216 ESSAY 



nights as are attended with some dew, and that 

 its great degrees are commonly attended with a 

 copious formation of that fluid ; since it cannot 

 be thought, that the same stratum of air will 

 deposit moisture on the ground, from an in- 

 sufficiency of heat, at the very time it is re- 

 ceiving moisture from the ground, in the state 

 of pellucid vapour, as this presupposes, that it 

 is not yet replete with water. 



Our atmosphere has been very generally re- 

 garded, as incapable of being heated directly 

 by the rays of the sun, principally because these 

 give no heat to any particular portion of it, in 

 which they are brought to a focus. I do not 

 know, whether this experiment was ever made 

 with all the accuracy of which it is susceptible ; 

 but, granting that it has been thus made, my 

 opinion is, notwithstanding, that no reliance 

 can be placed in it. For as air, if heated at all 

 by concentrated sunbeams, must be heated by 

 them in a very slight degree, during the time 

 that their focus may be looked upon as sta- 

 tionary, otherwise the present question would 

 not have arisen, it is necessary for conducting 

 the experiment properly, that, during the whole 

 of it, the same individual small portion of air 

 shall constantly receive that focus ; but this, 

 for various manifest reasons, cannot possibly 

 happen. Viewing, therefore, the argument 



