98 THE FITNESS OF THE ENVIRONMENT 



ing conversion of heat into latent heat, is a 

 continuous process. The phenomenon is a 

 variable one, however, for while at high tem- 

 perature, both because of the greater supply 

 of heat and because of the greater amount of 

 water vapor that the air can hold, the process 

 is very important and active, at low tem- 

 perature it is far less considerable. This in 

 itself is no doubt a benefit because it tends 

 especially to restrict the upward march of 

 temperature when the temperature is high, 

 but is of minor importance when the tempera- 

 ture is low. 



In view of the other favorable qualities of 

 water it is perhaps not surprising to find that 

 its latent heat of evaporation is by far the 

 highest known. So great, in truth, is this 

 quantity and so important the process that 

 the latent heat of evaporation is one of the 

 most important regulatory factors at present 

 known to meteorologists. 



When the sun shines upon a body of water, 

 only a small part of the energy which the 

 water receives contributes to the elevation of 

 its temperature. Thus Fitzgerald has con- 

 cluded from his studies of Lough Derg in 

 Ireland during clear hot summer weather * 



1 See Hann, "Handbook of Climatology," translated by 

 Ward, p. 131. 



