I 



76 Dr. Holyoke's Estimate of the Excess of Heat and Cotcl 



The quantity of evaporation in any country must, I think, 

 depend principally, if not entirely, upon the three circum- 

 stances of— dryness — ^hcat — and motion, of the air, pontig- 

 uoift to the evaporating surface* For the dryer the air, 

 the more capable it is of absorbing a certain quantity of 

 water in a given time ; for when fully saturated with w^ater, 

 as in a foggy season, little or no .evaporation takes place. 

 Heat too is found to promote evaporation, pirobably as it 

 lessens the cohesion of the particles of water : and 'the wind, 

 not only by agitating the evaj)o'rating surface, but also by 

 applying fresh portions of air to the same, tends greatly to 

 j)romote this process. 



Now, all these circumstances conspire with us in Ameri- 

 ca, in a greater degree than in Europe, to increase the dry- 

 ness upon the surface of the earth. And such a degree of 

 dryness does in fact take place here, as much more frequent- 

 to injure our crops, and frustrate the hopes of the hus- 

 bandman, than in Europe. 



The proof of this point however, from actual observa- 

 tion, according to the Ephemerides Meteorologies, is rath- 

 er lame ; for of the six or seven p!aces mentioned -in that 



work, which can be easily brought into a comparison with 

 those of Dr. Williams in the same work which are the only 



American ones, which I have met with, two, if I understand 

 them, exceed his considerably : all the rest indeed fall much 



short ; 



serrations have been made upon it for some competent time in both contlneHts ; tbis mode 

 ot detemmiiig it must reeiain a desideratum. 

 * Electricity may perhaps be considered as another cause promotive of cvapora- 



verv' . . ' "''"' ^' '"'''^'' '^^' ^^ ^^^'^^ '^^ I>^°^^^-S evaporation, x.ay be 

 ver, much m proportion to the dryness of the atmosphere. 



% 



\ 



