometer below zero, the observer must pay attention to the change in the direction of reading the scale, the fractions of a degree counting downward from the degree mark instead of upward from it, as in readings above zero. Readings below the zero of the scale are distinguished in recording them by prefixing the minus sign. The annexed figure shows the reading of two thermometers graduated to fifths of a degree, one showing a temperature of 1°.4, the other of -1°.4. The British Meteorological Office now recommends the use of the Centigrade thermometer graduated from the absolute zero, i.e., the freezing point is shown by 273°, the boiling point as 373°.
Care of Thermometers.—Mercurial thermometers will always be employed for ordinary purposes in places where the temperature is not likely to fall to -40°: i.e., everywhere except in the polar regions and the interior of continents north of 50° N. These thermometers are very strong and are not easily broken except by violence. The one vulnerable part is the bulb, which is of thin glass and filled with heavy mercury. Hence, in carrying thermometers, care has to be taken to protect the bulb from coming in contact with any hard object. The best way to carry an unmounted thermometer is in a closed brass or vulcanite tube with a screw top, the inside of the tube being lined with india-rubber and provided with a cushion of cotton-wool for the bulb to rest on. If the thermometer is mounted in a wooden frame it should be secured in a box so that the frame is firmly held and the bulb projects into a vacant part of the box, which may be lightly filled with cotton-wool or provided with a deep and well-padded recess. Every thermometer which is not graduated above 120° should have an expansion at the top of the tube which the mercury that may be driven beyond the scale by over-heating will not fill; otherwise any accidental over-heating will break the bulb.
The unavoidable shaking or any sudden shock during travelling is apt to cause the mercury column to separate, and a portion of it may be driven to the top of the tube, where it may remain unless looked for and brought back. Hence it is important to see that the top of the bore of the tube is visible, and not covered by any attachment holding the tube to a wooden frame. Thermometer readings are absolutely valueless unless the whole of the mercury fills the bulb and forms a continuous column in the stem. To bring a broken column together the best plan is to invert the thermometer, if necessary shaking it gently, until the mercury flows from the bulb and entirely fills the tube, leaving a little vacant dimple in the mass of mercury in the bulb. When this is done, the thermometer should be brought into its normal position bulb downwards, and the column will usually be found to have united. If this method does not succeed the thermometer may be held in the hand by the upper end, raised to the full stretch of the arm, and swung downwards through a wide arc with a steady sweep. I have never known this method to fail.
Thermometer Screens.—It is usual at fixed stations to expose the thermometer to the air by hanging it in a screen made of louvre-boards so arranged that the air penetrates it freely while the direct rays of the sun are cut off. The Stevenson screen, constructed on this plan with a door opening on the side away from the sun, is well adapted for use in temperate countries; but it is too cumbrous to carry on a journey and does not afford sufficient ventilation for use in tropical countries. An excellent substitute is the canvas screen devised by the late Mr. H. F. Blanford, which consists of a bamboo frame carrying the thermometers (with their bulbs four feet from the ground). The whole structure is five feet high, and is sufficient for any places where the wind is moderate. It is constructed of bamboos or rods of light wood, cords, and canvas, which may readily be made up before starting, and it is easily renewed or repaired. The canvas roof should be triple or quadruple according to the thickness of the material. Such a screen will afford sufficient protection at night, or even in the day, if set up in the shade, and it will throw off rain; but in the sun it will require a thick mat as an additional protection on or preferably stretched above the roof.
Fig. 2.—Mr. H. F. Blanford’s Portable Thermometer Screen.
For a more permanent station the form of exposure recommended by a committee of the British Association for use in tropical Africa will be found very suitable in hot countries.
Fig. 3.—Hut for Sheltering Thermometers.
The thermometers are placed in a galvanised iron cage, which is kept locked for safety. This cage is suspended under a thatched shelter, which should be situated in an open spot at some distance from buildings. It must be well ventilated, and protect the instruments from being exposed to sunshine or rain, or to radiation from the ground. A simple hut, made of materials available on the spot, would answer this purpose. Such a hut is shown in the drawing (Fig. 2). A gabled roof with broad eaves, the ridge of which runs from north to south, is fixed upon four posts, standing four feet apart. Two additional posts may be introduced to support the ends of the ridge beam. The roof at each end projects about eighteen inches; in it are two ventilating holes. The tops of the posts are connected by bars or rails, and on a cross bar is suspended the cage with the instruments. These will then be at a height of six feet above the ground. The gable-ends may be permanently covered in with mats or louvre-work, not interfering with the free circulation of the air, or the hut may be circular. The roof may be covered with palm-fronds, grass, or any other material locally used by the natives for building. The floor should not be bare but covered with grass or low shrubs.
The great object of these precautions is to obtain the true temperature of the air, and avoid the excessive heating due to the direct rays or reflected heat of the sun falling on the thermometers, and the excessive cooling due to the radiation of heat from the thermometers to a clear sky at night. Such a shelter is absolutely necessary when maximum and minimum thermometers are used; but can be dispensed with for the simple observation of the temperature of the air at a given time. This may be effected by securing a rapid flow of air over the thermometer, either by causing the air to flow past the instrument or by causing the instrument to move rapidly through the air. It has been found by experiment that the true temperature of the air is obtainable in this way whether the operation is performed in sunshine or in shade; but it is preferable to do so in the shade.
Sling Thermometer.—The sling thermometer is the most simple and convenient of all instruments for ascertaining the temperature of the air. It is an unmounted thermometer with a cylindrical bulb, and the degree-marks engraved on the glass stem. The upper end terminates in a ring to which a silk cord about two feet long is attached. As a precaution it is as well to secure the cord by a couple of clove hitches round the top of the thermometer stem as well as to the ring, as the thermometer would then be held securely even if the ring broke. The thermometer is used by whirling it in a vertical circle about a dozen times, the observer taking care, by having a loop of the string round the wrist or finger, that it is not allowed to fly off. Then the thermometer is read, swung once more, and read again. This process is repeated until two consecutive readings are identical; when this is the case the instrument shows the true temperature of the air. It is sufficient to note the final temperature in the observing book.
The risk of breaking a sling thermometer is the only drawback to its use. Only a silk cord should be used, and it should be examined frequently to see that it has not got chafed. In swinging the thermometer, an open place must be selected where it is not likely to come in contact with a branch or any other object.
Hygrometers.—As the humidity or degree of moisture in the atmosphere is a very important climatic factor it is necessary to measure it as carefully and as frequently as the temperature of the air. There are many instruments, called psychrometers or hygrometers, for doing this; but few of them are simple enough for the use of a traveller. The proportion of water-vapour in air is a little difficult to understand at first, because it is not a constant quantity as in the case of the other constituents of air, but varies according to the amount of water-surface exposed to the air and according to the temperature. The maximum amount of water-vapour which can be present in air varies with the temperature, being greater as the temperature is higher and less as the temperature is lower. Thus, if air at 50° F. contains the maximum amount of water-vapour which it can contain at that temperature, it is said to be saturated, for it will take up no more and evaporation stops; and if the temperature were to fall ever so little there would be more water-vapour present in the air than it could hold and some would separate out and condense into dew or rain, hence the temperature of saturation is called the dew-point. But if air saturated at 50° is warmed up say to 60° it can then contain more water-vapour than it has, and the temperature would require to fall 10° before dew or rain could form. When the air is not saturated water exposed to it evaporates rapidly until the maximum quantity of water-vapour is again present, a larger quantity corresponding to the higher temperature. At any given temperature the essential thing to know about the humidity of the air is the additional amount of water-vapour it could take up before becoming saturated, or in other words the humidity relative to the maximum humidity possible at the existing temperature. The relative humidity is expressed in percentages of the maximum humidity possible (saturation) at the actual temperature of observation. It may be measured by two methods, (1) finding the dew-point or temperature at which the amount of vapour present saturates the air; (2) by finding the rate at which the air allows evaporation to proceed; the farther the air is from saturation the more rapid is the rate.
The dew-point may be found directly by means of an instrument by which the air is cooled down until it begins to deposit moisture on a polished surface, but such an instrument is inconvenient to handle when travelling. It may also be found indirectly by calculation from the relative humidity.
The relative humidity is most easily calculated from the rate of evaporation. It is one of the laws of evaporation that heat is required to change liquid into vapour, and when evaporation is going on heat is being abstracted from surrounding bodies, and they are growing colder. By allowing evaporation to take place from the bulb of a thermometer the rate of evaporation may be measured by the fall of temperature produced, and tables have been constructed to convert the differences between the wet and dry bulb readings into relative humidities.
The wet-bulb thermometer consists of an ordinary thermometer, the bulb of which is covered with clean muslin and kept moist by means of a piece of cotton lamp-wick dipping into a small vessel of pure water. Care must be taken to have the water quite pure and free from salt, otherwise the true reduction of temperature will not be observed. Hence special precautions are necessary when observing at sea or in an arid country where the ground is covered with incrustations of salt.
In any form of wet bulb thermometer when the air is much below the freezing point, it will usually be found most satisfactory to remove the muslin covering and allow the bulb to become covered with a coating of ice, by dipping it into water and allowing the water to freeze upon it. Evaporation takes place from solid ice sufficiently rapidly to give the true wet-bulb readings at least with a sling thermometer.
When the air is saturated, i.e., relative humidity = 100 per cent., there is no difference in the reading of the wet and dry bulb thermometers, and the greater the difference between the readings at a given air temperature the smaller is the relative humidity of the air.
The wet-bulb thermometer has to be exposed to the air with the same precautions as are taken in the case of the dry bulb. The two may be hung side by side—but at least six inches apart—in the screen or cage described on [p. 15]; or the wet bulb may be employed as a sling thermometer. One way to do this is to tie a muslin cap on the bulb of the sling thermometer with a piece of wet lamp-wick coiled round the upper part of the bulb, and then whirl it until the reading becomes constant, taking care to moisten the bulb again if it should become dry. Another way is simply to twist a piece of filter-paper or blotting-paper round the bulb, and dip it in water before swinging.
Aspiration Psychrometer.—Perhaps the most convenient form of wet and dry bulb thermometer for use by a traveller is that known as Assmann’s Aspiration Psychrometer. It requires no protecting screen, is not subject to the risk attending the use of the sling thermometer, and gives an extremely close approximation to the true temperature and humidity. The principle of the instrument is very simple. The wet and dry bulb thermometers are enclosed separately each in an open tube (see Fig. 4) through which a current of air is drawn by means of a fan, actuated by clockwork in the upper part of the case. In making an observation, all that is required is to see that the water vessel for the wet bulb is filled and the bulb properly moist, and that the dry bulb is free from any condensed moisture. The instrument is then hung to a branch or other support placed in the open air (or even held in the hand), preferably in the shade, although this is not essential, and the clockwork wound up. Air will then be drawn over the bulbs for five minutes or more, and if the temperature of each thermometer has not become steady by the time the clockwork has run down, it must be wound up again.
Fig. 4.—Section of Assmann’s Aspiration Psychrometer.
The thermometers in Assmann’s Psychrometer are graduated according to the Centigrade scale, and each degree is subdivided into fifths on a slip of porcelain enclosed in the outer tube of the thermometer (see [p. 13]).
Minimum Thermometer.—The minimum temperature of the night can usually be ascertained by a traveller exposing a minimum thermometer when the camp is set up and reading it in the morning before starting on his way. There are several forms of minimum thermometer, but the only one likely to be used is that known as Rutherford’s. It is very delicate and liable to go out of order. The instrument should be of full size, as used in meteorological stations at home; it must be packed so as to be as free as possible from shock or vibration, and ought to be carried in a horizontal position. The bulb is filled with alcohol or some similar clear fluid, and within the column of spirit in the stem there is included a little piece of dark glass shaped like a double-headed pin. This is the index which continues pointing to the lowest temperature until the instrument is disturbed or re-set. The thermometer has to be hung in a horizontal position. When the temperature rises, the column of spirit moves along the tube, flowing past the index without disturbing it. When the temperature falls, the spirit returns towards the bulb, flowing past the index until the end of the column touches the end of the index. The phenomenon known as surface-tension gives to the free surface of any liquid the properties of a tough film, and the smaller the area of a free surface is, the greater is this effect of surface-tension. Hence it is that the inner surface of the column of alcohol is not penetrated by the glass index, but draws the index with it backwards towards the bulb. As soon as the temperature begins to rise, the alcohol once more flows past the index towards the farther end of the tube. The end of the index farthest from the bulb remains opposite the mark on the stem indicating the lowest temperature which had occurred since it was last set, and this reading must be taken without touching the thermometer.
To set the index it is only necessary to tilt the bulb end of the tube upwards, when the index will slide down by its own weight until it comes in contact with the inner surface of the end of the column of alcohol.
Care of a Minimum Thermometer.—The chief dangers to which a minimum thermometer are liable are three—(1) the index being shaken into the bulb, (2) the index being shaken partly or wholly out of the column of spirit, and sticking in the tube, and (3) the column of spirit becoming separated or a portion of the spirit evaporating into the upper end of the tube.
