6o 
THE FAIRY-LAND OF SCIENCE. 
leave off trying to fly away ; it is always pressing up* 
ward and outward with all its might, while the earth 
is doing its best to hold it down. 
The effect of this is, that near the earth, where the 
pull downward is very strong, the air-atoms are drawn 
very closely together, because gravitation gets the best 
in the struggle. But as we get farther and farther 
from the earth, the pull downward becomes weaker, 
and then the air-atoms spring farther apart, and the 
air becomes thinner. Suppose that the lines in this 
FIG. 12. 
diagram represent layers of air. Near the earth we 
have to represent them as lying closely together, but 
as they recede from the earth they are also farther 
apart. 
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 
