their ProdudioTij Suspension, and Destruction. 9 



in fact, from the same cause as heretofore stated, (the water 

 first condensed being followed by undiffuscd vapour ;) yet 

 this cloud speedily evaporates again at all tuDCs, except when 

 precipitation is actually going on at large in the atmosphere 

 next the ground ; when it li only dispersed therein. By 

 comparing the different effects of a clear frosty air, and of a 

 misty though much warmer one, on the perspiration and 

 breath of horses warmed by labour, we may be assisted in 

 reasoning on the great case of evaporation, which, in some 

 sense, is the perspiration of the earth. 



The most powerful predisposing cause of evaporation 

 appears to be a superior current in the atmosphere coming 

 from a region where the low temperature of the surface, or 

 its drv state, occasions a comparative deficiency of vapour. 

 Hence, after heavy rain in winter, we see the sudden evapo- 

 ration, lirst of the remaining clouds, then of the water on 

 the ground, followed by a brisk northerly wind and sharp 

 frost. 



The very snow which had fallen on its arrival is some- 

 times totally evaporated again during the prevalence of such 

 a wind. On the contrary, the first appearance of clouds 

 forming in cold weather gives us to expect a speedy remis- 

 sion of the frost, although the cause is not generally known 

 to be a chanee to a southerly direction already begun in 

 the superior altmosphere, which consequently brings on an 

 excess of vapour. 



This excess of vapour, coming with a superior current, 

 may be placed next to depression of temperature among the 

 causes of rain. The simultaneous decomposition of the 

 hieher imported vapour, and of that wliieh is formed on the 

 spot, or already diffused in the inferior current, would ne- 

 cessarily produce two orders of cloud, differing more or 

 less in electricity as well as in other respects. To the slow 

 action of these upon each other, while evaporation conti- 

 nues below, may be attributed the singular union which 

 constitutes the cumulo-stratus. It is too early n\ the pre- 

 sent state of the subject to attempt to define the precise 

 mode of this action, or to say by what change of state a 

 cumulus alreadv formed is thrown into this modification. 

 That the latter phaniomenon is an electrical eft'eet, no one 

 who has had opportunity to see its rapid progress during 

 the approach of a thunder storm can reasonably doubt. 



To assert that rain is in almost every instance the result 

 of the electrical action of clouds upon each other, might 

 appear to many too speculative, were we even to bring the 

 authority of Kirwau for it, which is decidedly in favour of 



this 



