232 DISSERTATION on the 



nifhed to find it at 5 ° above it, although the thermometer in 

 .air had not altered its pofition, but was ftill at 7 , nor had the 

 fun (hone out in the interval, fo that this furprifing change of 

 heat in the earth feems to have been effected by a fhower of 

 rain that fell between the two obfervations ; a ftrong confirma- 

 tion of the hypothefis that rain water contains a large portion 

 of latent heat, and probably of electric matter g fo that it is 

 not furprifing, if plants Ihould, under certain circumftances, 

 flart, as it were, fuddenly out of the earth after a fhower of 

 rain, as they receive fo large a fupply, not only of moifture, 

 but likewife of heat, and poflibly a vivifying principle from 

 the flimulus of the electric fluid carried down by the rain. 



General Obfervations on the Summer Atmofphere. 



The (late of the atmofphere during the fummer, is in gene- 

 ral pretty fixed, and the air very ferene and clear, both during 

 the day and night, in fpite of the heavy dew that falls from 

 the fetting to the rifing of the fun, which feems to ferve, as 

 before obferved, for watering the plants during this hot dry 

 feafon. I have been much {truck with obferving, that an ex- 

 cellent hygrometer, fent me by the learned Profeflbr Pictet of 

 Geneva, (hung within doors with the windows open) indicated 

 a greater degree of humidity, on a fine fummer evening, than 

 during the moft continued rainy weather ; fo much higher is 

 the faturation of the air with water, and fo much greater is its 

 difFufion through it, than when it falls in the form of rain. 

 This obfervation favours the modern hypothefis of the chemical 

 folution of water in air. 



I think alfo worthy of remark, the much greater effect 

 this fultry debilitating period has upon foreigners, (from even 

 warm countries) than on the natives of the north, as one 

 Ihould naturally think the former would fupport it better, than 

 a people unaccuftomed, for fo long a period of the year, to ex- 



ceflive 



