MARCH OF THE THUNDER-CLOUD. 217 



case of those clouds that ride quietly on " their own 

 wind." Its whiteness is a proof of its density ; for 

 it shows that it has body enough to reflect the entire 

 light of the sun, and so the shady side of it will be 

 as black as the sunny side is white. [It is the same 

 kind of treacherous appearance which we have in 

 the white curl-clouds, they are white, not because 

 they are rare, but because they are dense, and the 

 whiter the denser.] 



The firm outline is occasioned by the resistance 

 which the cloud encounters, and the pressing of it 

 between the two winds would bring it down in rain, 

 only that the opposing wind blows under it, and the 

 heat of that and of the earth repels it upwards. The 

 top projects the most, but both that and the under 

 side are turned back in a sort of head, like that of a 

 streamlet when it rolls before it a stone which its 

 force can barely roll, and no more ; so that, long 

 before it reaches the zenith, there is a deep shade 

 upon it, all but the front edge, which, as it pushes 

 on in curved scallops, shows white sometimes on 

 one, and sometimes on another. When its edge is 

 about the zenith, it appears to move with greater 

 velocity, as it is then nearest to the eye. As it ap- 

 proaches the place of the sun, the edge becomes 

 very splendid ; and as there are places which admit 

 only the red rays of the sun and the heating rays to 

 pass through, some of the tints are dismal. The red 

 light through the thinner parts of the cloud, mingling 

 with the reflected green from the earth, gives the 

 cloud and the air under it a very smouldering and 

 murky appearance, as if the sky were about to be 

 on fire. If the cloud is to break where the observer 

 is, the lightning usually begins about that stage ; the 

 first flashes being in the cloud, that is, through the 

 dry air that separates the different strata, and the 

 thunder is low and growling. But every flash brings 

 some of the strata together, and the collected mass 

 descends towards the earth with increasing velocity ; 

 T 



