A DROP OF WATER. 79 
the coast of America, or even, if the wind is from 
the north, of chilly particles gathered from the surface 
of Greenland ice and snow, and brought here by the 
moving currents of air. Only, of one thing we may 
be sure, that they come from the water of our earth. 
Sometimes, if the air is warm, these water-particles 
may travel a long way without ever forming into 
clouds ; and on a hot, cloudless day the air is often 
very full of invisible vapour. Then, if a cold wind 
comes sweeping along, high up in the sky, and chills 
this vapour, it forms into great bodies of water-dust 
clouds, and the sky is overcast. At other times 
Fig. 19. 
Clouds formed by ascending vapour as it enters cold spaces in the 
atmosphere. 
clouds hang lazily in a bright sky, and these show us 
that just where they are (as in Fig. 19) the air is cold 
and turns the invisible vapour rising from the ground 
into visible water-dust, so that exactly in those spaces 
we see it as clouds. Such clouds form often on a 
warm, still summer's day, and they are shaped like 
masses of wool, ending in a straight line below. They 
are not merely hanging in the sky, they are really 
resting upon a tall column of invisible vapour which 
stretches right up from the earth ; and that straight 
