THE AERIAL OCEAN IN WHICH WE LIVE. 6 1 
case with the atoms of the air. Only there is this 
difference, if the papers have lain for some time, 
when you take the top ones off, the under ones remain 
close together. But it is not so with the air, because 
air is elastic, and the atoms are always trying to fly 
apart, so that directly you take away the pressure they 
spring up again as far as they can. 
In this the ocean of air differs from an ocean of 
water, for water is neither elastic nor can it be com- 
pressed except to a very small extent. If it were 
otherwise the sea at great depths would be almost or 
quite solid under the pressure of the enormous weight 
of water above ; and even at a few fathoms below the 
surface would present great resistance to bodies pass- 
ing through it. Fish or marine animals could only 
exist at or near the surface. At any considerable 
depth the compressed water would hold sunken objects 
embedded in it as does ice ; nothing could reach the 
bottom below a certain depth. 
I have here an ordinary pop-gun. If I push the 
cork in very tight, and then force the piston slowly 
inward, I can compress the air a good deal. Now I 
am forcing the atoms nearer and nearer together, but 
at last they rebel so strongly against being more crowd- 
ed that the cork can not resist their pressure. Out it 
flies, and the atoms spread themselves out comfortably 
again in the air all around them. Now, just as I 
pressed the air together in the pop-gun, so the at- 
mosphere high up above the earth presses on 
the air below and keeps the atoms closely packed 
together. And in this case the atoms cannot force 
back the air above them as they did the cork in the 
