1820.] 



Mr. Dallon on Meteorology. 



257 



Mean Monlhlij and Annual Quantities of Rain at various Places^ 

 being the Averages for many Years. 



Observations on the Theory of Rain. 



Every one must have noticed an obvious connexion between 

 heat and the vapour in the atmosphere. Heat promotei? evapo- 

 ration, and contributes to retain the vapour when in the atnio^ 

 sphere, and cold precipitates or condenses the vapour. But 

 these facts do not explain the phenomenon of rain, which is as^ 

 frequently attended with an increase as with a diminution of the 

 temperature of the atmosphere. 



The late Dr. Button, of Edinburgh, was, I conceive, the first 

 person who published a correct notion of the cause of rain» 

 (See Edin. Trans, vol. i. and ii. and Mutton's Dissertations, &c.^ 

 Without deciding whether vapour be simply expanded by heal^ 

 and diffused through the atmosphere, or chemically combined 

 with it, he maintained from the phenomena that the quantity of 

 vapour capable of entering into the air increases in a greater 

 ratio than the temperature ; and hence he fairly infers,, that 

 whenever two volumes of air of different temperatures are mixed 

 together, each being previously saturated with vapour, a preci- 

 pitation of a portion of vapour must ensue, in consequence of 

 the mean temperature not being able to support the vtean quan- 

 tity of vapour. 



This explanation may be well illustrated by contemplating a 

 curve, convex towards its axis, in which case the ordinate* 

 increase in a greater ratio than the absciisste. The abscissae 

 represent temperature, -and the ordinatea tb.e quantity of steam 

 which the corres|)ondii:g temperatures tire capable of retaining. 



In 179:3 I published my iVIeteo.rological Observations and 

 Vo... XV.NMV. K 