The thermometer should be so constructed as to make it impossible for the index to get into the bulb, or with an index so long as not wholly to leave the tube, and this should be seen to before purchasing. When any of the other derangements occurs the natural instinct of an observer is to immerse the thermometer in warm water until the spirit entirely fills the tube, and then allow it to cool. The only drawback to this simple method is the almost inevitable bursting of the bulb and destruction of the thermometer. This method should never be attempted; but if the warning were not given, the idea would be sure to occur to the observer some time or other, and he would proceed to destroy his thermometer with all the fervour of a discoverer. The only satisfactory way to rectify a deranged minimum thermometer is as follows:
If the column is separated, but the index remains in the spirit, grasp the instrument firmly by the upper end and swing it downwards with a jerk (as in the case of the mercurial thermometer mentioned on [p. 23]). If the index has been shaken out of the spirit and remains sticking in the upper part of the tube, or if a little spirit has volatilised into the top of the tube and cannot be shaken down by the first method, a quantity of spirit should be passed into the upper end of the tube by grasping the thermometer by the bulb end of the frame and swinging in the same way. When the index is immersed or the drop of volatilised spirit joined on to the column, the first process of swinging by grasping the upper end of the tube will bring the instrument into working order. After any operation of this kind the thermometer should be kept in a vertical position bulb downwards, to allow the spirit adhering to the sides of the tube to drain back completely. Then the thermometer should be brought into the horizontal position and set by allowing the index to slide down to the end of the column of spirit. The end of the column of spirit farther from the bulb should always show the same temperature as the dry-bulb thermometer. If it should be observed to read a degree or two lower, it will be found that some of the spirit has volatilized and condensed at the end of the tube.
The minimum thermometer should be exposed to the air four or six feet from the ground under a screen or roof, like that described on [p. 15], so that it is not exposed to the open sky, and the ground under the shelter should be covered with grass or leaves, not on any account left bare. The loss of heat by radiation of the ground to the open sky will produce a night temperature much lower than that of the air a few feet above the ground, and a radiation thermometer is often employed, laid on the grass and exposed to the sky to measure this effect. Travellers, however, can rarely be expected to make observations of such a kind, as the instrument is one of extreme delicacy.
Maximum Thermometers.—Maximum registering thermometers are filled with mercury, and are less liable to get out of order than spirit-thermometers. The simplest and best form for use by travellers is Negretti and Zambra’s. Its principle is very simple. When the temperature rises and the mercury in the bulb expands, it forces its way along the stem in the usual manner; but there is a little constriction in the tube just outside the bulb which breaks the column as the temperature begins to fall, and so prevents the mercury in the bulb from drawing back the thread of mercury from the tube. The thermometer is hung horizontally, and the end of the mercury farthest from the bulb always shows the highest temperature since it was last set. Before reading the thermometer, it is well to take the precaution of seeing that the inner end of the thread of mercury is in contact with the constriction in the tube, and if, by the shaking of the instrument or otherwise, the mercury has slipped away from this position, it should be brought back to it by tilting the thermometer bulb downwards very gently, then returning it to the horizontal position and reading.
To set this thermometer, it is only necessary to hold it vertically bulb downwards and shake it slightly, if necessary striking the lower end of the frame carrying the instrument, gently against the palm of the hand. This causes the mercury to pass the constriction and re-enter the bulb. When set, the end of the column farther from the bulb should indicate the same temperature as the ordinary dry-bulb thermometer.
Another form of maximum thermometer is known as Phillips’. It is an ordinary mercurial thermometer, but a short length of the upper part of the column in the tube is separated from the rest by a little bubble of air. It is used in the horizontal position, and as the temperature rises the whole column moves forward, while, when the temperature falls, only that portion behind the air-bubble retires towards the bulb. The tip of the column thus remains to mark the maximum temperature to which its farther end points. The instrument is set by gently tilting the bulb end downwards, when the detached portion of the column at once runs back until stopped by the air-bubble. This is the most convenient instrument to use at a fixed station; but in travelling it is apt to get out of order as shaking may have the effect of allowing the air-bubble to escape into the upper part of the tube, or into the bulb, and the instrument cannot easily be brought into working order again.
Rain-Gauge.—While measurements of rainfall can possess no climatological value unless they are carried on continuously at a fixed station, some very interesting observations may be made by the traveller both during the night when in camp, and during heavy showers when compelled to stop on the march. The rain-gauge is in itself the most simple of all scientific instruments, for it consists essentially of a copper funnel to collect the rain as it falls, and a bottle to contain what has been collected. A graduated measuring glass is the only accessory required. Rain is measured by the depth to which the water would lie on level ground if none soaked in, evaporated or flowed away. On an emergency, a rain-gauge can be improvised out of a biscuit tin, or any vessel with vertical sides and an unobstructed mouth. Such a vessel standing level would collect the rain, the depth of which might be measured by an ordinary inch-rule. It is rare, however, to find rain so heavy as to give any appreciable depth when collected in a vessel freely open to evaporation, and in order to estimate the amount of rainfall to small fractions of an inch, the device is employed of measuring the water collected in the receiver of the gauge in a glass jar of much smaller diameter than the mouth of the collecting funnel. Thus, if the funnel exposes a surface of fifty square inches, and the measuring glass has a cross-section of one square inch, the fall of 1/50 of an inch of rain on the funnel will give a quantity of water sufficient to fill the measuring glass to the depth of an inch. In this way the actual rainfall may be read to the thousandth part of an inch without trouble. The smallest diameter for a serviceable rain-gauge is five inches, and this size is well adapted for the traveller. A three-inch rain-gauge might be employed, but the results obtained with it are not so satisfactory. The rain-gauge should be placed in an open situation, so that it is not sheltered by any surrounding trees or buildings, and it ought to be firmly fixed by placing it between three wooden pegs driven securely into the ground. The mouth of the gauge should be level, and when the instrument is fixed, the rim of the funnel ought to be one foot above the ground. A spare measuring glass should be carried, but as there is always a considerable risk of breaking such fragile objects, it is well to carry also one or two small brass measures of the capacity of half an inch, two-tenths of an inch, and one-tenth of an inch of measured rainfall. In this way, although no satisfactory record could be kept of light rainfall, a very fair estimate may be made of any torrential showers, the half-inch measure being used first, and then the smaller measures, finally estimating by eye the fraction of the tenth of an inch that remains over. It must, however, be distinctly borne in mind that an estimate formed in this way is not an accurate measurement, and the fact of using the rough method must be stated in the note-book.
When snow falls along with rain, the melted snow is measured as equivalent to rainfall, and if the funnel of the rain-gauge should contain some unmelted snow at the time of observation, it should be warmed until the snow melts before a measurement is taken. When snow falls in a strong wind the drift that occurs makes it almost impossible to measure the amount accurately, but an effort should be made to estimate the average depth of the snow over a considerable area.
If the receiving bottle of the rain-gauge should be broken by frost or accident, any other bottle may be used, or in default of a bottle, the copper case itself will act as a receiver, although the risk of loss by evaporation, and by the wetting of a large surface in pouring out the water, is considerably increased.
At a fixed station the rain-gauge should be read every morning. The traveller who only exposes his rain-gauge during a halt should be careful to state the hours when it was exposed and when it was read.
Barometers.—The barometer is the most delicate, and at the same time the most important, instrument which a meteorologist has to employ. It requires particular care in transport, and must be very carefully mounted and read, while several accessory observations have to be made at each reading in order to ascertain the corrections required for the subsequent calculation of the results. The function of the barometer is to measure the pressure of the air at the time of observation, and this purpose may be carried out by the use of two different principles. The oldest and best method is to measure the height at which a column of heavy fluid is maintained in a tube entirely free from air. The weight of this column is equal to the weight of a column of the atmosphere of the same sectional area. Mercury being the densest fluid is the only one usually employed, because the column balancing a column of the atmosphere of equal sectional area is the shortest that can be obtained, and, consequently, a mercurial barometer is the most portable that can be constructed on this principle. The mercurial barometer has come to be recognised as the standard in all parts of the world.
The average height of the column of mercury in a barometer is about thirty inches, and, consequently, the whole instrument cannot well be made less than three feet long, so that when account is taken of the glass tube, and the amount of mercury it contains, it is long, fragile and heavy. To avoid the disadvantages inherent in such an instrument, the method of measuring the pressure of the air by the compression of a spring holding apart the sides of an air-free flexible metallic box was devised, and the aneroid barometer invented. The aneroid is graduated on the dial in “inches,” i.e., divisions each of which corresponds to a change of atmospheric pressure, equal to that measured by one inch of mercury in a standard barometer. Although a carefully constructed aneroid is a very useful instrument indeed, it is not to be trusted like a mercurial barometer kept in a proper place. But a good aneroid is likely to be much more serviceable to the ordinary traveller on the march than a standard mercurial barometer, every packing and unpacking of which exposes it to the risk of breakage, or to the equally fatal risk of air obtaining access to the vacuous space at the top of the tube. The scale of a barometer may be divided into millimetres, or, as now recommended by the British Meteorological Office, into millibars or thousandths of a hypothetical “atmosphere.” We shall describe the Fortin barometer, which is best adapted for use at a fixed station, and one devised by Prof. Collie and Capt. Deasy, which is portable enough for the use of travellers.
The Fortin Barometer.—The barometer must be kept in a room with as equable a temperature as possible; the instrument must be absolutely vertical—hence it should be hung freely and not touched while it is being read; it must be in a good light, and yet be sheltered from the direct rays of the sun. The measurement of the height of any mercurial barometer is that of the difference of level between the surface of the mercury in the tube and the surface of the mercury in the cistern. When the mercury rises in the tube it falls in the cistern, and vice versâ, although when the cistern is much wider than the tube the changes of level there are much less than those in the tube. In most barometers an arbitrary correction is made to allow for this change, the “inches” engraved on the scale not being true inches. In the Fortin barometer, however, the lower end of the measuring rod is brought in contact with the mercury in the cistern before every reading, and then the scale of inches engraved on the upper part of the measuring rod gives the true height of the column of mercury. In calculating the barometric pressure for purposes of comparison, five corrections have to be applied: (1) for temperature, which requires the temperature of the barometer at the time of reading to be observed, (2) for altitude, which necessitates knowing the elevation of the place of observation above sea-level, (3) for the force of gravity at sea-level, which requires the latitude to be known, (4) for the capillary attraction between the mercury and the glass tube, which is a constant for each barometer, (5) for the slight imperfection in engraving the scale (index error), which is also a constant for each instrument.
Fig. 5.—Two Readings of the Barometer Vernier.
It is enough for the observer at a fixed station, and to such alone can the use of a Fortin barometer be recommended, to read the temperature on the thermometer attached to the barometer and to read the height of the mercury in the barometer tube. These two figures he is to enter in his note-book, and unless he is himself discussing the results, he should apply no correction whatever to them. The rules for observing, then, are:—
1. Read the attached thermometer and note the reading.
2. Bring the surface of the mercury in the cistern into contact with the ivory point which forms the extremity of the measuring rod by turning the screw at the bottom of the cistern. The ivory point and its reflected image in the mercury should appear just to touch each other and form a double cone.
3. Adjust the vernier scale so that its two lower edges shall form a tangent to the convex surface of the mercury. The front and back edges of the vernier, the top of the mercury, and the eye of the observer are then in the same straight line.
4. Take the reading, and enter the observation as read without either correcting it to freezing point or reducing it to the sea-level.
The scale fixed to the barometer is divided into inches, tenths, and half-tenths, so that each division on this scale is equal to 0.050 inch.
The small movable scale or vernier attached to the instrument enables the observer to take more accurate readings; it is moved by a rack and pinion. Twenty-four spaces on the fixed scale correspond to twenty-five spaces on the vernier; hence each space on the fixed scale is larger than a space on the vernier by the twenty-fifth part of 0.050 inch, which is 0.002. Every long line on the vernier (marked 1, 2, 3, 4, and 5) thus corresponds to 0.010 inch. If the lower edge of the vernier coincides with a line on the fixed scale, and the upper edge with the twenty-fourth division of the latter higher up, the reading is at once supplied by the fixed scale as in A (Fig. 5), where it is 29.500 inches. If this coincidence does not take place, then read off the division on the fixed scale, above which the lower edge of the vernier stands. In B (Fig. 5) this is 29.750 inches. Next look along the vernier until one of its lines is found to coincide with a line on the fixed scale. In B this will be found to be the case with the second line above the figure “2.” The reading of the barometer is therefore:—
| On fixed scale | 29.750 |
| On vernier (12 × .002) | .024 |
| Correct reading | 29.774 |
Should two lines on the vernier be in equally near agreement with two on the fixed scale, then the intermediate value should be adopted.
5. Lower the mercury in the cistern by turning the screw at the bottom until the surface is well below the ivory point; this is done to prevent the collection of impurities on the surface about the point.
