72 THE FAIRY-LAND OF SCIENCE. 
skelter, with such force that you have great difficulty in 
standing against them, and catch hold of something to 
support you for fear you should be blown down. 
But how come they to find any empty space to 
receive them. To answer this we must go back 
again to our little active invisible fairies the sunbeams. 
When the sun-waves come pouring down upon the 
earth they pass through the air almost without heating 
it. But not so with the ground ; there they pass down 
only a short distance and then are thrown back again. 
And when these sun-waves come quivering back they 
force the atoms of the air near the earth apart and 
make it lighter ; so that the air close to the surface of 
the heated ground becomes less heavy than the air 
above it, and rises just as a cork rises in water. You 
know that hot air rises in the chimney ; for if you put 
a piece' of lighted paper on the fire it is carried up by 
the draught of air, often even before it can ignite. 
Now just as the hot air rises from the fire, so it rises 
from the heated ground up into higher parts of the 
atmosphere. And as it rises it leaves only thin air be- 
hind it, and this cannot resist the strong cold air whose 
atoms are struggling and trying to get free, and they 
rush in and fill the space. 
One of the simplest examples of wind is to be 
found at the seaside. There in the daytime the land 
gets hot under the sunshine, and heats the air, making 
it grow light and rise. Meanwhile the sunshine on 
the water goes down deeper, and so does not send 
back so many heat-waves into the air; consequently 
the air on the top of the water is cooler and heavier, 
and it rushes in from over the sea to fill up the space 
