156 ESSAY 



even when the grass was wet from preceding 

 rain, and when, consequently, it must have 

 been, in some measure, cooled by evaporation. 

 On one such night, the grass was found to be 

 4 colder than the earth an inch beneath the 

 surface of the plat, which afforded a sufficient 

 reason for the grass itself being warmer than 

 the air. In windy weather, however, if the sky 

 was clear, some degree of cold, in addition to 

 that of the air, was always observed upon the 

 grass ; and in calm weather, very high clouds, 

 though sufficiently extensive and dense, to con- 

 ceal the sky completely, would yet frequently 

 allow of the grass being several degrees colder 

 than the air. I once observed, upon a night of 

 this kind, a difference of 5 between the tem- 

 peratures of those bodies. 



If the night became cloudy, after having 

 been very clear, though there might be no 

 change with respect to calmness, a considerable 

 alteration in the temperature of the grass always 

 ensued ; and this sometimes very suddenly. 

 Upon one such night, the grass, after having 

 been 12 colder than the air, became only 2 

 colder than it, the temperature of the air being 

 the same at both observations. On a second 

 night, grass became 9 warmer in the space of 

 an hour and a half. On a third night, in less 

 than 45 minutes, for the whole change occurred 



