The process of observation is extremely simple. The first thing is to determine, either by observation or estimation, the air temperature likely to prevail during the series of observations: if this is done within 5° it will be sufficiently accurate (within about 1 per cent.). The scale must then be set to this temperature in the manner above explained. Subsequently the readings must be taken from the outer scale of feet, and the difference will give the difference of elevation. The following example, given by the maker, Mr. Casella, of actual observations taken between Hampstead and London will explain the proceeding :

Temperature of 40° and scale set accordingly.

The true difference of altitude, according to the Ordnance levels, is 428 feet, showing an error of only four feet. The accuracy of the result will be further increased if the observations are repeated more than once, and the average of the results taken.

It should be mentioned that the above principle of adjustment can only be correctly applied to aneroids in which the graduation is nearly uniform, and therefore extreme care is taken in the selection of suitable instruments for this purpose.

(79) Hygrometers.

An hygrometer is an instrument for ascertaining the amount of vapour in the air. Hygrometers are either constructed on the principle of absorption, of condensation, or evaporation.

The hair hygrometer of Saussure is an example of absorption; when the air is damp the hair absorbs moisture and shrinks in size, returning to its original length when dry.

One of the most accurate and modern of hygrometers is that of Dynes, which acts on the principle of condensation (see Fig. 22). Its indications are obtained with unusual facility by means of a

little water and ice, or cold water only. This is put into the cup A, and allowed to flow gently through the small chamber D, whence it rises through a perforated diaphragm into the space above. In this space rests the bulb of a sensitive thermometer, the space being covered water-tight by a thin smooth piece of silver or black glass. By turning the tap B the water will flow

gently from the spout, as shown, thus cooling the cover; when the temperature reaches the dew-point, a strong film of vapour or dew will be visible, the temperature being shown on the graduated stem of the thermometer (Figs. 22 and 23), either horizontal or vertical form.

In the vertical form of this hygrometer (Fig. 23) ether may be employed, and a constant supply of it kept in the instrument for use whenever required.

CHAPTER XI.

THE DEW-POINT-RAINFALL-SUNSHINE-WIND.

(80) Determination of the Dew-point.

THE dew-point is ascertained indirectly by evaporation by means of two ordinary mercurial thermometers placed under precisely the same conditions, save that in the one "the wet bulb" has its bulb constantly kept moist by being covered with a thin piece of muslin, a thread from which dips into pure distilled water. As the water from the muslin is ever evaporating, the wet bulb always, save when the air is already saturated, gives a lower reading than the dry; and from the difference between the two readings the dewpoint can be ascertained by means of tables or by calculation.

From these observations the following may be ascertained:(1) The dew-point; (2) the elastic force of vapour, or the amount of the barometric pressure due to the vapour present in the atmosphere; (3) the quantity of vapour in a cubic foot of air; (4) the additional vapour required to saturate a cubic foot of air; (5) the relative humidity; and (6) the weight of a cubic foot of air at the pressure prevailing when the observation is made.

The formula for reduction, when the reading of the wet bulb is above 32°, is

when the wet bulb is below 32° the formula is

The dew-point is found from F by using Table XXI. reversely, and finding the temperature opposite the elastic force calculated. To borrow an example from Buchan's Meteorology. Suppose the dry bulb to read 50°, and the wet 45°, barom. 29 inches, then f=299 inch (from Table XXI.); d= 50°-45° = 5°; and h 29 inches. Hence,

=

from Table XXI., the temperature opposite 244 is seen to be 39.7°, which is therefore the dew-point. A more convenient method of calculation is to use Glaisher's factors, in which, by simple multiplication of the difference between the dry and wet bulbs, and subtracting the product from temperature of the dry bulb, the dew-point is found (see Table XXII.). For instance, in the above example the dew-point would be found as follows:—

The factor opposite the dry-bulb (temperature 50°) is 2-06, the difference between the two thermometers is 5°; 206 x 5 = 10:3, and 103-50° = 39 7°.

The relative humidity of the air is calculated on the assumption that dry air = 0, and absolutely saturated air = 100. It is found by dividing the elastic force corresponding to the temperature of the dew-point by the elastic force corresponding to the temperature of the air, and multiplying the quotient by 100. For example, 244 is the elastic force at 397; at 50 it is 361. Then x 100 – 67 6, or in round numbers 68; 68 is therefore the S61 elative humidity.