CHAPTER XXI. 
THE UNIVERSAL FORCE. 
The force bj which all objects are held to the 
surface of the earth as it whirls through space, is of 
such importance that a chapter must be given up to 
it. This force does not belong to the earth only, for, 
so far as we know, it holds all worlds in their ap¬ 
pointed places. 
In obedience to this force a stone falls to the 
earth; the waters seek the valleys and the sea. A 
balloon rises in the air because this force draws the 
air toward the earth more strongly than it does the 
balloon, bulk for bulk; that is, the air outside the 
balloon is heavier than the hot air or gas inside 
the balloon, and hence it crowds up the balloon. 
This force acts upon us whether we wake or 
sleep. It draws the cannon-ball toward the earth’s 
center as strongly when it flies through the air at the 
rate of a mile a minute as it does when the ball lies 
quietly in the gun. Such a universal force certainly 
requires some attention. 
When we speak of this force as acting upon the 
earth, we call it gravity; when we speak of its action 
upon the spheres in the universe, we call it gravitation. 
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