The transport of barometers requires very great care in order to prevent the introduction of air into the tube or the fracture of the tube by the impact of the mercury against the top. To reduce the risk of these accidents, the barometer must be carried with the tube quite full of mercury, and in an inverted position, at least with the cistern end kept higher than the top of the tube. The flexible cistern of the Fortin type of barometer allows of it being screwed up tight so as to fill the tube and close the lower end of it. In case of breakage, the operation of fitting a new tube is not very difficult, but unless the tube has been carried out ready filled with mercury, this cannot well be attempted. In order to drive out the film of air adhering to the glass on the inside, it is necessary, after filling the tube, to raise its temperature to the boiling-point of mercury. No one should attempt either to fill or to change a barometer tube unless he has had practice in doing so under expert supervision beforehand.
The Collie Portable Mercurial Barometer.—This instrument is not likely to be broken in travelling. It is quickly set up, and from such tests as have been applied, it appears to give excellent results. The cistern and vacuum tube at the top are of equal diameter, and are connected by a flexible tube, and the difference in level of the mercury may be measured directly by means of a graduated rod, or as in Deasy’s mounting by means of a vernier. There is no attached thermometer, but if the instrument be used in the open air, and is exposed for ten minutes or a quarter of an hour before using, it will be sufficient to note the temperature of the air in the usual way.
The upper end (Fig. 7) is about 2.5 inches long, and contains an air-trap, into which all the air that may accidentally enter the barometer, either by the tap leaking, through the rubber tubing, or through either of the joints, must find its way. The lower or reservoir end (Fig. 8) is about 4.5 inches long, and has an air-tight glass tap about an inch below the broad part. These ends are forced into the rubber tubing, and, as an additional precaution against leakage, copper wire is bound round the joints. The scale is cut on an aluminium bar, along which two carriages, to which the barometer is attached, move up and down, and they can be clamped to the bar at any place (Fig. 6). By means of the verniers attached to the carriages, which are divided to 0.002 of an inch, it is easy to estimate the height of the mercury to 0.001.
To use the barometer, the carriages are put on the scale bar; the lower one is clamped at the bottom of the bar, and the upper one some inches higher up; the barometer is attached to the carriages by clamps which fit over the joints; the rubber cap is removed from the reservoir end, the tap opened, the verniers put in the middle of their runs, and the upper carriage moved up the bar until there is a vacuum. By means of the screws on the right of the carriages the verniers are moved up or down until the top of the mercury at each end is in line with the edges of the rings attached to the verniers, which fit round the glass ends. Both verniers are then read, and the difference gives the height of the barometer. The rubber cap on the reservoir end is merely to prevent the small quantity of mercury, which should be left above the tap when it is closed, from being shaken out when travelling.
Fig. 6. The Collie Barometer, with the Deasy Mounting, in its Normal Working Position.
Fig. 7. The Upper Carriage and Vernier on a larger scale, with Barometer Attached.
Fig. 8. The Lower Carriage and Vernier, with Reservoir End of Barometer Attached. (Same scale as Fig. 7.)
To pack up the barometer, lower the upper carriage very slowly until the mercury has touched the top of the glass; then detach the barometer from this carriage, and either let the upper end hang vertically below the reservoir, or detach the reservoir end from its carriage and raise it till the barometer hangs vertically. By this means the barometer is completely filled with mercury, and then the tap must be closed. The tube is then to be coiled away in its padded box. When too much air is found in the trap, it must be extracted by means of the air-pump.
The Aneroid Barometer.—The aneroid barometer is so convenient on account of its portability that, although much less trustworthy than a mercurial barometer, it is much more likely to be used by a traveller. Care should be taken in using it to see that the pointer has come to a position of equilibrium, and it should be tapped gently before reading. The eye must be brought directly over the end of the pointer, and the reading made to one-hundredth of an inch, the barometer being held in a horizontal position. Every opportunity of comparing the aneroid with a standard mercurial barometer should be taken, and a note made of the readings of both. The mercurial barometer will require to be corrected for temperature before its indications can be used for correcting the aneroid, as all good aneroids are compensated for changes of temperature. The readings of an aneroid give a very fair idea of the changes of atmospheric pressure, and are very much better than none at all, although they cannot in any case be accepted as of the highest order of accuracy.
The Watkin mountain aneroid, which is so constructed as to be thrown into gear at the moment when it is read, appears to be free from the worst errors of the ordinary aneroid.
For climatological purposes, it is impossible to make barometric observations of value while travelling unless the altitude of each camping-place is accurately known. This is practically never the case except when travelling along the sea-shore or the margin of a great lake the elevation of which has been determined. But, meteorology apart, barometric readings in any little known country are of value, because by comparing them with simultaneous readings taken at a neighbouring fixed station, new data as to the altitude of the country may be obtained. While in camp, it would be an extremely useful thing to make barometer readings, even with an aneroid, every two hours, in order to get some information as to the normal daily range of atmospheric pressure.
The Boiling-point Thermometer.—The temperature at which water boils depends on the pressure of the atmosphere, so that an accurate observation of the boiling-point of water enables the pressure of the atmosphere at the moment of observation to be determined with the utmost accuracy. This method of determining atmospheric pressure having been used hitherto almost solely for the purpose of measuring altitudes, the boiling-point thermometer is usually known as the Hypsometer, but its records are quite as valuable for use at fixed stations as in mountain climbing. Mr. J. Y. Buchanan recommends the use of a boiling-point thermometer with a very open scale graduated to fiftieths of a degree Centigrade and entirely enclosed in a wide glass tube through which steam from water boiling in a copper vessel is passing. On a thermometer of this kind change of pressure can be measured by the change of boiling-point more accurately than with the aid of a mercurial barometer. See Table, Vol. I., p. 293.
2. Observations for Forecasting the Weather.—The familiar name of “weather-glass” is appropriately applied to the barometer, for in most parts of the world it is the surest indicator of any approaching storm.
The scientific prediction of the weather by means of the barometer involves the comparison of the simultaneous readings of barometers over as wide an area as possible, and can only be carried out where there is a complete telegraph system and a public department charged with the work. The storms of wind and rain which break the more usual steady weather are usually associated with the formation of centres of low atmospheric pressure towards which wind blows in from every side. These atmospheric depressions move, as a rule, in fairly regular tracks, the rate of movement of the centre of the depression having no relation to the rate at which the wind blows or to the direction of the wind. The term cyclone is usually applied to such a moving depression, because of the rotating winds round the centre; but the size of a cyclone may vary from a vast atmospheric eddy extending across the whole breadth of the Atlantic to one only a few miles in diameter. The strength of the wind in a cyclone depends on the barometric gradient; in other words, the greater the difference in atmospheric pressure between two neighbouring points the stronger is the wind that blows between them. Or, when a cyclone is passing over an observer, the more rapidly the barometer falls or rises the stronger may the wind be expected to blow.
In direct contrast to the cyclone or depression is the system of high pressure rising to a centre from which the wind blows out on every side. This is called an anticyclone, and is a condition which, once established, may last for many days, or even weeks, without change. It is the typical condition for dry calm weather in all parts of the world.
Fig. 9.—Cyclone Paths and Circulation of Winds in Cyclones in the Northern and Southern Hemispheres.
The direction of movement of the centres of cyclones in the northern hemisphere is usually westward and northward near the equator, the path of the centre bending to the right as it proceeds, and becoming ultimately eastward and southward. In the southern hemisphere the direction of the centre near the equator is westward and southward, turning towards the left as it proceeds. The rotation of the wind about the centre of a cyclone in the northern hemisphere is inwards towards the centre in the direction opposite to the hands of a watch, and in the southern hemisphere it is in the direction in which the hands of a watch move (Fig. 9). In the centre of a cyclone there is a calm, a well-known danger to sailing ships caught in such a storm at sea, because there is no wind to move the vessel, but a tremendous sea driven in from the gale which rages all round from every point of the compass. The law of storms has been very fully studied, and rules have been drawn up to enable sailors to ascertain the direction in which the centre of an approaching cyclone lies and the direction in which it is moving. In a work intended mainly for travellers on land it is not necessary to give these rules; all that is required is to tell how the approach of dangerous storms may be ascertained some time in advance. The fact that the barometer is high or low is in itself of no value for prediction. The important thing to know is the distribution of atmospheric pressure at a given moment over a considerable area. To the isolated observer this is impossible, and he can only judge of the state of the atmosphere by observing the rate at which the barometer is falling or rising. Thus, if for several days the barometer has been steadily and slowly rising, he will probably be right in believing that an anticyclonic condition is establishing itself, and that the weather may be expected to continue fine for many days to come, even after a gradual fall of the barometer begins. A sudden fall of the barometer, on the other hand, is always a sign of wind, and usually of wet weather as well. This is a particularly valuable sign of approaching storm in those parts of the world where, as in the tropics, the normal weather is very uniform and steady. In such places a very sudden fall, say one-tenth of an inch in an hour, is a sure precursor of a violent storm. As the barometer continues to fall, the wind will probably continue to increase in force, and when the barometer reaches its lowest point it will either fall calm (if the centre of the cyclone is passing over the observer) or suddenly change in direction. The rapid rise of the barometer after a great depression is also always accompanied by strong wind, though not so frequently by rain.
It must be clearly understood that these remarks refer only to observations at a fixed station. If a fall of barometer is observed in travelling, it may be due either to a change in the state of the atmosphere or to a change in the traveller’s height above sea-level. This is the reason why it is absolutely essential, in making barometric estimates of height (or boiling-point determinations), to have simultaneous observations going on at a base-station, or preferably at a series of intermediate stations.
The ordinary prognostics of the approach of rain or bad weather differ in different localities, and require a considerable amount of local knowledge before they can be utilised. The peculiar absorption band in the solar spectrum due to the water vapour of the Earth’s atmosphere, and called the rain-band, is a valuable guide to an experienced observer with a spectroscope in predicting rain. The only instrument, however, likely to be useful to the ordinary traveller is the wet and dry bulb thermometer. When the two thermometers have the same reading, indicating saturation of water vapour, or when they approach at temperatures above 60° F. within two degrees or so, rain may be expected, or possibly mist. The appearance of low clouds clinging to the hillsides is an indication that the temperature at the place where they are is below the dew-point. The appearance of the upper clouds, taken in conjunction with the readings of the barometer, is a valuable indication of forthcoming weather changes. The increase of cirrus clouds in a clear sky with a falling barometer, or the appearance of a solar or lunar halo, may be taken as a sure sign of an approaching cyclone, the intensity of which may be foreseen by the rate at which the barometer is falling.
While the weather of places on the west coasts of temperate continents exposed to the prevailing sea-wind is usually made up of a succession of cyclones of different degrees of intensity, and of the anticyclonic intervals between them, over the greater part of the Earth’s surface the climate is much more uniform, and the seasonal changes are the principal cause of changes of weather. To understand these general conditions it is necessary to consider the elements of climatology.
3. Outlines of Climatology.—The air is in constant movement on account of the unequal way in which the heat of the sun falls on different parts of the Earth’s surface, and at different seasons of the year. All the conditions of the atmosphere show a certain diurnal periodicity which is most marked in the regions of steady climate between and near the tropics. Thus, as a rule, the minimum temperature of the air occurs just before sunrise, the maximum temperature from two to three hours after noon. The amount of difference between the maximum and minimum temperature of the day (daily range) is least near the sea or in wet regions (a maritime climate) and greatest in the interior of the continents, especially where the rainfall is slight (a continental climate). Over the sea itself the daily range of air temperature averages only 3 Fahrenheit degrees; but in the heart of a continent, especially in a desert, it may exceed 60 Fahrenheit degrees.
Diurnal changes of pressure are proportionally much smaller in amount than changes of temperature, and are to be observed as a regular phenomenon only in the tropics, or elsewhere during very settled weather. There are usually two maxima daily, about 10 A.M. and 10 P.M., and two minima occurring about 4 A.M. and 4 P.M. It is only in rare cases that the total barometric range exceeds 0.10 inch, very frequently it is not greater than 0.04 inch. Still it is convenient to remember in the tropics that a fall of the barometer not greater than 0.10 inch between 10 A.M. and 4 P.M. is to be expected, and does not indicate either the approach of a storm (if the observer is at rest) or the ascent of 100 feet (if he is on the march).
Associated with the diurnal changes of temperature in settled weather are changes of wind due to local configuration of the ground. The wind, for example, usually blows up a mountain side, or up a steep valley, during the day, and down a mountain, or down a steep valley, during the night. So, too, the regular land and sea breezes found on the borders of the sea or of great lakes blow from water to land in the day time and from land to water at night. Here the determining cause is the fact that land is warmed and cooled by radiation, and in turn heats or chills the air much more than water does. In the settled climates of high tropical plateaus a regular diurnal change of wind direction has been observed, the wind blowing successively from all points of the compass.
A similar diurnal periodicity occurs in the amount of cloud, in the moisture of the air, the fall of rain, the occurrence of thunderstorms, etc. It is also to be noticed in the flow of rivers in mountainous regions where the streams take their rise from glaciers or snow, the rapid melting of which by the heat of the sun causes the volume of water to increase greatly in the afternoon, while the cessation or reduction of the rate of melting at night diminishes the volume of the river in the morning and forenoon.
