THE AERIAL OCEAN IN WHICH WE LIVE. 57 
have to represent them as lying closely together, but 
as they recede from the earth they are also farthei 
apart. 
Fig. u. 
But the chief reason why the air is thicker or 
denser nearer the earth, is because the upper layers 
press it down. If you have a heap of papers lying 
one on the top of the other, you know that those at 
the bottom of the heap will be more closely pressed 
together than those above, and just the same is the 
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. 
I have here an ordinary pop-gun. If I push the 
cork in very tight, and then force the piston slowly 
inwards, I can compress the air a good deal. Now I 
am forcing the atoms nearer and nearer together, but at 
