250 ESSAY 



countries, who sleep at nights on the tops of 

 their houses, are cooled, during this exposure, 

 by the radiation of their heat to the sky ; or, 

 according to his manner of expression, by re. 

 ceiving frigorific rays from the heavens. An- 

 other fact of this kind seems to be the greater 

 chill, which we often experience upon passing, 

 at night, from the cover of a house into the 

 open air, than might have been expected from 

 the cold of the external atmosphere. The 

 cause, indeed, is said to be the quickness of 

 transition from one situation to another. But, 

 if this were the whole reason, an equal chill 

 would be felt in the day, when the difference, 

 in point of heat, between the internal and ex- 

 ternal air, was the same as at night, which is 

 not the case. Besides ; if I can trust my own 

 observation, the feeling of cold from this cause 

 is more remarkable in a clear than in a cloudy 

 night, and in the country, than in towns. The 

 following appears to be the manner, in which 

 these things are chiefly to be explained. 



During the day, our bodies while in the open 

 air, although not immediately exposed to the 

 sun's rays, are yet constantly deriving heat from 

 them, by means of the reflection of the atmo- 

 sphere. This heat, though it produces little 

 change on the temperature of the air which 

 it traverses, affords us some compensation for 



