354 Of fh£,Modiftcalio)is of Clouds, and 



sition, extending through the portion thus thrown up, ant?, 

 in short, a recommencement in the superior region, of the 

 same process which in the vicinity ot the earth furnished 

 the dew of the night. In this case, however, the particles 

 of water cannot arrive at the earth, as they arc necessarily 

 evaporated again in their descent. 



It appears that tliis second evaporation takes p?ace pre- 

 cisely at that elevation A\hcrc the temperature derived from 

 the action of the sun's rays upon the earth, and decreasing 

 upward, becomes just sufficient to counterbalance the pres- 

 sure of the superior vapour. 



Here is formed a sort of boundary between the region of 

 cloud and the region of permanent vapour, which for the 

 present purpose, and until we are furnished with a nomen- 

 clature for the \\ )iole science of meteorology, may be deno- 

 minated the evaporating plane, or, more simph', the vapour 

 plane *. 



Immediately above the vapour plane, then, the formation 

 of the cunmlus commences (as soon as a sufficient quantity 

 of vapour has been thrown up) by the mixture of descend- 

 ing minute drops of water with vapour newly formed, and 

 just diffusing itself, as in the case of the stratus before de- 

 scribed, 



A contiimance of this process might be expected to pro- 

 duce a uniform sheet of cloud, in short a stratus, only differ- 

 ing in situation from the true one. Instead of which we see 

 the first-formed small masses become so many centres, to- 

 wards which all the water afterwards precipitated appears to 

 be attracted from the space surrounding them ; and this at- 

 traction becomes more powerful as the cloud increases in 

 magnitude, insomuch that the small clouds previously 

 formed disappear when a large one appl-oaclies them in its 

 increase, and seem to vanish instead of joining it. This is 

 probably owing to the small drops composing them having 

 passed in a loose manner, and successively, into the laraje 

 one, and consequently so as not to be traced in their pas- 



Are the distinct masses into which the drops form them- 

 selves, in this case, due to the attraction of aggregation alone, 

 t)r is the operatimi of any other cause to be adniitled ? 



A rigid mathematician would perhaps answer the latter 

 clause in the negative ; and with such a conclusion we 

 should have great reason to remain satisfied, as cutting short 



-r wfg ujjg ji^g word plane here in the same sense in which we apply 

 it to a surface of water. Strictly speaking, it is a portion of a sphrrc. 



much 



