264 EVAPORATION. 



different degrees of velocity. Hence clouds situated in 

 these portions of air, divide, collect, and change form, 

 according to the foice acting upon them. Water-spouts 

 are usually attended by a thick black cloud, formed pro- 

 bably by the vapor condensed by opposite currents of air 

 meeting. New accessions of vapor often change the 

 form of clouds ; also the dissolving of vapor or a dimi- 

 nution of its density. Sometimes probably a cloud meets 

 with a stratum of air sufficiently warm to dissolve it. In 

 this case it will vanish by degrees. Different parts of a 

 cloud may be in strata of air of different warmth or den- 

 sity. The cloud will then partly dissolve, and the part 

 dissolved will perhaps rise and become visible in a higher 

 portion of the air, where the heat is not sufficient to ren- 

 der it invisible. In the spring it is often cloudy in the 

 morning, and clears off towards noon. The heat of the 

 sun dissolves the moisture which arose in great quantities 

 from the damp earth in the morning. 



Clouds near the horizon generally appear to extend 

 much farther horizontally than perpendicularly. This is 

 probably, in part, an illusion. If the lower surface of a 

 cloud is nearly parallel to the surface of the earth, its 

 extent towards the zenith will appear much less than it 

 really is, while this will not be the case with its extent in 

 the direction of the horizon. As it approaches the zenith, 

 it will appear more nearly of its true dimensions. Hence 

 clouds in the zenith seldom, or never appear to be of this 

 form. Clouds often move in opposite directions. Dif- 

 ferent portions of air often move in different directions 

 above one another, on account of their being unequally 

 rarefied by heat. They of course carry the clouds with 

 them. This may be readily illustrated. If, in cold 

 weather, the door of a warm room be opened a little, 

 and a candle held near the bottom of the opening, and 

 another near the top, the flame will often be blown in op- 

 posite directions. The cold air rushes in at the bottom, 

 and the warm air being lighter goes out at the top. 



The color of clouds depends on the rays of light which 

 they reflect. Dark clouds often precede wind. But 

 although they are seen before the wind is felt, they are 

 not the cause, but the effect of the wind. As the wind 

 moves on, it presses upon that portion of the air, which 



