68 THE FAIRY-LAND OF SCIENCE. 
weight of the air above, and they take every oppor- 
tunity, when they can find more room, to spread out 
violently and rush into the vacant space, and this rush 
we call a wind. 
Imagine a great number of active schoolboys all 
crowded into a room till they can scarcely move their 
arms and legs for the crush, and then suppose all at 
once a large door is opened. Will they not all come 
tumbling out pell-mell, one over the other, into the hall 
beyond, so that if you stood in their way you would 
most likely be knocked down ? Well, just this happens 
to the air-atoms ; when they find a space before them 
into which they can rush, they come on helter-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 
