HYGROMETERS. 



after frost, thick stone walls which continue to retain their low 

 temperature, are covered by a profusion of moisture. 



Hygrometers. As water evaporates at all temperatures, how- 

 ever low, the atmosphere cannot be supposed to be ever en- 

 tirely destitute of moisture. The proportion present varies 

 with the temperature, the direction of the wind, and other 

 circumstances, but is generally greater in summer than in 

 winter. There are various means by which the moisture in 

 the air may be indicated and its quantity estimated, affording 

 principles for the construction of different hygroscopes or hy- 

 grometers. 



1st. Many solid substances swell on imbibing moisture, and 

 contract again on drying, such as wood, parchment, hair, 

 and most dry organic substances. The hygrometer of Deluc 

 consisted of an extremely thin piece of whalebone, which 

 in expanding and contracting moved an index. The prin- 

 ciple of this instrument is illustrated in the transparent 

 shavings of whalebone cut into figures, which bend and crumple 

 up when laid upon the warm hand. Saussure made use of 

 human hair boiled in caustic ley, as a hygrometric body, and 

 it appears to answer better than any other substance of the 

 class. Instruments of this kind are graduated experimentally 

 from observations made on placing them in air kept at a known 

 state of dryness by the presence of deliquescent salts. But all 

 such instruments alter in their indications after a time, and 

 should be viewed as hygroscopes rather than hygrometers. 



2ndly. The degree of dryness of the 

 air may be judged of by the rapidity of 

 evaporation. Leslie made use of his 

 differential thermometer as a hygro- 

 meter, covering one of the bulbs with 

 muslin, and keeping it constantly 

 moist by means of a wet thread from 

 a cup of water placed near it. The 

 evaporation of the moisture cools the 

 ball and occasions the air in it to 

 contract. This instrument gives use- 

 ful information in regard to the rapi- 

 dity of evaporation, or the drying 

 power of the air, but does not indi- 

 cate directly the quantity of moisture 

 in the air. The wet-bulb hygrometer 