Periodic changes of greater amount but similar in kind are produced by the alternation of the seasons, the difference between the mean values of the months in which the phenomena are at a maximum and minimum respectively being termed the annual range. With regard to temperature, very moderate changes occur in the tropical zones where the altitude of the noon-day sun is always great, and the length of day and night varies little with the season (for the most part less than 5 Fahrenheit degrees); but in the temperate and frigid zones there are strongly marked annual changes. As in the case of daily range, proximity to the sea is a controlling factor in the annual range of temperature. To take a very characteristic instance, the annual range between the mean temperature of July and January is about 23 Fahrenheit degrees in the Lofoten islands on the margin of the Atlantic, while it is 120 Fahrenheit degrees at Verkhoyansk in the same latitude, but in the centre of the Asiatic continent.
The extreme months for air temperature are January and July in almost every part of the world, the maximum occurring north of the equatorial belt in July and south of it in January.
The annual changes in barometric pressure and wind are equally marked. The belt of low pressure which lies nearly under the vertical sun moves northward over the surface of the globe in the northern summer, coming to its most northerly position in July: returning southward after the sun, it reaches its most southerly position in January. This belt of low pressure is also a belt of calms, known by sailors as the Doldrums, and it is a belt of frequent rains, so that as it approaches and passes over a place there is a rainy season, followed by a dry season when it retires. Near the mean position of the belt of low pressure, where it passes over a place twice in the year, there are two rainy seasons. The low pressure belt is bordered to north and south by belts of high atmospheric pressure, from which the trade-winds blow towards the equator, and the westerly anti-trades blow towards the poles. These are also subject to the annual change; but the different action of land and sea on the distribution of pressure exercises a greater influence than does the difference of latitude. As the greater heating and cooling of the land each day causes the phenomena of daily land and sea breezes, so the greater heating and cooling of the land between summer and winter causes seasonal land and sea winds, blowing from land to sea in winter, from sea to land in summer. Generally speaking, the pressure is greater—in the same latitude—where the air is cooler, so that outside the frigid zones cold areas are usually areas of high pressure, from which wind blows out in every direction, while warm areas are areas of low pressure towards which wind blows in on every side.
The distribution of rainfall on the land is dependent on the direction of the rain-bringing wind and the configuration of the surface. Thus when the rain-bringing wind meets a mountain range, it deposits a great rainfall on the exposed slopes, but passes over as a dry wind which yields little rain to the region beyond. In places where the wind changes with the season, as in southern Asia, the distribution of rainfall is entirely different during the continuance of the different monsoons.
All these questions of normal climate can be more easily illustrated on maps than explained by words. But the reader must be cautioned against taking the condensed and generalised representations of small-scale maps as showing all that is known on the subject. Even the magnificent plates in the ‘Atlas of Meteorology,’ which forms part of Bartholomew’s Physical Atlas, cannot show everything that is known; and in many parts of the world so little has yet been ascertained as to the climatic conditions that generations of observers will be required to make it possible for meteorologists to draw a uniform trustworthy map of the whole world showing the distribution of any one element of climate.
Isothermal maps.—The principle of an isothermal map is that of representing the distribution of temperature by drawing lines through all the places where the temperature is the same at a given time. It is usual to take this time as an average month in an average year. Thus in a map of isotherms for January (see [p. 50]), what is shown is not the temperature of any particular day in any particular January, but that of an average day in a long series of Januaries. Hence it is not likely that the exact distribution of temperature shown in the map will ever be found on any January day; but it is to be expected that most days in every January will have a distribution of temperature which is very similar to that shown. The same is of course true of maps showing pressure, or rainfall, or any other average condition.
Again, the isotherm is necessarily constructed from average temperatures which have been corrected so as to be applicable to the same level. On the equator, for instance, the summit of a lofty mountain is seen by the snow on it to have a temperature not exceeding 32° F., while at sea-level the temperature may be 90°. But observations have been made showing the rate at which the temperature of the air diminishes as the height increases, and although the rate varies in different places and at different seasons, it may be taken roughly as one Fahrenheit degree in 300 feet. Now if the mountain top with a temperature of say 30° F. is known to be 18,000 feet above the sea, the addition of 1° for every 300 feet, or 60° altogether, would give the temperature of 90° as that corresponding to sea-level. By applying such corrections, the isothermal maps have been constructed to show the distribution of temperature at the level of the sea. In order to compare the temperature he has observed with that on the map the observer must calculate the average of his daily observations for the month in question, and then make the correction for the altitude of his station.
Similarly, in ascertaining from an isothermal map the mean temperature of a particular place, care must be taken to subtract from the number of degrees of the isotherm passing through the place one degree for every 300 feet of elevation. Of course it will usually happen that no isotherm as shown on the map runs through the point the mean temperature of which it is desired to obtain. In that case the temperature at the point will be found by considering its relative position between the two nearest isotherms. Thus, if it lie half-way between the lines of 60° and 70°—measured perpendicularly to the isotherms—the temperature of 65° may be assumed; if it lies one-tenth of the distance from 60° and nine-tenths from 70°, it is safe to assume 61°; if three-tenths from 70° and seven-tenths from 60°, then assume 67°. If the point lie in a loop of a single isotherm, e.g., Cape St. Roque, the eastern point of South America in the map for January, lying within the 80° isotherm, one can only guess that the temperature is above 80° and it may be assumed to be below 85°. The method of representation is unsatisfactory in such a case.
These facts being borne in mind, the study of isotherm maps will be found to give an excellent general idea of the distribution of climate at sea-level, and if the contour lines of 600 and 6000 feet are traced on the maps the areas within which corrections of over -2° and -20° have to be applied to the isothermal values to get the temperature at the place will be easily recognised.
Isobaric Maps.—Isobars are drawn from the data of the height of the barometer corrected to sea-level values and to the temperature of 32° F., exactly in the same way as isotherms are drawn from the data of thermometer readings or contour lines from data of altitude measurements. The practical value of the study of isobars is very great, because of the importance of assuming a probable value of sea-level pressure in reducing the barometric or boiling-point thermometer readings for determining elevation, and also because of the intimate relation between the form and proximity of isobars and the direction and force of the winds.
Barometric gradient is measured by the difference between the isobars per unit of length. For instance, gradient is frequently expressed in the number of hundredths of an inch difference between barometers fifteen nautical miles apart. The greater the gradient of pressure is, the more closely together must the isobars be drawn in order to represent it. For example, in the isobaric map for January ([p. 50]) a very steep gradient is shown on the east coast of Asia, north of Japan, and a remarkably gentle gradient in the interior of Asia from the Black Sea eastward. The steeper the gradient the stronger is the wind.
The arrows in the isobaric maps (which are represented flying with the wind) show the average directions of the wind over the world for the months in question. The relation they bear to the isobars becomes clear on inspection, although, on account of the greater number of observations available for some parts of the world than for others, all the arrows are not drawn with the same amount of certainty, and the direction of a few contradicts that of most. As a general rule, the following facts may be taken as absolutely established: (1) Wherever there is a region of high pressure the wind blows out from it in all directions. (2) Wherever there is a region of low pressure the wind blows in towards it from every side. (3) The wind never blows perpendicularly to the isobars or directly from higher to lower pressure, but always in a curved or spiral path inclined to the isobars. (4) In the northern hemisphere the wind blows out from a high pressure area in the same direction as the hands of a watch move, but in the southern hemisphere in the opposite direction. Also in the northern hemisphere the wind blows into a low-pressure area in the direction opposite to that of the hands of a watch and in the southern hemisphere in the same direction as the hands of a watch move. (5) Recognising that the wind blows nearly parallel to the direction of the isobars, the following statement (known as Buys Ballot’s Law) expresses its direction both for high-pressure and for low-pressure areas: If you stand with the lower pressure on your left hand, and the higher pressure on your right hand, in the northern hemisphere the wind will be blowing on your back, but in the southern hemisphere in your face.
Rainfall Maps.—Rainfall is represented on maps by lines of equal precipitation termed Isohyets. These represent actual figures without reduction for elevation or other local conditions, and a rainfall map can consequently be studied as a direct record of observed facts. The map ([p. 50]) of mean annual rainfall brings out clearly the equatorial zone of heavy rains crossing the Amazon valley, the Congo valley, the southeastern peninsulas of Asia and the Malay archipelago. North and south of this belt are the nearly rainless regions of the tropical deserts, extended northward and southward over the continents, and merging nearer the poles into the fairly-watered temperate zones. The rainfall maps for separate months show the intimate relation between rainfall and the direction of the wind taken in conjunction with the configuration of the land. Even on the coast, when the prevailing wind is off shore, there may be scarcely any rain, as on the west coast of tropical South America. In the very heart of a continent the rainfall may be very heavy where the sea-wind blows across a great plain before striking the mountains, as is illustrated by the eastern slope of the Andes. Rainfall is, however, one of the most inconstant elements of meteorology, and the actual rainfall of any year may differ very widely from the average. The practical value of exact statistics of rainfall is, however, greater than that of any other climatological condition; for the water supply and the fertility of the land depend in every case on the rain that falls either locally or on the heights of the water-sheds.
In order to pursue the subject further the chapters on the atmosphere in the writer’s ‘Realm of Nature’ (London, Murray; New York, Scribner) and Mr. L. C. W. Bonacina’s ‘Climatic Control’ (London, A. and C. Black) may be useful. The most systematic treatment of climatology will be found in Hann’s ‘Handbuch der Klimatologie,’ 3 vols. (Stuttgart), which contains numerous references to special works; the essential part of this treatise is translated by Professor R. de C. Ward, under the title of ‘Handbook of Climatology,’ Part I. (Macmillan). The most important work of all is the great ‘Atlas of Meteorology’ by Dr. A. Buchan and Dr. A. J. Herbertson, forming Vol. III. of Bartholomew’s ‘Atlas of Physical Geography’ (London, Constable), which gives an unrivalled series of climate and weather-maps with explanatory letterpress.
The recent great advances in meteorology have rendered the old textbooks obsolete, while the new data, especially those regarding the upper regions of the atmosphere, have not yet been built into a coherent system. Sir Napier Shaw’s ‘Forecasting Weather’ (London, Constable & Co.) is a pioneer discussion, and the numerous publications of the Meteorological Office, South Kensington, London, S.W.7, may be consulted with much advantage.
Daily synoptical weather-maps are published by the Weather Service of almost every civilised country. Those for the United Kingdom may be obtained from the Meteorological Office of the Air Ministry. The only weather-maps of large areas produced regularly are the Pilot Charts of the North Atlantic and the North Pacific, published monthly by the Hydrographic Office at Washington, and those of the North Atlantic and Mediterranean published monthly by the Meteorological Office in London. These show the tracks of cyclones, and give a great deal of information as to the meteorology and currents of the oceans. They are intended primarily for the use of sailors.
The following list gives the name of the official weather service of all countries outside Europe and the town in which the head office is situated. Application might be made to any of these offices for information as to the stations where standard instruments are established in the country in question.
Extra-European Weather Services.
In addition to the above, which are regular government services specially organised for publishing and utilising the data from numerous observing stations, there are many isolated stations in all parts of the world. There are no colonies which do not possess some meteorological stations, and at many mission stations meteorological observations are made. It would always be well for a traveller to try to ascertain where in the vicinity of his route meteorological stations have been established and over what period of time their records extend.
In the Table ([pp. 44-49]), taken from Marriott’s “Hints to Meteorological Observers,” is given the relative humidity for every 2° of temperature from 20° to 80°, and for every two-tenths of a degree of difference between the dry and wet-bulb readings from 0°·2 to 18°·0.
To use the Table: Look in the column on the left or right for the nearest degree to the dry-bulb reading; then carry the eye horizontally along until the column is reached corresponding to the difference between the readings of the dry and wet-bulb thermometers, when the relative humidity will be found. Intermediate readings can be interpolated in the usual way.
Example: Dry-bulb 58°·5, wet-bulb 52°·7, the difference is 5°·8. Having found 58° in the column on the left or right, run the eye along this line until the column under 5°·8 is reached, when the relative humidity will be found, viz., 67.
Table of Relative Humidity.
