HISTORY OF SCIENCE. 



moisture it contains. The oldest instrument for indicating this con- 

 dition is the hair hygrometer of De Saussure. It is represented in 

 Fig. 326, and consists essentially of a hair c attached to a fixed support 

 at E, and fastened below to an arm of the index K, which moves over 

 the graduated arc G. The hair is kept in a state of slight tension by 

 a small weight D. A thermometer B is usually attached to the appara- 

 tus. The scale on each instrument must be graduated by actual 

 trial with air of known humidity, and, even after every precaution has 

 been taken, little reliance can be placed upon the quantitative indica- 

 tions of the scale, . It is, however, still much used in cold countries, 

 especially in Russia. An instrument of 

 much greater accuracy is the hygrometer 

 invented by Darnell. It is represented in 

 Fig. 327, and consists of a tube twice bent 

 at right angles, and terminated by a bulb at 

 each end. The bulb a is of black glass, and 

 is half filled with ether, into which dips the 

 bulb of a small thermometer enclosed in 

 the bent tube. The other bulb b is covered 

 externally by a piece of muslin. A thermo- 

 meter is attached to the stand of the instru- 

 ment to indicate the temperature of the air. 

 In making an observation, ether is dropped 

 upon the muslin, and the cold occasioned 

 by the evaporation of the ether causes a 

 condensation in b of some of the ether- 

 vapour which fills the instrument. There- 

 upon some of the liquid in a evaporates, 

 and by abstracting heat, soon so far cools 

 the exterior of the bulb that the moisture of 

 the external air condenses upon it. The 

 moment this occurs the temperature of the 

 enclosed thermometer is noted; again, at 

 the instant the moisture disappears, the en- 

 closed thermometer is read. The mean of 

 the two readings is taken as the "dew-point." 

 From their observation the quantity of moisture present in the atmo- 

 sphere may be inferred. Another instrument called the psychrometer, 

 or the Wet and Dry Bulb Hygrometer, is shown in Fig. 328. It has 

 two similar thermometers close to each other, the only difference being 

 that one of them has the bulb covered with muslin, which is connected 

 by some cotton wick with a vessel containing water. The evaporation 

 which takes place from the moistened bulb lowers the temperature to 

 a degree which depends on the quantity of moisture in the air. 



The measurement of the rainfall is perhaps the simplest of all the 

 determinations which the meteorologist is called to constantly make. 



FIG. 328. 



