A HISTORY OF SCIENCE 



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 expres- 

 sion, by receiving frigorific rays from the heavens. 

 Another 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 air than might have been 

 expected from the cold of the external atmosphere. 

 The cause, indeed, is said to be the quickness of transi- 

 tion 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 in- 

 ternal and external air was the same as at night, which 

 is not the case. Besides, if I can trust my own obser- 

 vation, the feeling of cold from this cause is more re- 

 markable 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 ex- 

 plained : 



" 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 atmosphere. This heat, though it 

 produces little change on the temperature of the air 

 which it traverses, affords us some compensation for 

 the heat which we radiate to the heavens. At night, 

 also, if the sky be overcast, some compensation will be 

 made to us, both in the town and in the country, 

 though in a less degree than during the day, as the 

 clouds will remit towards the earth no inconsiderable 

 quantity of heat. But on a clear night, in an open part 

 of the country, nothing almost can be returned to us 



190 