Table showing the Pressure of Saturated Aqueous Vapour in inches of Mercury at Latitude 45° for each degree Fahrenheit from -30° to 119°.
| ° | Inch. | ° | Inch. | ° | Inch. | ° | Inch. | ° | Inches. |
|---|---|---|---|---|---|---|---|---|---|
| -30 | 0.0099 | 0 | 0.0440 | 30 | 0.1665 | 60 | 0.5192 | 90 | 1.4128 |
| -29 | 0.0105 | 1 | 0.0461 | 31 | 0.1738 | 61 | 0.5379 | 91 | 1.4578 |
| -28 | 0.0111 | 2 | 0.0482 | 32 | 0.1815 | 62 | 0.5572 | 92 | 1.5040 |
| -27 | 0.0117 | 3 | 0.0504 | 33 | 0.1888 | 63 | 0.5771 | 93 | 1.5514 |
| -26 | 0.0123 | 4 | 0.0527 | 34 | 0.1964 | 64 | 0.5976 | 94 | 1.6001 |
| -25 | 0.0130 | 5 | 0.0551 | 35 | 0.2043 | 65 | 0.6187 | 95 | 1.6502 |
| -24 | 0.0137 | 6 | 0.0577 | 36 | 0.2125 | 66 | 0.6405 | 96 | 1.7017 |
| -23 | 0.0144 | 7 | 0.0604 | 37 | 0.2210 | 67 | 0.6630 | 97 | 1.7546 |
| -22 | 0.0152 | 8 | 0.0632 | 38 | 0.2297 | 68 | 0.6862 | 98 | 1.8088 |
| -21 | 0.0160 | 9 | 0.0661 | 39 | 0.2388 | 69 | 0.7101 | 99 | 1.8646 |
| -20 | 0.0168 | 10 | 0.0691 | 40 | 0.2482 | 70 | 0.7347 | 100 | 1.922 |
| -19 | 0.0177 | 11 | 0.0723 | 41 | 0.2579 | 71 | 0.7601 | 101 | 1.980 |
| -18 | 0.0186 | 12 | 0.0756 | 42 | 0.2679 | 72 | 0.7862 | 102 | 2.041 |
| -17 | 0.0196 | 13 | 0.0790 | 43 | 0.2783 | 73 | 0.8131 | 103 | 2.103 |
| -16 | 0.0206 | 14 | 0.0825 | 44 | 0.2890 | 74 | 0.8409 | 104 | 2.166 |
| -15 | 0.0217 | 15 | 0.0862 | 45 | 0.3001 | 75 | 0.8695 | 105 | 2.231 |
| -14 | 0.0228 | 16 | 0.0901 | 46 | 0.3116 | 76 | 0.8989 | 106 | 2.298 |
| -13 | 0.0239 | 17 | 0.0942 | 47 | 0.3235 | 77 | 0.9292 | 107 | 2.366 |
| -12 | 0.0251 | 18 | 0.0985 | 48 | 0.3358 | 78 | 0.9604 | 108 | 2.437 |
| -11 | 0.0263 | 19 | 0.1030 | 49 | 0.3485 | 79 | 0.9925 | 109 | 2.509 |
| -10 | 0.0276 | 20 | 0.1076 | 50 | 0.3616 | 80 | 1.0255 | 110 | 2.583 |
| - 9 | 0.0289 | 21 | 0.1124 | 51 | 0.3751 | 81 | 1.0595 | 111 | 2.659 |
| - 8 | 0.0303 | 22 | 0.1174 | 52 | 0.3891 | 82 | 1.0945 | 112 | 2.736 |
| - 7 | 0.0318 | 23 | 0.1226 | 53 | 0.4036 | 83 | 1.1305 | 113 | 2.817 |
| - 6 | 0.0333 | 24 | 0.1282 | 54 | 0.4186 | 84 | 1.1675 | 114 | 2.898 |
| - 5 | 0.0349 | 25 | 0.1339 | 55 | 0.4341 | 85 | 1.2056 | 115 | 2.982 |
| - 4 | 0.0366 | 26 | 0.1399 | 56 | 0.4501 | 86 | 1.2447 | 116 | 3.067 |
| - 3 | 0.0383 | 27 | 0.1461 | 57 | 0.4666 | 87 | 1.2850 | 117 | 3.156 |
| - 2 | 0.0401 | 28 | 0.1526 | 58 | 0.4836 | 88 | 1.3264 | 118 | 3.246 |
| - 1 | 0.0420 | 29 | 0.1594 | 59 | 0.5011 | 89 | 1.3690 | 119 | 3.338 |
Transcriber’s Note: Maps are clickable for larger versions.
ISOTHERMAL LINES SHOWING THE MEAN TEMPERATURE (FAHR.) OF THE GLOBE FOR JANUARY.
Published by the Royal Geographical Society in “Hints to Travellers.” 1921.
ISOTHERMAL LINES SHOWING THE MEAN TEMPERATURE (FAHR.) OF THE GLOBE FOR JULY.
Published by the Royal Geographical Society in “Hints to Travellers.” 1921.
ISOBARIC LINES AND PREVAILING WINDS OF THE GLOBE FOR JANUARY.
Published by the Royal Geographical Society in “Hints to Travellers.” 1921.
ISOBARIC LINES AND PREVAILING WINDS OF THE GLOBE FOR JULY.
Published by the Royal Geographical Society in “Hints to Travellers.” 1921.
MEAN ANNUAL RAINFALL OF THE GLOBE.
Published by the Royal Geographical Society in “Hints to Travellers.” 1921.
II.
PHOTOGRAPHY.
By J. Thomson, formerly Instructor in Photography R.G.S.
Revised by J. McIntosh,
Secretary of the Royal Photographic Society of Great Britain.
The photographic camera should form an essential part of the traveller’s outfit, as it affords the only trustworthy means of obtaining pictorial records of his journey, and it is also helpful in making the survey of a new region, delineating its contours, its geological and botanical features, and ethnographical types of race. The camera and materials necessary for a journey may be readily obtained, so designed as to minimise space and weight, and in every way so perfectly adapted to the traveller’s needs as to ensure successful results in every variety of climate, and render the operation of taking a photograph extremely simple. It is necessary, however, that the traveller should make himself master of the principles involved in the production of a successful photograph, as he will have to depend on his personal effort in exposing and developing the plate, etc. He should also acquire a knowledge of the construction of the camera, to enable him to effect slight repairs when necessary.
In selecting an outfit he must first decide upon the size of plate to be carried, and that need not exceed what is termed “half-plate,” 6½ × 4¾ inches; which is large enough for the best work. The smallest effective size for scientific work may be “quarter-plate,” 4¾ × 3¼ inches; in use in many hand-cameras. Negatives on this scale, if perfect, may be enlarged for book illustration, or printed as lantern slides. The two sizes given form a very complete outfit when extra weight may be conveniently carried.
Selecting a Camera.—The cameras should have bellows bodies of Russian leather which folds into small space, the woodwork must be well seasoned to prevent warping, or cracking under a hot sun. The framework should be metal-bound at the corners, and the camera fitted with a rising front and swing back, although the swing back is not indispensable. A reversible back (now universal) is of advantage, as it enables the operator to take vertical or horizontal views without turning the camera on its side. If the camera is fitted with a swing-front, the swing-back becomes unnecessary. The swing-front is in all ways preferable to the swing-back.
Bellows Camera.
The Hand-camera.—Hand-cameras are designed to carry a dozen or more plates or films in flat sheets or in spools, so arranged inside the camera as to be changed after each exposure by simply turning a milled head, or moving a lever. Rolled films are not recommended for travellers’ use in hot and humid climates. Sir Martin Conway says: “A traveller who carries glass plates and flat films will probably bring home a larger percentage of good negatives from a long mountain journey than one who relies upon spools of films.” There can be no question about the force and accuracy of this statement. My own experience goes to prove that a camera arranged for glass plates and flat films is best.
Twin-Lens Camera (open).
The twin-lens hand-camera made by Ross, of New Bond Street, London, offers several advantages in its design. It is fitted with a focal-plain shutter which is in every way simple and effective. The twin lenses are of equal focal length, enabling the object to be photographed to be seen on the same scale as it will appear in the finished negative, so that just what is required may be embraced in the field. It consists of a stout body of thoroughly seasoned hardwood, not easily damaged by rough usage. The principal fittings required for adjusting the instrument are inside, protected by the outer case. The exception to this arrangement is the milled head by means of which the two lenses are focussed at the same time. The lenses are of uniform focal length, so that the image transmitted by the “finder” is a counterpart of the image which falls upon the sensitive plate. The advantage of this is that the object to be taken is seen on the finder-screen to scale exactly as it will appear in the finished photograph. By this means the operator has it in his power to place the object in the required position on the screen at the moment of exposure. This is of signal importance if the object is moving about. It must also be noted that this form of camera may be used when the operator is facing at right angles to the object to be photographed. I have frequently found that natives of foreign countries resent the liberty taken of pointing a camera at them, and fly as if they expected to be shot. The slides are each made to hold two plates, or flat films. They are strong, serviceable, and easily managed, while the body of the camera is so arranged as to carry a roll holder. The camera can also be adapted to stereoscopic work, and fitted for the use of glass plates, flat films, or rolled films.
A light tripod stand should be taken for supporting the camera when longer exposures are required than can be given in the hand. A very satisfactory compromise has been adopted by Sir W. Abney between using the camera in the hand and on a tripod. He rests the camera on top of a walking-stick when making hand exposures, with the result that he overcomes all tremor caused by pulsation, and so secures photographs full of sharp detail.
The same object is obtained by the reflex type of camera in which the image formed by the lens is thrown upon a silver-faced mirror and reflected upward to the focussing screen. Focussing can be carried out, and a moving object followed till the moment when the exposure is made. The reflex type of camera is a trifle less bulky than the twin-lens, and, of course, only one high class and expensive lens is required.
A Focussing-cloth.—This is used for keeping out the light while focussing, being thrown over the camera and the head of the operator. It is generally made of black velvet, but waterproof sheeting is much better. It should have rings sewn on to one edge, or some arrangement by which it may be attached to the camera so as not to be blown away.
Camera-stand.—There are many varieties of tripod stands, with legs either folding or sliding into a small compass. For mountainous country it is of great advantage to have a stand with telescopic legs, as they can be readily altered in length so as to stand firmly on slopes or rocky ground. The smallest size, weighing about 3 lbs., and measuring 33 in. long when closed, and standing about 4 ft. 6 in. high, is steady enough to support a 6½ × 4¾ camera without perceptible vibration in a moderate wind.
A Small Circular Cup Level, let into the wood of the camera, for levelling the camera on the tripod, is a useful addition.
Lenses.—There are many lenses in the market, and as it is impossible to do good work with an inferior lens, it is necessary to exercise great care in selecting this part of a photographic outfit. Lenses known as rectilinear or symmetrical are useful to a scientific explorer, and are well fitted for producing pictorial effect in his work.
Ordinary portrait lenses are designed specially for rapid work, and this is attained at the cost of qualities in a lens most useful to an explorer. The so-called portrait combination should therefore be avoided.
Rectilinear and symmetrical lenses give true images of objects to be photographed free from distortion, so that straight lines are reproduced as straight lines. In this way they are invaluable where accurate measurements have to be taken from photographs produced by them.
Homocentric Lens.
Ross’s homocentric lens is one of the most useful lenses. It has a flat field, is free from what is called “coma” and astigmatism, and is so perfectly corrected as to fit it for interior and exterior work alike. It is also a rapid hand-camera lens. The homocentric are made in series to suit all cameras. Other lenses may also be noted, viz.: c. Dallmeyer’s rapid rectilinear, including about 37°. d. Zeiss’s anastigmat, made by Ross, consists of a double front lens and a triple back lens. It is intended for portraits, groups, copying, and general outdoor work. The combinations being brought closely together, gives them great illuminating power. They have an angular aperture of from 858 to 908, and can therefore be used as wide-angle lenses when desired. In consequence of the peculiar system of correction for oblique pencils adopted in these lenses, they behave somewhat differently from the usual types with regard to the mode of compensating the effect of the resulting aberrations between centre and margin of the field. This is, of course, only possible in the case of perfectly plane objects. In all other cases—landscape, instantaneous work, or interiors—the centre should be focussed rather than objects at a distance or foreground.
Focus.—It will be sufficient to say that focal length means the distance between the diaphragm of a lens, and the ground-glass screen when the image of a distant object is seen most distinctly on the screen.
Exposure Tables.—Exposure tables are based on the focal length of a lens, in relation to the diameter of the diaphragm of a lens. Thus, if the focus is eight inches and diameter of diaphragm one inch, the relationship will be expressed by f /8 or by the uniform standard number 4, and so on, as in table. The diaphragms are so arranged for size of opening that each succeeding number requires double the exposure necessary with the one preceding it. f /11.3, for example, requires double the exposure wanted with f /8.
| U. S. Nos. | 4 | 8 | 16 | 32 | 64 | 128 | 256 |
| f | f | f | f | f | f | f | |
| Ratio of Stops. | — | — | — | — | — | — | — |
| 8 | 11.3 | 16 | 22.6 | 32 | 45.2 | 64 |
Such tables are useful guides to the relative duration of exposure with diaphragms of different sizes applied to the same lens. They afford no clue, however, to time of exposure to be given with any particular lens or diaphragm. This can be best ascertained by experience, as duration of exposure of a plate or film in the camera depends on the sensitiveness of the plate, the time of day, the sun, the state of the atmosphere, the nearness or distance of the object to be photographed, etc. To take an extreme case of the difference of time required to impress the plate with the image of an exterior view and that of an interior, a landscape open and well lighted may be taken in the fraction of a second, while a dimly-lighted interior with the same lens would require an hour, both being taken with plates coated with the same emulsion. The duration of exposure may be approximately estimated by using an exposure meter such as may be obtained from any photographic dealer’s. It is useful to keep an exposure record; a handy book for this purpose is published by Messrs. Burroughs & Welcome. In this book rules are set down for exposure during different months of the year and for different latitudes. These are apt to prove misleading to the amateur. The simplest method of measuring the actinic power of light in any latitude, and at any moment, is by actinometer, giving plate-speeds, focus of lens, etc. Watkins “Bee Meters” are excellent for this purpose.
