ON DEW, &c. 221 



temperature, as the distance from the earth 

 increased. Mr. Six likewise found, that, on 

 cloudy nights, the air was sometimes colder 

 220 feet above the ground, than at the distance 

 of 9 feet from it. When, therefore, the earth 

 has become colder, from radiation, than the 

 neighbouring air, in consequence of the latter 

 having, by reason of its small radiating power, 

 emitted a less proportion of its heat to the 

 heavens, the warmer air must radiate a part of 

 its heat to the earth, without receiving a full 

 compensation, and will therefore become colder, 

 than it otherwise would have been. In propor- 

 tion too as the air is nearer to the earth, must 

 the cold of the former from this cause be the 

 greater. My own conception of this matter is 

 facilitated*, by contemplating the occurrence 

 of an opposite effect, when the earth is warmer 

 than the air. Let it be supposed then, that 

 while the earth, in this state, radiates upwards 

 a quantity of heat, a foot in depth of the in- 

 cumbent air is capable of stopping a 1000th of 

 what it hence receives, and of converting it 

 into heat of temperature. The consequence 

 must be that the next foot, from receiving only 

 999 parts of what had been emitted by the 

 earth, will not be so much heated as the first 



* The same facility is afforded by considering cold as a 

 body. 



