ON DEW, &c. 213 



little or no cold will be observed upon them 

 above that of the atmosphere, as the frequent 

 application of warm air must quickly return a 

 heat equal, or nearly so, to that which they had 

 lost by radiation. A slight agitation of the air 

 is sufficient to produce some effect of this 

 kind ; though, as has already been said, such 

 an agitation, when the air is very pregnant with 

 moisture, will render greater the quantity of 

 dew, one requisite for a considerable produc- 

 tion of this fluid being more increased by it, 

 than another is diminished. 



VI. A small body, as a thermometer, sus- 

 pended in the air, will even in the calmest night 

 exhibit but little cold from radiation, since it is 

 continually exposed to the application of fresh 

 parcels of warmer air, both from the progres- 

 sive motion of this fluid, and from the down- 

 ward motion produced in it by the superior 

 gravity of such portions, as have been cooled 

 by contact with the suspended body. On. the 

 other hand, a thermometer upon a board, raised 

 above the earth and possessing a surface of 

 several square yards, will have its cold from 

 radiation much less diminished than the former, 

 as it is exposed to no loss from a downward 

 motion of the air, and as the air, which ap- 

 proaches it horizontally, must, almost always, 

 have had its temperature previously lowered, 