Sensitive Plates or Films.—Gelatine plates are now made commercially by a large number of firms and of great excellence; they keep indefinitely before exposure, and for a long time afterwards and before development and under some circumstances (as for instantaneous pictures, portraits, and dimly-lighted interiors) will give results which could hardly be obtained at all on collodion plates. Gelatine plates are made of various degrees of sensitiveness; the slowest are best for ordinary landscape work. They are generally supplied in parcels of a dozen each, packed face to face with strips of folded paper between opposite edges. The card boxes in which they are usually packed are an insufficient protection against injury and damp. In all cases it is advisable, and for sea voyages and damp climates essential, to have each package of a dozen plates soldered down in a tin case, and afterwards packed in a light wooden box with tow or cotton wool, and the box screwed (not nailed) down. In packing them up again after exposure or after development, a good plan (due to Sir W. Abney) is to provide oneself with a number of cardboard frames exactly the size of the plates, made of strips of card about ¼ in. wide, one of which is inserted between every two plates film to film. The packages thus made up should be soldered down again, and treated with at least as much care as the original plates in packing. Should there be no available means of resoldering the boxes, it will be better to have tin boxes with the lid turned well down, the joinings to be closed by strong well-gummed paper or medical rubber strapping. It will also be well to be provided with a supply of waterproof paper, or cloth, as an additional precaution in packing and in case of emergencies.
Sensitive films in rolls or spools are made by the Kodak and other companies, and may be used successfully in their proper roll-holders when they can be kept perfectly dry in temperate climates. Flat films made by Kodak, Ltd., and others have many advantages for travellers. The celluloid of which they are made is very much lighter than glass, and in exposure and development may be treated in the same way as a glass plate. When plates can be carried, the extra weight is compensated for by greater certainty of success, and general excellence in the photographs.
How to Keep Plates and Films Dry.—When the traveller has a long journey before him, and the prospect of storing his plates and films for months both before and after exposure, it is of the greatest importance that precautions should be taken against the inroads of damp. This applies with full force when the country to be explored has a hot, humid climate. Plates and films that have absorbed moisture, causing decomposition in the sensitive gelatine coating, are frequently brought back to this country to be developed, and are the most fruitful cause of failure. The remedy is simple, but can only be applied when packing and repacking the plates. Some guarantee should be sought from makers of plates and films that they are packed perfectly dry, and that the packing used is also dry. Assuming that work has to be done in a damp climate and that the plates have been exposed in the camera and require to be repacked, they should be dried in a box containing a small quantity of chloride of calcium. The box used for drying may be also designed to carry the camera and outfit. It should have a lid with a rim of rubber padding, so that by putting the lid on and a weight on it, the box would be fairly air-tight. Stack the exposed plates, or films, in the bottom of box, so separated as to permit the passage of air between. Place a cup or saucer on the bottom of box containing chloride of calcium. (The chloride should be first dried on a piece of iron over a fire.) Put on the lid and allow the plates to remain for an hour or more. Dry all the packing materials, remove the plates from the box and repack. The chloride will have absorbed the moisture in the plates, and rendered them quite dry and safe for preserving for an indefinite length of time.
Apparatus and chemicals for development.—The development of the plates or films after exposure in the camera requires practice and experience in order to secure the best results. Instructions for development are sent out with all commercial plates or papers, but many failures would certainly result from attempting to work by these without some preliminary practice at home. As plates, &c., will keep after exposure (if well protected from damp) for 18 months, or longer if properly packed, it is not, of course, necessary to develop them en route, although if the traveller possess sufficient skill, and if ample water-supply and other facilities can be secured, it will be advantageous for many reasons to do so. On a long journey, when the temperature is not too high, use of convenient resting-places may be made to develop from time to time a few plates selected from the whole, both as tests for exposure and as proof that all the apparatus is in order. The following list comprises all that is absolutely required for developing 8 or 10 dozen gelatine plates:—Three papier-mâché dishes, two 3-ounce glass measures, three 6-ounce bottles, containing strong solutions of pyrogallic acid preserved with potassium meta-bisulphite, potassium bromide, and sodium carbonate respectively, 1 lb. hyposulphite of soda, and ¼ lb. alum, both in crystals, 4 or 5 feet of india-rubber tubing and a spring clip, to make a syphon for a water supply from a jug or can, a basin or tub to serve as a sink, a folding rack for draining the plates.
There are several convenient new developing agents in the market: Hydro-Kinone, Eikonogen, Glycin, Metol, Rodinal, and Amidol. Some of these were made and named in Germany, they are no longer procurable under their German names. English manufacturers are however making substitutes quite as good under new names, and these can be obtained from the dealers. Many of them can be obtained in compressed form.
The traveller is recommended for advanced study of photography, such works as Instructions in Photography by Sir Wm. Abney, The Science and Art of Photography by Chapman Jones, and The Manual of Photographic Exposure and Development by Alfred Watkins, which may be had from any photographic dealer’s.
The aim of the traveller-photographer should be the production of good negatives. It often requires years of study on the part of professional operators (with advantages impossible to the traveller) before thoroughly good negatives are habitually produced; and it must not be supposed that a person taking up photography for the first time, in a few hurried moments before departure on a journey, will attain other than very unsatisfactory results.
The operations necessary for taking a picture are briefly as follows:—Having selected the position from which the view is to be taken (for valuable hints as to the artistic production of pictures see Robinson’s ‘Pictorial Effect in Photography’), the tripod stand is first set up, and the head approximately levelled by means of the pocket level, altering the position or length of the legs as may be necessary. The camera is next screwed on to the stand, and the lens selected which on trial is found to include the required amount of subject. For groups or portraits a long focus lens with wide aperture, such as Dallmeyer’s “Rapid rectilinear,” 11 in. focus, should be used. The next operation is to focus the picture accurately on the ground-glass screen of the camera. The focussing-cloth is thrown over the head and the camera, so as to exclude the light as much as possible, and while looking at the inverted image on the ground glass, the milled head of the rack adjustment is turned till the image appears as sharp as possible. The camera is now turned about on its vertical axis till it exactly includes the view intended to be taken, and the screw is tightened. It may be necessary to raise or lower the front of the camera carrying the lens in order to include objects at a high or low elevation; if the vertical range of this sliding front is insufficient, the camera must be tilted; but, if this is done, care must be taken to set the focussing-screen vertical again by means of the swing back, and to readjust the focus. The full aperture of the lens should always be used for focussing, and if the image is not sharp all over the plate it will be necessary to insert a diaphragm in the lens, using the largest that will effect the required object. Having then put the cap on the lens, the hinged frame carrying the focussing-glass is turned over, and one of the slides carrying the sensitive plates is inserted in its place. The slides should be exposed as little as possible to the light, especially avoiding direct sunlight; however carefully constructed, it is difficult to make them absolutely light-tight. The shutter of the slide is then withdrawn, and the exposure made by removing the cap from the lens for time exposure, and by a spring shutter for instantaneous work. The time of exposure must be estimated according to circumstances, and it requires considerable experience to judge of it accurately. A record should be kept in a note-book of every plate exposed, giving the number, date, time, exposure, subject, &c. If the plates cannot be developed the same evening, and the slides are wanted for fresh plates, they must be packed up again, and should be numbered. This is best done by marking the number on the back with a bit of dry soap, or in the film with a lead pencil. The image on the plate after exposure is latent and invisible, and has to be developed. This is effected by pouring on the plate, laid in one of the flat dishes, a dilute solution containing pyrogallic acid, soda, and potassium bromide. The excellence of the result largely depends on the due proportion between these constituents, and here more experience is perhaps necessary than in any other part of the process. The image having been fully developed, the plate is well washed, and then immersed in a solution of alum, which hardens the film. After another thorough washing it is “fixed” by immersion in a solution of sodium hyposulphite, which dissolves out the unchanged bromide of silver, and, being once more well washed, it is finished, and must be set up in the rack to dry spontaneously. On no account must heat be applied, not even the warmth of sunlight, or the wet film will melt. When dry it must be varnished to protect the film. The printing operations are best deferred till the return home, as they would involve the carriage of a large amount of extra apparatus. It is generally best to get the printing done by a professional printer; but if the traveller prefers to print from his own negatives he will find full instructions in each packet of paper which he buys.
Colour sensitive plates are now much used and, when a yellow glass filter is placed immediately in front of, or behind, the lens, will give in the print the same variations in depth of tint which the eye sees in the landscape, or other object. The nearest to perfection among these plates are those known as Panchromatic, but they must be developed in total darkness. The subject is a large one and should be studied in the pamphlets on ‘orthochromation’ published by Kodak (Wratten Division), and Ilford, Ltd.
As regards the expense of a photographic outfit, at the present time the manufacturing trade is still very unsettled, some goods cannot be supplied, and all have been greatly increased in price. Quarter plates which used to be sold at a shilling per dozen are now about 3s. 6d. Application should be made to the dealers for current prices.
The camera, slides and lenses may be arranged to pack into a solid leather case, conveniently in the form of a knapsack, measuring about 16 in. wide, 12 in. high, and 5 in. deep. This can easily be carried on the back of one man, and is of a more convenient shape than the cases generally sold for the purpose.
The plates and other apparatus, with the exception of the knapsack and its contents, and the tripod stand, are best packed for travelling in a strong basket, which is much better than a box, being more elastic and lighter. It will weigh, when packed with the apparatus, and a gross of 7½ × 5 plates, about 60 lbs.
Photography in Natural Colours.
It is now possible for the traveller to bring home records of what he has seen in natural colours. There are two or three known methods by which this may be done, but few are available for the work of exploration. The picture obtained by the method patented by Mr. Ives and named Krōmskōp Photography is produced by three monochrome images. These have however been taken through three tinted glasses in a camera of a special kind. The images may be thrown upon a screen by means of a special lantern; the light passes through tinted glasses of colours complementary to those employed in making the negatives with the result that the picture on the screen exhibits all the varied hues of nature. The devices however are exceedingly expensive and can be satisfactorily employed only by those who thoroughly understand the apparatus and the problems which have to be solved.
Simpler processes are the Autochromic and The Paget Colour Plate process. In the Autochromic process the manufacturers coat a sheet of glass with minute specks of three colours, blue, violet green and orange, irregularly spread, and lay a coat of panchromatic emulsion on top. The photographer exposes through the glass, thus obtaining a negative. The developed silver, the negative image, is dissolved away and the silver bromide remaining is developed giving a positive image in natural colours.
In the Paget process the glass is coated with points of colour laid in a regular pattern. This is called the screen plate and is placed in the dark slide face to face with a colour-sensitive plate, and the expression is made through the screen plate. From the negative so obtained a monochrome positive plate is made, and this is bound up in contact with one of the screen plates. Some little experience is necessary to obtain correct register.
Each process has its difficulties. Most photographers consider that the Autochrome plate gives the nearest approach to natural colours, but the slides are very dense and require an intense light to show them well. The Paget plate is much more transparent and possesses the advantage that any number of positives may be made from the negative, whereas the Autochrome plate having been converted into a positive cannot be multiplied, resembling in that particular the earliest form of photography, the Daguerrotype.
None of these colour processes are suitable for photographing objects in motion. The exposures may be reckoned as from fifteen to twenty times as long as with an ordinary slow plate.
III.
GEOLOGY.
By the late W. T. Blandford, F.R.S.
Revised by Prof. E. J. Garwood, F.R.S.
A traveller who has not devoted some time to studying geology in the field must not be surprised or disappointed if the rocks of any country which he may happen to traverse appear to him a hopeless puzzle. If he desires to investigate the geological structure of an unknown region, he should previously devote some time to mastering, with the aid of a good geological map and description, the details of some well-known tract.
Under the term “Geological Observations,” two very distinct types of inquiry are commonly confounded. The first of these, to which the name of Geological Investigation ought properly to be restricted, consists in an examination of the rocks of a country as a whole, so as to enable a geological map, or, at all events, geological sections, to be constructed. This demands a knowledge of rocks (petrology), some acquaintance with the details of geological surveying, and, usually, with the elements of palæontology—a science that, in its turn, requires a preliminary study of biology, and especially of zoology. Despite all these hard terms, any intending traveller who has a taste for geology—if he has none he had better not waste time upon the subject—will find that a few months’ study in any good museum, a course of geological lectures, and, above all, a few days in the field with a good geologist, will start him very fairly equipped with the great requisite to all successful scientific investigation, a knowledge of how to observe, and what to observe.
The term “Geological Observations” is, however, often, but incorrectly, used in a second sense, which implies a restriction of the observations to the useful minerals found in any country, or to what is termed economic geology. Here also a preliminary knowledge of the elements of geological science will be found very useful, and will frequently enable the traveller to form much more trustworthy conclusions as to the nature and value of mineral deposits than he could without such a guide. But the essential point is to recognise a valuable mineral when seen, and for this some knowledge of mineralogy is requisite.
