570 HYGKOMETRY IIooloo), employed by Captain Kater on account of its remarkable property of twisting and untwisting according to the dampness of the air ; a slip of whalebone cut across the fibres, used by De Luc ; and the hygrometer of Saussure, which was formed of a hair from which the oily matter had been previously removed, and which stretches when moist and contracts when dry. Experience has, however, shown that none of these hygrometers are satisfactory instruments, seeing they give inconsistent results, and are liable when in use to great and uncertain changes. Hence, while they serve to give the roughest idea of the state of the air as regards moisture, they have fallen into disuse as accurate instruments of observation of the hygrometry of the atmo sphere. But in the intensely cold climates of Russia and Siberia, the hair hygrometer still continues to be used as an instrument of observation ; and when we consider the tediousness and difficulty of making hygrometric observa tions with the hygrometer in most general use when the temperature of the air descends below the freezing-point of water, the hair hygrometer should perhaps be considered as good an instrument as is available to put into the hands of ordinary observers in times of low temperature. The most accurate hygrometers are those which are con structed, not as the above, on the principle of absorption of vapour, but on the principles of condensation or evaporation. The well-known fact that the tempera ture of a wet body is lower than that of a dry one when under the same atmospheric conditions was applied by Sir John Leslie to mea sure the humidity of the air. Leslie s hygrometer (fig. 1), which is an adaptation of his differential thermometer, is formed by uniting two tubes having a ball blown on the end of each, into which some coloured sulphuric ether has been previously introduced. When both bulbs are at the same temperature, the fluid stands at the zero of the scale, but when one of them is covered with wetted paper or muslin the instrument shows the depression of temperature of the wetted bulb. In order to ascer tain the quantity of moisture cor responding to the reading of a Leslie s hygrometer, we must de duct from the total quantity of moisture which the air of the tem perature at the time of observa tion is capable of holding the deficiency due to the degree of cooling shown by the hygrometer. As no air is ever absolutely dry, but contains more or less moisture, it is evident that if any mass of air be cooled sufficiently it may be made to deposit its moisture. A familiar example of the condensation of vapour is seen in the formation of dew on a tumbler filled with cold water and brought into a warm room. This dew is caused by the deposition of moisture from the air in contact with the cold surface of the glass, which is cooled down below the point of saturation. The temperature of the glass at the instant dew begins to form on its surface is termed the dew-point, which corresponds with the point of saturation of the air. Daniell s and Regnault s hygrometers are constructed on the principle of this simple phenomenon, various contriv ances being used for lowering the temperature quickly to any point that may be desired, and for observing with FIG. 1. Leslie s Hygrometer. requisite precision the temperature at which the dew begins to form. In both cases ether is employed to lower the temperature. Daniell s hygrometer consists of a glass tube bent at right angles at two points with a bulb at each extremity, one bulb being of black and the other of clear glass, the latter covered with muslin. The liquid within the bulbs is ether, which at the time of being sealed is made to boil for the purpose of expelling the air. If the temperature of the two bulbs be made to differ from each other, all the ether is transferred from the warmer to the colder bulb. In making an observation the whole of the ether is first trans ferred to the black bulb, arid ether is then dropped on the muslin covering outside the clear bulb. This ether quickly evaporates, and in doing so rapidly lowers the temperature of the clear bulb so that the ether inside the black bulb distils over into the clear bulb. The result is a lowering of the temperature of the black bulb, and, as soon as this falls to the temperature of the dew-point of the air where the experiment is conducted, a ring of vapour begins to be formed outside the black bulb, more or less dulling its surface. At this instant a thermometer placed inside the tube with its bulb immersed in the ether filling the black bulb is read, and the reading gives the dew-point of the air at the time. Regnault s hygrometer is a little more complicated than Daniell s, but its indications are much more trustworthy. It consists (fig. 2) of a glass tube or cap- sule A, having on the bottom and a little way up a highly polished silver surface, and closed by a cork with two holes. Through one of these holes the stem of a thermometer B passes, having its bulb at the bottom of the silvered capsule, while through the other hole passes a narrow metallic tube C, one end of which opens close to the bottom of the capsule, and the other end may, if desired, be connected with an aspirator or air-pump. In making an observation as much ether is introduced into the capsule A as will cover the bulb of the thermometer, and then by trans mitting air through the tube C the ether vapour is withdrawn from the capsule A through another tube D. By this means the temperature of the ether is very rapidly reduced, and since the whole mass of the ether is agitated by the air-bubbles which rise through it from the bottom of the capsule, the cooling of the ether is equal throughout. The thermometer is then read quickly, but to ensure an accuracy to the tenth of a degree a second and a third experiment, conducted more slowly, should be made. As showing the rapidity with which observations can be made with this hygrometer, Henry F. Blanford on one occasion made six observations in six minutes in the dry climate of Secundera- bad, when the temperature of the air was 93, the dew- point 51, and the relative humidity consequently 24. The temperature of the air at the time of observation may be ascertained in the usual way, or by means of the thermo meter E, if care be taken that its temperature is unaffected by the proximity of the person of the observer. Owing to the expense and great trouble attending the use of hygrometers which give the dew-point directly by con densation, another hygrometer has come into extensive use by which the dew-point is determined indirectly by evapo ration. This is August s hygrometer, sometimes called Fro. 2. Kegnault s
Hygrometer.
