v.] 



EVAPORATION. 



depend on the effect of moisture upon pieces of catgut or 

 of twisted string. 



True hygrometers, or instruments for measuring humidity 

 with considerable precision, have been constructed by 

 Daniell, Regnault, and Mason, and are daily employed by 

 meteorologists. Some of these instruments effect their 

 purpose by indicating the dew-point directly, whilst others 

 depend for their indications on the rapidity of evaporation. 

 Daniell's hygrometer, a common in- 

 strument of the former class, is repre- 

 sented in Fig. 13, and described on p. 

 52. The form of hygrometer now 

 commonly used in this country is 

 known as Mason's Dry-and-wet bulb 

 Ther7no7neters, a name sufficiently de- 

 scriptive of its construction. It consists, 

 in fact, of two thermometers, placed 

 side by side as represented in Fig. [9; 

 one of the instruments has its bulb 

 free, whilst the other is covered with 

 muslin, which is connected, by means 

 of a strand of cotton, with a small re- 

 servoir of water : the thread constantly 

 sucks up the liquid, just as the wick of 

 a candle draws up the melted wax or 

 tallow, and the bulb is, in this way, 

 constantly kept moist. Whenever a body passes from the 

 state of liquid to that of vapour heat is absorbed : hence 

 a little water dropped upon the hand gives rise to the sen- 

 sation of cold as it evaporates ; a sprinkling of Eaii de 

 Cologne, or other liquid containing spirit of wine, produces 

 greater cold, since it is more volatile than water and dries 

 up much more rapidly; a little ether, again, being still 



Fig. 19. — Dry and wet 

 bulb thermometers. 