Outfit.—The essential articles of a geologist’s outfit are neither numerous nor cumbrous. A very large proportion of the known geology of the world has been made out with no more elaborate appliances than a hammer, a pocket compass with a small index to serve as a clinometer, a pocket-lens, a note-book and a pencil. No scientific observer has to depend more on his own knowledge and faculty for observation, and less on instrumental appliances, than a geologist.
The best hammer for general purposes should weigh from 12 to 24 oz. and should have a square flat end, and a straight cutting end—the latter should be horizontal, and the inner face of the hammer a continuous plane surface. The ends should be of steel, not too highly tempered. The hole for the handle should be as large as possible (with a small hole the handles are so weak as to be liable to break), and the handle should be secured in the hole by a wooden wedge, and an iron one driven into and across the wooden one. It is advisable to take a few spare ash handles and iron wedges. Cut a foot-measure in notches on the handle—this is very useful for measuring thickness of beds, &c. It is as well to have more than one hammer in case of loss, and if fossil-collecting is anticipated, at least one heavy hammer, with one end fashioned to serve as a pick, three or four cold chisels of various sizes, and a short crow-bar will be found useful. In London, hammers, chisels, &c., may be procured of Messrs. Buck, 242, Tottenham Court Road.
A very good pocket compass, the shape and size of a watch, with a clinometer arm, is made by Troughton and Simms, 138, Fleet Street. The use of the clinometer is for measuring the angle of dip in rocks. If more accuracy of measurement is required than is afforded by looking at a bed, a section, or a hill-side, and holding the straight-edge attached to the compass parallel to the dip, and if a surface can be found that affords the exact inclination, it is usually practicable, by means of a note-book laid on the rock surface, or, better still, a folding two-foot rule with a slot for sliding in the compass-clinometer, to obtain a plane sufficiently close to that at which the beds dip to enable the angle to be determined with a very short straight-edge. As a rule, except with very low angles of dip, the variation in the inclination of the rocks themselves exceeds the limits of error of the instrument. A little care, however, is necessary in taking dips; for the apparent dip seen in a section, such as is often exposed in a cliff, may differ widely from the true dip, which will only be shown if the section runs at right angles to the strike of the beds. Dips seen on the sides of hills at a distance are but rarely correct for the same reason.
A prismatic compass and an aneroid are frequently of great service: the former to determine the position on the map, if one exists, and to aid in making a rough map, if there is none; and the latter to estimate roughly the heights on the road travelled, especially in mountainous countries, and also to measure the thickness of horizontal beds. Both form a part of the outfit of most modern travellers. A good aneroid gives sufficiently accurate determinations of height for a rough but adequate geological section across any country, if the distances are known. The Watkin mountain aneroid of Hicks and Co., Hatton Garden, is the most accurate for considerable heights.
Collections.—Geological specimens require little more than paper and boxes, or biscuit tins, for packing. Occasionally fossils or minerals are fragile, and need tow or grass to protect them from injury; but there is no risk from the animal and vegetable enemies of zoological or botanical collections. The only important point to be borne in mind is that every specimen should be labelled on the spot, or, at all events, in the course of the day on which it is collected. Strong paper is best for labels, and these should not be put up in contact with the rock-fragments themselves, or they will be worn by sharp edges and become illegible, if not rubbed to fragments. Always wrap each specimen in paper, or some substitute, then add the label, and then an outer covering. The label,[3] if nothing else is written, should always record the locality distinctly written.
A collection of rock specimens may show what kinds of rock occur in a country, but the information afforded is very meagre, and, in general, of very small value. Such collections, indeed, unless made by a geologist, and accompanied by notes, are scarcely worth the carriage. If such specimens are taken, care should be used to select them from the rocks in place, not from loose blocks that may have been transported from a distance. In certain cases, however, where the traveller does not intend to penetrate beyond the low ground, pebbles brought down by streams give some indication of the strata which occur higher in the drainage basin, and the information may be useful to future explorers, but they should always be labelled as such. No fragments of spar or crystals should be collected merely because they are pretty.
In taking specimens of useful minerals, such as coal or metallic ores, the traveller should always endeavour to procure them himself from the place of occurrence, and if such are brought to him by natives, he should, if practicable, visit the locality whence the samples were procured. The value of all useful minerals depends both on quality and quantity; the former can to some extent be ascertained from a sample, but the amount available can only be estimated after a visit to the locality. Most metallic ores occur in veins or lodes. These were originally cracks in the rock, and have been irregularly filled with minerals, different from those in the neighbourhood. It is, however, very difficult, and often impossible, to estimate from surface examination whether the quantity of ore occurring in veins is likely to prove large; some idea may possibly be obtained if underground workings exist. Many of the ores of iron and manganese, some of those of other metals, and all coal and salt occur in beds, and here it is important to see what is the thickness, and to ascertain whether the mineral is equally pure throughout. Iron ores occur in most countries, and unless very pure and within easy reach of water-carriage, are not likely to be worth transport. The value of salt also depends on facilities for carriage. Coal, however, may be of value anywhere; but it is improbable that seams of less thickness than four or five feet can be of much use, except in countries where there is a skilled mining population and a considerable demand for the mineral. It does not follow because much thinner seams are sufficiently valuable to be worked in Western Europe, that they would pay for extraction in a country where the mechanical arts are less advanced. Still the occurrence of thin seams is worthy of record, as thicker deposits may exist in the neighbourhood. It must not be inferred, however, that a seam of small thickness at the surface will become thicker below. The reverse is equally probable.
A blow-pipe is extremely useful for ascertaining the nature of ores, and for determining minerals generally, and a small blow-pipe case might be added to a traveller’s kit, if he thinks it probable that he may meet with minerals in any quantity. But in general they are not to be found in such profusion as to render it difficult to carry away specimens sufficient for determination at leisure. A blow-pipe, too, is of no use to any one unacquainted with the method of employing it, though this is easy to acquire.[4]
To form a rough idea of the value of iron ore, see whether it is heavy; to form some notion of the quality of coal, pile up a heap and set fire to it. If it does not burn freely, the prospects of the coal being useful are small. It may be anthracitic, and very valuable with proper appliances; but anthracite is not of the same general utility as bituminous coal. Good coal should burn freely, with more or less flame, and should leave but little ash, and it is preferable that the ash should be white, not red, as the latter colour is often due to the presence of pyrites, a deleterious ingredient.
Gold and gems have, as is well known, been procured in considerable quantities from the sands of rivers and alluvial deposits. The deposits known to the natives of any country are often of small value, and the rude methods of washing prevalent in so many lands suffice to afford a fair idea of the wealth or poverty of the sand washed. Gold and, wherever it is found, platinum occur in grains and nuggets, easily recognised by their colour and their being malleable; but gems, such as diamond, ruby, sapphire, are not so easy to tell from less valuable minerals. They may be recognised by their crystalline form and hardness. A diamond is usually found in some modification of an octahedron, and the crystalline facets are often curved; rubies and sapphires are really differently coloured varieties of corundum or emery, and occur, when crystalline, in six-sided pyramids or some modification. A diamond is the hardest of known substances; nothing will scratch it, and it will scratch all other minerals. Sapphire will scratch everything except diamond. Topaz will scratch quartz.
In collecting fossils, it is useless to take many specimens of one kind unless carriage is exceptionally plentiful. Two or three good examples of each kind are usually sufficient, but as many kinds as possible should be collected. Great care is necessary that all specimens from one bed be kept distinct from those from another stratum, even if the bed be thin and the fossils in the two beds chiefly the same species. If there is a series of beds, one above the other, all containing fossils, measure the thickness roughly, draw a sketch-section in your note-book, apply a letter or a number to each bed in succession on the sketch, and label the fossils from that bed with the same number or letter.
Remains of Vertebrata, especially of mammals, birds and reptiles, are of great interest; but it is useless to collect fragments of bones without terminations. Skulls are much more important than other bones, and even single teeth are well worth collecting. After skulls, vertebræ are the most useful parts of the skeleton, then limb bones. If complete skeletons are found, they are usually well worth some trouble in transporting. If fossil bones are found abundantly in any locality, and the traveller has no sufficient means of transport, he will do well to carry away a few skulls, or even teeth, and carefully note the locality for the benefit of future geologists and explorers. The soil of limestone caverns, and especially the more or less consolidated loam, rubble, clay, or sand beneath the flooring of stalagmite, if it can be examined, should always be searched for bones, and also for indications of man or his works.
The foregoing remarks are intended for all travellers, especially for those who have paid little or no attention to geology. It would be far beyond the object of the present notes to attempt to give instruction in the methods of geological observation; all who wish to know more fully what questions are especially worthy of attention, should consult the article on Geology by the late Dr. Charles Darwin and Professor. J. Phillips in the ‘Admiralty Manual of Scientific Enquiry.’ But a few hints may be usefully added here for those who have already some knowledge of geology, who do not require to have such terms as dip, strike, fault, or denudation explained to them, and who are sufficiently conversant with geological phenomena to be able to distinguish sedimentary from volcanic, and metamorphic from unaltered rocks, and to recognise granite, gneiss, schist, basalt, trachyte, slate, limestone, sandstone, shale, &c., in the field. Assuming then that a traveller with some knowledge of field geology is making a journey through a tract of the earth’s surface, the geology of which is unknown, what will be the best method of procedure and the principal points to which he should direct his attention?
On the whole, the most useful record of a journey, whether intended for publication or merely as a memorandum, is a sketch geological map of the route followed, with the dips and strikes of the rocks and approximate boundaries to the formations, supplemented by notes and sketch-sections. Where, as is commonly the case in mountain-chains, and frequently in less elevated portions of the country, the rocks are much disturbed, and especially if the number of systems exposed is large and the changes frequent, no traveller can expect to do more than gain a very rough and general idea of the succession of beds in detail, and of the structure; but by making excursions in various directions, whenever a halt is practicable, by searching for fossils as a guide to the age and for the identification of beds with each other, and by carefully noting the general dip and strike of the more conspicuous beds, it is often possible, especially if an opportunity occurs of retracing the road followed, or of traversing a parallel route, to make out the structure of a country that at first appears hopelessly intricate. Dense forest is perhaps the worst obstacle to geological exploration; snow is another, though not quite so serious a disadvantage. It is always a good plan to climb commanding peaks; the general direction of beds, obscure from the lower ground, not unfrequently becomes much clearer when they are seen from above.
In level and undulating regions, on the other hand, it frequently happens that enormous tracts of country are occupied by the same formation, and if the rocks are soft, and especially if they are horizontal, or nearly so, little, if any, rock is to be seen in place. In this case water-courses should be searched for sections, and the pebbles found in the stream-beds examined, care being taken not to mistake transported pebbles derived from overlying alluvium or drift for fragments of the underlying rock. Where the same formation prevails over large tracts, it is usually easy, by examining the stones brought down by a stream, to learn whether any other beds occur. It is astonishing how even a small outcrop of hard rock at a remote spot in the area drained by a stream will almost always yield a few fragments that can be detected by walking two or three hundred yards up the stream-bed and carefully examining the pebbles.
Not infrequently different rocks support different vegetation, and by noting the forms that are peculiar, the constitution of hills at a considerable distance may be recognised. Thus some kinds of rock will be found to support evergreen, others deciduous trees, others grass, whilst a fourth kind may be distinguished by the poverty or want of vegetation. It is not well to trust too much to such indications, but they may show which hills require examination and which do not. The form assumed by the outcrop of some hard beds is often characteristic, and may be recognised at a considerable distance.
One most important fact should never be forgotten; mineral character, whether of sedimentary or volcanic rocks, is absolutely worthless as a guide to the age of beds occurring in distant countries. The traveller should never be led to suppose, because a formation, whether sedimentary or volcanic, in a remote part of the world, is mineralogically and structurally identical with another in Europe, or some country of which the geology is well known, that the two are of contemporaneous origin. The blunders that have been made from want of knowledge of this important caution are innumerable.
There are a few points of geological interest well worthy of the investigation of those who traverse unexplored, or partially explored, tracts of the earth’s surface. Amongst these are the following:—
Mountain-Chains.—Few, if any, geologists now believe that mountains were simply thrust up from below; all admit that, at least in the majority of cases, where great crumpling of the strata has taken place, there has been lateral movement of the earth’s crust. But the causes, extent and date, of the lateral movements are still, to a great degree, matters of conjecture, and every additional series of observations bearing on the question is of importance. There are many mountain-chains of which very little is yet known. In every case good sections are required, drawn as nearly to scale as practicable, through the range from side to side, and including the rocks at each base. The nature and distribution of all volcanic and crystalline rocks, both in the range and throughout the neighbouring areas, are especially noteworthy, and also the relations of the later beds, if any, on the flanks of the mountains, to those constituting the range itself. The derivation of the materials of the former from the latter, and the relative amount of disturbance shown by the two, and by the different members of each, will afford a clue to the date of upheaval; and two or more periods of movement may thus be determined, where intrusive igneous rocks, such as granite, occur, their relations to the surrounding rocks should be carefully noted, and specimens at the contact of the two rocks collected. If altered sedimentary rocks are found these should be traced, if possible, away from the igneous rock until some indication of their age is obtained from included fossils.
