82 THE FAIRY-LAND OF SCIENCE. 
in the Mediterranean, or in the Gulf of Mexico off 
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 
FIG. 20. Clouds formed by ascending vapour as it enters cold 
spaces in the atmosphere. 
this vapour, it forms into great bodies of water-dust 
clouds, and the sky is overcast. At other times clouds 
hang lazily in a bright sky, and these show us that 
just where they are (as in Fig. 20) 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 
