56 INTRODUCTION TO GENERAL SCIENCE 



39. UNIVERSAL GRAVITATION 



Gravitation is the name given to the force which acts among 

 all bodies of matter, tending to bring them together. We 

 who live on the earth are inclined to think that the earth 

 attracts us. While this is true, it is just as much a fact that 

 the earth is attracted by us. Gravitation, then, is the mutual 

 action which takes place between every two bodies of matter. 

 By careful measurements, it has been found that the changes of 

 gravitation vary as the product of the masses of the two bodies, 

 and inversely as the square of the distance between them. 



Not only is this force of gravitation acting between the earth 

 and the bodies on it, but it acts between all of the heavenly 

 bodies. Thus the sun attracts the earth, and the earth at- 

 tracts the sun ; the moon attracts the earth, and the earth 

 attracts the moon. The question which naturally arises is, 

 Why do not these bodies come together? The answer is 

 that they would come together if they were not in rapid 

 motion, whirling around each other. Part of the year the 

 earth and the sun do come closer together than at other 

 seasons, but the velocity of the earth is great enough to take 

 it entirely around the sun, and thus avoid any collision. 

 This is one of the proofs that the earth revolves around the 

 sun once a year, for if this revolution did not take place, the 

 earth and the sun would fall together. Yet as the earth re- 

 volves around the sun, its path is riot that of a smooth curve, 

 nor is it the same from year to year. The other planets at- 

 tract the earth, and the earth attracts the other planets, so 

 that the earth moves in a wavy path approximating an ellipse. 

 In the same way, the moon does not revolve around the earth 

 in a symmetrical curve, but moves in a wavy path. 



