Chap. 5-] Dr.Hutton'sVbeoryofRain. 485 



turn, a fall of rain is occafioned, becauie the w^rr 

 ftratum is cooled. To this it has been replied, that 

 by this mixture the colder ftratum mufr be as much 

 warmed as the warmer is cooled, and, therefore, that 

 no precipitation ought to take place. In order to 

 obviate this difficulty, Dr. Hutton fuppofes that heat 

 enables the atmofphere to fuipend water in an in- 

 creafmg ratio, according to the quantity of it. Thus, 

 if two degrees of heat enable a certain quantity of 

 air to fufpend two given meafures of water, three de- 

 grees of heat will enable the fame quantity of air to 

 fufpend more than three fuch meafures. Upon this 

 fupporkion, if two ftrata of air, each of which is charged 

 with as much water as its temperature enables it to 

 fufpend, meet together, and are reduced to a mean 

 temperature, they will not be able to fuipend as 

 much water as in their feparate ftate, and a preci- 

 pitation of part of the water will confequently be 

 produced *. 



Others confider rain as an electrical phenomenon f, 

 and it is very generally allowed, that electricity at 

 leaft concurs with other caufes in producing it. Bo- 

 dies charged with the fame electricity, whether it is* 

 negative or pofitive, always repel each other, and the 

 aqueous particles which compofe clouds, being acted 

 on in this way, will be prevented from uniting into 

 drops fufficiently large to fall to the earth. The ra- 

 pidity with which rain falls after a difcharge of light- 

 ning from a cloud, tends much to confirm this opi- 

 nion. Rain alfo falls heavieft from the center of a 



* That heat has a considerable influence in fufpending aqueous 

 vapour, appears from the phenomenon of dew ; which, the reader 

 will remember, is part of the water taken up by the air during 

 the day time, but which is again depofited during the coolnefs oi 

 the iiight. 



f See Book IV. chap. 6. 



I i 3 cloud, 



