72 PHYSIOGRAPHY. [chap. 



more volatile, reduces the temperature 3'et lower. The 

 evaporation of the water from the wet bulb therefore 

 lowers its temperature, and the more rapid the evaporation 

 the greater will be this difference of temperature between 

 the wet and the dry bulbs. If the air were saturated 

 with moisture there could be no evaporation, and conse- 

 quently the two thermometers would stand exactly alike. 

 When, on the other hand, the air is very dry, evaporation 

 becomes exceedingly rapid, and the temperature of the wet 

 bulb' consequently falls very lov/. From a comparison of 

 the temperatures shown by the two thermometers, the dew- 

 point, the relative humidity of the atmosphere, and the 

 quantity of vapour in a given volume of air, can be deter- 

 mined by simple methods of calculation. Such an instrument 

 as that just described is sometimes called a Psychrometer} 



From what has been advanced in this chapter, it is evident 

 that more or less watery vapour is always to be found in the 

 atmosphere ; its presence is constant, but its proportion 

 variable. It may perhaps be said that the air of England 

 contains on an average something like \\ per cent, of 

 aqueous vapour. This vapour is intimately associated with 

 the other constituents of the atmosphere, all being gaseous 

 bodies existing in a state of mechanical mixture. The 

 composition of the atmosphere, however, is so important a 

 subject that its full discussion must be reserved for the next 

 chapter. 



When the temperature of the air is sufficiently reduced in 

 any given locality, the watery vapour which it contains 

 condenses as a liquid, while the other constituents retain 

 their gaseous state. The liquid drops of water thus con- 

 densed as rain are said to be distilled. In fact, the process 

 carried on in nature is precisely similar in principle to the 

 1 Psychrometer, from -i/vxphs, psuchros, cold. 



