120 ELEMENTARY SCIENCE 



it was the fall of an apple which set that great scientist 

 Sir Isaac Newton (1642-1727) to pondering on this prob- 

 lem; the problem of why the apple falls. The great con- 

 clusion which he reached is known as Newton's law of uni- 

 versal gravitation. It may be stated as follows: 



Every mass of matter in the universe attracts every other 

 mass with a force which is directly proportional to the product 

 of these masses, and is inversely proportional to the square 

 of the distance between their centers. 



According to this, you see, the apple attracts the earth 

 as well as the earth attracts the apple. But the apple is 

 so very, very small as compared with the earth that its 

 effect is not noticed. 



It is surprising how many things the law of gravitation 

 explains. You have learned that the tides are explained 

 by the mass attraction or gravitation of the moon acting 

 on the water of the seas. But a far greater thing is the 

 fact that the paths of the heavenly bodies are explained 

 by gravitation. You have heard of the "stars in their 

 orbits." By orbit is meant the path or course which is 

 determined by effects of gravitation, or "pull" of heav- 

 enly bodies upon one another. The moon is held in its 

 orbit by the mass attraction of the earth, and the planets 

 (of which earth is one) are held to their orbits around 

 the sun by the attraction which its mass exerts upon 

 them. So, also, the stars in their courses are believed 

 to be suns around which other planets wheel in response 

 to the same law. 



Weight. But to come back to earth. We can see now 

 that the weight of any object is nothing more than the effect 

 upon it of the law of gravitation. Weight is an expres- 



