210 



HEAT. 



clouding is seen the thermometer is read and the aspirator is closed. 



But the observed temperature will be a little too low, as the very first 



deposition of dew will be too small 

 to be visible. Communication 

 with the aspirator being cut off, 

 the ether gradually rises in tem- 

 perature, and when the dew-point 

 is exceeded the dew will evaporate 

 again. When the surface is once 

 more clear the thermometer is 

 again read, but the temperature 

 now observed will be slightly above 

 the dew-point, as the evaporation 

 will not be instantaneous. After 

 a few trials the two tempera- 

 tures, one above and the other 

 below the true dew-point, are 

 made to close in upon it until the 

 mean of the two can only differ 

 from it by a very small quantity. 

 The tube A'B', similar to AB, is 

 not connected with the apparatus. 

 It only contains a thermometer T', 

 which gives the temperature of 

 the air, while the silver surface 

 always remains bright and serves 

 as a standard with which to com- 

 pare the surface on which the dew 

 is deposited. 



The form of the Kegnault 

 hygrometer has been modified in 



FlG. 122. Regnault's Dew-point 

 Hygrometer. 



various ways so as to make the deposition of dew on the silver surface as 

 evident as possible, and with practice an observer determines the dew- 

 point speedily and accurately. 



Dines's Hygrometer. A somewhat simple instrument is that devised 



FlO. 123. Dines's Hygrometer. 



by Dines. V is a vessel containing water in which ice is placed to cool 

 it. T is a tap which allows the cold water to trickle out of the vessel 