The distinction between a contemporaneous lava flow and an intrusive sheet of igneous rock is not always at first sight apparent; if the latter, it may pass from one bed to another or send tongues upwards into the overlying strata. Search should be made in the beds overlying the igneous rock for signs of alteration by heat. Thus limestone may be re-crystallised into marble, or shales altered into flinty hornstone.
Volcanoes and Volcanic Rocks.—It is almost needless to say that any additional information on the distribution of volcanic vents, recent or extinct, is of interest. In the case of extinct vents, the geological date of the last eruptions should be ascertained if practicable. This may sometimes be determined by finding organic remains or sedimentary beds of known age interstratified with the ashes or lava-streams near the base of the volcano.
Coasts.—The subject of the erosion of coasts is now fairly understood, and there is no doubt that the relative importance of this form of denudation was greatly overrated by many geological writers, who took their ideas of geological denudation generally from the phenomena observed in the islands, and on some of the coasts of Western Europe. Still, wherever cliffs occur, they afford good sections, and deserve examination. One question will usually present itself to almost every geological observer, and that is, whether any coast he may be landing upon affords evidence of elevation or depression. In the former case, beds of rolled pebbles or of marine shells, similar to those now living on the shore, may be found at some elevation above high-water mark. Very often the commonest molluscs in raised beds are the kinds occurring in estuaries, which are different from those inhabiting an open coast. Caution is necessary, however, that heaps of shells made by man, or isolated specimens transported by animals (birds or hermit-crabs), or by the wind, be not mistaken for evidence of raised beds.[5] If the shore is steep, terraces on the hillsides may mark the levels at which the sea remained in past times, but some care is necessary not to mistake outcrops of hard beds for terraces. If dead shells of species of mollusca, only living in salt-water estuaries, are found in places now beyond the influence of the tide, it is a reasonable inference that elevation has taken place.
The evidence of depression, on the other hand, unless there are buildings or trees partly sunk in the water, is much less readily obtained, and neither trees nor buildings are available as evidence, unless the depression is of comparatively recent date. The best proof is the form of the coast. If deep inlets of moderate breadth occur, with numerous branches, a little examination will frequently show whether such inlets are valleys of subaërial erosion, as they not unfrequently are, that have been depressed below the sea. A good and familiar example of such a depressed valley is to be found at Milford Haven in South Wales. In higher latitudes, the coast should be examined for signs of the action of sea ice, and stones should be collected from icebergs which have drifted from outside the accessible area; the shape of these stones and the proportion of those having only one smoothed side should be noted.
Rivers and River-Plains.—At the present time a question of much interest is the antiquity of existing land-areas, and some light may be thrown upon this, if the relations of existing river-basins to those of past times can be determined. If a stream cuts its way through a high range, it is probable that the stream is of greater antiquity than the range, and either once ran at an elevation higher than the crest of the ridge now traversed, or else has cut its way through the range gradually during the slow elevation of the latter. Where a river traverses a great alluvial plain, it may fairly be inferred that a long time has been occupied in the accumulation of the deposits to form the plain; but it remains to be seen whether those deposits are not partly marine or lacustrine. If upheaval has taken place over any portion of the plain, or if the river has cut its bed deeper, sections may be exposed, and these should always be examined for fossil remains. Bones of extinct animals are not unfrequently found in such deposits.[6]
Lakes and Tarns.—The mode of origin of lakes is always a subject of considerable geological interest, and any evidence which bears on the origin of a particular lake should be carefully noted. Lakes may be divided broadly into two classes: (1) Rock basins, (2) impounded hollows. Lakes of the latter class may, as a rule, be readily recognised and accounted for. The material forming the barrier may be due to a moraine, screes, or a landslip, or may result from the presence of a glacier in the main valley damming back the drainage of a lateral tributary. When lakes occur near the summit of a pass, they may often be traced to deposition of delta material on the floor of the valley brought down by a tributary stream. In this case, the inability of the stream to remove the material may often be traced to the abstraction of the head waters of the valley by the encroachment of the stream on the other side of the watershed. Rock basins, on the other hand, are frequently difficult to account for. They may occupy volcanic craters, or lie in areas of special depression (earthquake districts), or synclinal folds. They may be due to upheaval of old valley systems, causing reversal of drainage, or to subsidence at the upper end of a valley.
In special cases they may be due to the solution of soluble rock (rock-salt, gypsum, limestone, or dolomite). Many not otherwise explicable have been attributed to ice erosion during glacial periods, and it is still a moot point how far these lakes are due to partial changes in the elevation of the country, some observers having adopted, while many others dispute, the views of the late Sir A. Ramsay, who believed all these hollows to have been scooped out by ice moving over the surface in the form of a glacier or an ice-sheet. The origin of any lake met with should, if possible, be investigated and assigned to one of these causes.
Evidence of Glacial Action.—Closely connected with the subject of lakes is that of glacial evidence generally. There is probably no geological question which has produced more speculation of late years than the inquiry into the traces of a comparatively recent cold period in the earth’s history, and the former occurrence of similar glacial epochs at regular or irregular intervals of geological time.
The evidence of the last glacial epoch may be traced in two ways—by the form of the surface, which has been modified by the action of ice, and by changes that have taken place in the fauna and flora of the country in consequence of the alteration in the climate. The effects of an ice-sheet, like that now occurring in Greenland, if such formerly existed in comparatively low latitudes, must have been to round off, score and polish the rocks of the country in a peculiar manner, easily recognised by those familiar with glaciated areas. Care should be taken that the peculiar scoring and grooving of rock surfaces produced by the action of sand transported by the wind be not mistaken for glacial evidence. Cases also occur where movement among a mass of unconsolidated conglomerate or scree material has produced striation of the pebbles; in this case, however, careful observation will disclose a similar striation in the material of the matrix as well. Glaciers, properly so called, are confined to hilly or mountainous countries, and the valleys formerly occupied by them retain more or less the form of the letter U instead of taking the shape of the letter V, as they do when they have been cut out by running water. The sides of the valley, when modified by a glacier, have a tendency to assume the form of slopes unbroken by ravines, and with all ridges planed away or rounded, whilst in ordinary valleys of erosion by water, the sides consist of a series of side valleys or ravines, divided from each other by sharp ridges running down to the main valley. Large and small masses of rock, preserving to a considerable extent an angular form, but frequently polished and grooved by being ground against the sides or bottom of the valley, are carried down by the ice, and either left behind, perched up high on the slopes of the valley, or accumulated in a vast heap or bank, known as a terminal moraine, at the spot where the ice has terminated, or as lateral moraines on the sides of the valley. The nature of the rock will usually show whether the fragments on the side of a hill or at the bottom of a valley are derived from the higher parts of the drainage area, or whether they have merely fallen down from the neighbouring slopes. In the latter case, they may be due to landslips; in the former, their shape and the erosion they have undergone will aid in showing whether they have been transported by water or ice.
The surfaces that have been modified by earlier glacial epochs must in general have been long since removed by other denuding agencies. The most important evidence of former ice action consists in the occurrence, embedded in fine sediment, of large boulders, occasionally preserving marks of polish and striation, and usually, though not always, angular. Accumulations of this kind afford evidence of transport by two different agencies, water, which has brought the silt, and ice, which has carried the boulders. If the water had been in rapid movement, and thus capable of moving the boulders, it would have carried away fine silt or sand, instead of depositing it. Evidence of ice action has thus been traced equally in the boulder clay of North-Western Europe, and in the Palæozoic boulder beds of India, South Africa, and Australia, and probably of South America.
It is well to search in all mountain ranges for traces of glacial action. In many mountain chains, even in comparatively low latitudes, proofs have been found of the existence of glaciers, at a much lower level than at present, dating from a comparatively recent geological period, whilst in other mountain regions none have been recognised. The question also whether glacial action has been contemporaneous in the two hemispheres is of the greatest importance, and the evidence hitherto adduced is of a very conflicting character.
Deserts.—The great sandy or salt plains, with a more or less barren surface, that occupy a large area in the interior of several continents, have only of late years received due attention from geologists. A great thickness of deposits must occur in many of these vast, nearly level, tracts, for the underlying rocks are often completely concealed over immense areas. The investigation of the deposits is frequently a matter of great difficulty for want of sections; but, where practicable, a careful examination should be made, and exact descriptions of the formations exposed recorded. Some, at all events, of these beds appear to be entirely deposited from the air, and consist of the decomposed surfaces of rocks and the sand and silt from stream deposits, carried up by wind and then redeposited on the surface of the country. Such deposits are very fine, formed of well-rounded grains, and, as a rule, destitute of stratification. The geologist who has especially described these formations, Baron F. von Richthofen, in his work on China, attributes to the loess of the Rhine and Danube valleys a similar origin. It is usual to find beds due to water-action, rain-wash and stream-deposits, interstratified with the subaërial accumulations. Further observations on these formations are desirable. The occurrence of blown sands, the origin of these accumulations, and the peculiar ridges they assume, usually at right angles, but in some remarkable cases parallel to the prevailing winds, are questions deserving of additional elucidation.
Early History of Man in Tropical Climates.—Very little has been discovered as to the races of men formerly inhabiting tropical regions. It is evident that a race unacquainted with fire could only have existed in a country where suitable food was procurable throughout the year, and this must have been in a region possessing a climate like that found in parts of the tropics at the present day. It is possible that an investigation of the cave deposits in the tropics may throw some light on this subject. “Kitchen middens,” as they are termed—the mounds that have once been the refuse heaps of human habitations—are also worthy of careful examination.
Permanence of Ocean-Basins.—Within the last few years some geologists have adopted the theory that all the deep-sea area has been the same from the earliest geological times, and that the distinction between the depressions occupied by the oceans and the remaining undepressed portion of the earth’s crust, constituting the continents and the shallow seas around their coasts, is permanent. This view is very far from being universally or even generally accepted amongst geologists, although many who hesitate to accept the theory as a whole admit that parts of the oceans may have been depressions since the earth’s crust was first consolidated.
The argument on both sides depends upon theories to which travellers can contribute but little except by observations on the geology, fauna, and flora of oceanic islands, and by the investigation of coral-reefs and especially of atolls. In ranges of hills or mountains near the coast both of continents and islands and in all tracts where evidence of recent elevation exists, search should be made for deep-sea deposits. These are fine calcareous or argillaceous beds, often containing small Foraminifera or Radiolaria, which, however, are generally extremely minute, and require microscopical examination for detection. If any beds of consolidated calcareous or siliceous ooze or especially if red or gray clay (in older rocks, slate, or even quartzite) be found associated with pelagic deposits, such as coral limestone, a few small fragments of the beds should always be brought away for examination, and any distinct fossil remains found in such beds, for instance echinoderms (sea-urchins or star-fishes) or sharks’ teeth, should be carefully preserved with some of the matrix. Deep-sea deposits have recently been discovered in several parts of the world, for instance, the West Indies, the Solomon Islands, the islands of Torres Straits and Southern Australia, as well as in Europe.
Atolls or Coral-Islands.—Each of the remarkable coral-islands of the Pacific and Indian oceans consists usually of an irregular ring, part or the whole of which is a few feet above the sea, and which encircles an inner lagoon of no great depth. The outer margin of the reef around each island slopes rapidly, sometimes precipitately, to a depth of, usually, several hundred fathoms. Darwin, taking these facts into consideration, together with the circumstance that no coral-reefs are known to be formed at a greater depth than about 15 to 20 fathoms (90 to 120 feet), showed that all the facts of the case could be explained by the theory that coral-islands were formed in areas of subsidence. This view was generally accepted until Prof. A. Agassiz, Sir John Murray, and other writers brought forward evidence in favour of coral-islands being founded on shoals that may be areas of elevation.
Much light has been thrown on this subject by recent exploration. Two instances in especial may be mentioned. The examination of Christmas Island in the Indian Ocean, South of Sumatra, by Mr. Andrews, has shown it to be a raised atoll, founded on a volcanic base, whilst, on the other hand, borings on the atoll of Funafuti, one of the Marshall Islands in the Western Pacific, carried to a depth of over 1000 feet on the ring itself, and to 245 feet below sea-level in the middle of the lagoon, have yielded results which, in the opinion of the geologists engaged, Prof. Sollas and Prof. Edgeworth David, completely confirm Darwin’s theory.
It is probable that atolls originate in more than one way, some being formed in rising or stationary tracts, others in areas of depression. The important question, from a geographical point of view, is not so much how isolated atolls were formed as whether the great tracts in the Pacific and Indian oceans in which no islands occur except atolls, for instance, the Marshall, Gilbert and Low archipelagoes in the former, and the Laccadives and Maldives in the latter, have been areas of extensive subsidence during the later geological periods. Further evidence on this question may perhaps in time be furnished by additional borings, for one of which an island of the Maldive group would furnish an excellent locality, since there is in this case independent evidence to indicate that the archipelago occupies part of a sunken tract. Meantime any additional details would be useful, such as careful soundings around those atolls which have not been fully surveyed, so as to give an accurate profile of the sea-bottom in the neighbourhood.