In the pursuits and investigations of the science of Meteorology, which is essentially a science of observation and experiment, instruments are required for ascertaining, 1. the pressure of the atmosphere at any time or place; 2. the temperature of the air; 3. the absorption and radiation of the sun’s heat by the earth’s surface; 4. the humidity of the air; 5. the amount and duration of rainfall; 6. the direction, the horizontal pressure, and the velocity of winds; 7. the electric condition of the atmosphere, and the prevalence and activity of ozone.
CHAPTER I.
INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE.
Fig. 1.
1. Principle of the Barometer.—The first instrument which gave the exact measure of the pressure of the atmosphere was invented by Torricelli, in 1643. It is constructed as follows:—A glass tube, CD (fig. 1), about 34 inches long, and from two to four-tenths of an inch in diameter of bore, having one end closed, is filled with mercury. In a cup, B, a quantity of mercury is also poured. Then, placing a finger securely over the open end, C, invert the tube vertically over the cup, and remove the finger when the end of the tube dips into the mercury. The mercury in the tube then partly falls out, but a column, AB, about 30 inches in height, remains supported. This column is a weight of mercury, the pressure of which upon the surface of that in the cup is precisely equivalent to the corresponding pressure of the atmosphere which would be exerted in its place if the tube were removed. As the atmospheric pressure varies, the length of this mercurial column also changes. It is by no means constant in its height; in fact, it is very seldom stationary, but is constantly rising or falling through a certain extent of the tube, at the level of the sea, near which the above experiment is supposed to be performed. It is, therefore, an instrument by which the fluctuations taking place in the pressure of the atmosphere, arising from changes in its weight and elasticity, can be shown and measured. It has obtained the name Barometer, or measurer of heaviness,—a word certainly not happily expressive of the utility of the invention. If the bore of the barometer tube be uniform throughout its length, and have its sectional area equal to a square inch, it is evident that the length of the column, which is supported by the pressure of the air, expresses the number of cubic inches of mercury which compose it. The weight of this mercury, therefore, represents the statical pressure of the atmosphere upon a square inch of surface. In England the annual mean height of the barometric column, reduced to the sea-level and to the temperature of 32° Fahrenheit, is about 29·95 inches. A cubic inch of mercury at this temperature has been ascertained to weigh 0·48967 lbs. avoirdupois. Hence, 29·95 × 0·48967= 14·67 lbs., is the mean value of the pressure of the atmosphere on each square inch of surface, near the sea-level, about the latitude of 50 degrees. Nearer the equator this mean pressure is somewhat greater; nearer the poles, somewhat less. For common practical calculations it is assumed to be 15 lbs. on the square inch. When it became apparent that the movements of the barometric column furnished indications of the probable coming changes in the weather, an attempt was made to deduce from recorded observations the barometric height corresponding to the most notable characteristics of weather. It was found that for fine dry weather the mercury in the barometer at the sea-level generally stood above 30 inches; changeable weather happened when it ranged from 30 to 29 inches, and when rainy or stormy weather occurred it was even lower. Hence, it became the practice to place upon barometer scales words indicatory of the weather likely to accompany, or follow, the movements of the mercury; whence the instruments bearing them obtained the name “Weather Glasses.”
2. Construction of Barometers.—In order that the instrument may be portable, it must be made a fixture and mounted on a support; and, further, to render it scientifically or even practically useful, many precautions are required in its construction. The following remarks apply to the construction of all barometers:—Mercury is universally employed, because it is the heaviest of fluids, and therefore measures the atmospheric pressure by the shortest column. Water barometers have been constructed, and they require to be at least 34 feet long. Oil, or other fluids, might be used. Mercury, however, has other advantages: it has feeble volatility, and does not adhere to glass, if pure. Oxidised, or otherwise impure mercury, may adhere to glass; moreover, such mercury would not have the density of the pure metal, and therefore the barometric column would be either greater or less than it should be. The mercury of commerce generally contains lead; sometimes traces of iron and sulphur. It is necessary, therefore, for the manufacturer to purify the mercury; and this is done by washing it with diluted acetic, or sulphuric acid, which dissolves the impurities. No better test can be found for ascertaining if the mercury be pure than that of filling a delicate thermometer tube; if, on exhausting the air from this thermometer, the mercury will freely run up and down the bore, which is probably one thousandth of an inch in diameter, the mercury from which this thermometer was made will be found fit for any purpose, and with it a tube may be filled and boiled, not only of one inch, but even of two inches diameter. In all barometers it is requisite that the space above the mercurial column should be completely void of air and aqueous vapour, because these gases, by virtue of their elasticity, would depress the column. To exclude these the mercury is introduced, and boiled in the tube, over a charcoal fire, kept up for the purpose. In this manner the air and vapour which adhere to the glass are expanded, and escape away. One can tell whether a barometer has been properly “boiled,” as it is termed, by simply holding the tube in a slanting direction and allowing the mercury to strike the top. If the boiling has been well performed, the mercury will give a clear, metallic sound; if not, a dull, flat sound, showing some air to be present.
When the mercury in a barometer tube rises or falls, the level of the mercury in the cup, or cistern, as it is generally termed, falls or rises by a proportionate quantity, which depends upon the relative areas of the interior of the tube and of the cistern. It is necessary that this should be taken into consideration in ascertaining the exact height of the column. If a fixed scale is applied to the tube, the correct height may be obtained by applying a correction for capacity. A certain height of the mercury is ascertained to be accurately measured by the scale, and should be marked on the instrument as the neutral point. Above this point the heights measured are all less, and below, all more, than they should be. The ratio between the internal diameters of the tube and cistern (which should also be stated on the instrument, as, for instance, capac. 1⁄50) supplies the data for finding the correction to be applied. This correction is obviated by constructing the cistern so as to allow of the surface of the mercury in it being adjustable to the commencement of the fixed scale, as by Fortin’s or Negretti’s plan. It is also unnecessary in barometers constructed on what is now called the “Kew method.” These will all be detailed in their proper place. The tube, being fixed to the cistern, may have a moveable scale applied to it. But such an arrangement requires the utmost care and skill in observing, and is seldom seen except in first-class Observatories.
