252 ESSAY 



does not form upon a healthy human body, in 

 temperate climates ; but they may, notwith- 

 standing, arise from the same cause, that pro- 

 duces dew on those substances, which do not, 

 like the human body, possess the power of ge- 

 nerating heat, for the supply of what they lose 

 by radiation or any other means. 



III. I had often, in the pride of half know- 

 ledge, smiled at the means frequently employed 

 by gardeners, to protect tender plants from 

 cold, as it appeared to me impossible, that a 

 thin mat, or any such flimsy substance, could 

 prevent them from attaining the temperature 

 of the atmosphere, by which alone I thought 

 them liable to be injured. But, when I had 

 learned, that bodies on the surface of the 

 earth become, during a still and serene night, 

 colder than the atmosphere, by radiating their 

 heat to the heavens, I perceived immediately a 

 just reason for the practice, which I had before 

 deemed useless. Being desirous, however, of 

 acquiring some precise information on this sub- 

 ject, I fixed, perpendicularly, in the earth of a 

 grassplat, 4 small sticks, and over their upper 

 extremities, which were 6 inches above the 

 grass, and formed the corners of a square, the 

 sides of which were 2 feet long, drew tightly 

 a very thin cambric handkerchief. In this 



