BEARINGS BY COMPASS, SUN, ETC.
Pocket Compass.--A pocket compass should not be too small; if one of the little toy compasses be carried in the pocket, it should be as a reserve, and not for regular use. A toy compass will of course tell N. from N.N.E., and the like; and that may be very useful information, but the traveller will find that he constantly needs more precise directions. He doubts the identity of some hill or the destination of some path, and finds on referring to his map, that the difference of bearing upon which he must base his conclusion, is small: he therefore requires a good sized compass, to determine the bearing with certainty. One from 1 1/2 to 2 inches in diameter is practically the best. It should have plenty of depth, so that the card may traverse freely, even when the instrument is inclined: it should be light in weight, that it may not be easily jarred by a blow; the catch that relieves the card, when the instrument is closed, should be self-acting and should act well: lastly the movements of the needle should be quick; one that makes slow oscillations should be peremptorily refused, whatever its other merits may be: the graduation of the degrees on the card should be from 0 degrees to 360 degrees, North being 0 degrees and East 90 degrees. I wish some optician would make aluminum cards. The material can be procured as foil, like tinfoil. It can then be stamped and embossed, in which case it retains its shape perfectly, but I cannot satisfy myself as to a good pattern, nor do I see how to make the North and South halves of the disc sufficiently different in appearance.
Compass for use at night.--The great majority of compasses are well-nigh useless in the dark, that is, when it is most important to be able to consult them. They are rarely so constructed, that the difference between the north and south sides is visible by moonlight or by the light of a cigar or piece of tinder. The more modern contrivances are very effective; in these the southern half of the compass card is painted black, the northern being left white. With a very faint light, this difference can be appreciated. In compasses consisting simply of a needle, the north end of the needle should have a conspicuous arrow-head. It is extraordinary how much the power of seeing a compass or a watch at night is increased by looking nearly at it through a magnifying-glass. Thus, young people who can focus their vision through a wide range may be observed poring with their eyes close to their books when the light wanes. So again, at night-time, a placard, even in large type, is illegible at a short distance, but easily read on approaching it. It seems, in order that a faint image on the retina should be appreciated by the nerves of sight, that image must have considerable extent.
Moonlight or the light of a cigar may be condensed on the compass by a burning glass, or other substitute for it. (See "Burning Glass.")
True and Magnetic Bearings.--The confusion between true and magnetic bearings is a continual trouble, even to the most experienced travellers. Sir Thomas Mitchell's exploring party very nearly sustained a loss by mistaking the one for the other. I recommend that the points of the compass, viz. North, N.N.E., etc., should be solely used for the traveller for his true bearings; and the degrees, as 25 degrees (or N. 25 degrees E.), for his magnetic. There would then be no reason why the two nomenclatures should interfere with one another, for a traveller's recollection of the lay of a country depends entirely upon true bearings--or sunrise, sunset, and the stars--and is expressed by North, N.N.E., etc.; but his surveying data which find no place in his memory, but are simply consigned to his note-book, are necessarily registered in degrees. To give every facility for carrying out this principle, a round of paper should be pasted in the middle of the traveller's pocket-compass card, just large enough to hide the ordinary rhumbs, but leaving uncovered the degrees round its rim. On this disk of paper the points of the compass (true bearings) should be marked so as to be as exact as possible for the country about to be visited.
Errors in Magnetic Bearings.--The compass-needle is often found to be disturbed, and sometimes apparently bewitched, when laid upon hill-tops; even when they consist of bare masses of granite. The disturbance is easily accounted for by the hornblende in the granite, or by other iron-bearing rocks. Explorers naturally select hills as their points of triangulation; but compass observations on hill-tops, if unchecked by a sextant observation of the sun's bearings, are never so reliable as those taken on a plain.
Bearings by Sun and Stars.--It requires very great practice to steer well by stars, for, on an average, they change their bearings even faster than they change their altitudes. In tropical countries, the zodiacal stars - as Orion and Antares--give excellent east and west points. The Great Bear is useful when the North Pole cannot be seen, for you may calculate by the eye whereabout it would be in the heavens when the "pointers" were vertical, or due north; and the Southern Cross is available in precisely the same way. The true North Pole is about 1 1/2 degree or 3 diameters of the full moon, apart from the Pole star; and its place is on a line between the Pole Star and the Great Bear. An almanac, calculated to show the bearing, and the times of moonrise and moonset, for the country to be travelled over, as well as those of sunrise and sunset, would be a very great convenience; it would be worth while for a traveller accustomed to such calculations to make one for himself.
Diagram.--The diagram (preceding page) is intended to be traced in lines of different colours, when it will be found to be far less confused than at present.
Its object is to enable a traveller to use the sun, both as a rude watch and as a compass. The diagram is calculated for the latitude of London, but will do with more or less accuracy for the whole of England. A traveller going to other countries may easily draw up one for himself, and on a larger scale if he prefers it, by using the Azimuth tables and the Horary tables of Lynn.
The diagram represents, 1st, circles of equal altitudes; 2ndly, the path of sun, stars, etc., for each 10th degree of declination; 3rdly, the hour angles, all projected down upon--4thly--the level compass card.
Thus, six circles are drawn round the centre of the compass card at equal distances apart, each ring between them representing a space of 15 degrees in altitude.
The following angles were then calculated for each 10th degree of declination in turns, viz.:--The height of the sun, etc., when above the horizon at each point of the compass. 2ndly, the bearing of the sun at each consecutive hour. These points were dotted out; and, by joining the several sets of them, the drawing was made.
The broken lines which diverge in curves from P are hour lines; those which surround P in more or less complete ovals, are the paths of the sun and stars, for each 10th degree of declination; the prominent line running from E. round to W. being its path when on the Equator.
The diagram, when it is traced out for use, should have the names of the months written in coloured ink on either side of the south line at places corresponding to the declination of the sun during those months: viz.:--
January S. 23 degrees to S. 17 degrees
February S. 17 " S. 8
March S. 7 " N. 4
April N. 5 " N. 15
May N. 15 " N. 22
June N. 22 " N. 23
July N. 23 " N. 18
August N. 18 " N. 8
September N. 8 " S. 3
October S. 3 " S. 14
November S. 15 " S. 22
December S. 22 " S. 23
To use the card.--Draw a broad pencil line, which may afterwards be rubbed out, corresponding to the date of travel, and there will be no further confusion.
Then, to know what o'clock it is, "span out" (see "Spanning") roughly the altitude of the sun. The point in the diagram where the altitude so obtained crosses the pencil mark, corresponds to the position of the sun. The hour is then read off; and the compass bearings on the diagram are adjusted by holding it level, and turning it round until a line, drawn from its centre through the point in question, points towards the sun. As to the moon or a star, if its declination be unknown, but its bearing and altitude being given, its declination and path may be found, and therefore the time since its rising or before its setting; a most useful piece of information to a traveller. Watches break, and compasses cannot be used on horseback without stopping, and therefore a diagram of this description, of which any number of copies can be traced out, may be of use for rough purposes.
Other Signs of Direction.--Bearings by the Growth of Trees.--In exposed situations and near the sea, the growth of trees is rarely symmetrical; they betray by their bent heads and stunted branches the direction of the prevalent influences most adverse to their growth. This direction is constant over wide districts in a flat country, but cannot be equally relied upon in a hilly one, where the mountains and valleys affect the conditions of shade and shelter, and deflect the course of the wind.
Moss grows best where there is continuous damp, therefore it prefers that side of a tree which affords the most suitable combination of exposure to damp winds and shelter from the sun. When the winds do not differ materially in dampness, the north side of the forest trees are the most thickly covered with moss.
Bearings by the shape of Ant-hills.--That most accurate observer, Pierre Huber, writes as follows concerning the nests of the yellow ants, which are abundantly to be found in the Swiss Alps and in some other mountainous countries. It must be recollected, in reading his statement, that the chief occupation of ants is to move their eggs and larvae from one part of the nest to another, to ensure them a warm and equable temperature; therefore, it is reasonable to expect that the nests of ants should be built on a uniform principle as regards their shape and aspect. Huber says "they serve as a compass to mountaineers when they are surrounded by thick mists, or have lost their way during the night; they do so in the following manner:--The ant-hills (of the yellow ants), which are by far more numerous and more high in the mountains than anywhere else, are longer than they are broad, and are of a similar pattern in other respects. Their direction is invariably from east to west. Their highest point and their steepest side are turned towards the point of sunrise in the winter-time (au levant d'hiver), and they descend with a gradual slope in the opposite direction. I have verified these experiences of the shepherds upon thousands of ant-hills, and have found a very small number of exceptions; these occurred only in the case where the ant-hills had been disturbed by men or animals. The ant-hills do not maintain the constancy of their form in the lowlands, where they are more exposed to such accidents."
Ripple-marks on Snow or Sand.--The Siberians travel guided by the ripples in the snow, which run in a pretty fixed direction, owing to the prevalence of a particular wind. The ripples in a desert of sand are equally good as guides; or the wind itself, if it happens to be blowing, especially to a person pushing through a tangled belt of forest. Before leaving a well-known track, and striking out at night into the broad open plain, notice well which way the wind blows as regards the course you are about to pursue.
Flight of Birds.--I have read somewhere that in the old days coasting sailors occasionally took pigeons with them, and when they had lost their bearings they let one fly, which it did at once to the land.
To follow a Track at Night.--Where the track is well marked, showers of sparks, ably struck with a flint and steel, are sufficient to show it, without taking the pains of making a flame.
Smell of an Old Track.--The earth of an old and well-trodden road has a perceptible smell, from the dung and trampling of animals passing over it, especially near to encampments. It is usual at night, when a guide doubts whether or no he is in the track, to take up handfuls of dirt and smell it. It is notorious that cattle can smell out a road.