PROPERTIES OF MATTER 47 



and the liquids tended to move away in a straight line, but 

 they were kept from it by the band and by the globe. 

 What happens when there is not sufficient restraining force 

 is seen when the mud flies from the tires of a rapidly moving 

 vehicle. 



Newton many years ago discovered that all bodies of 

 matter have an attraction for one another. What causes 

 this no one knows, but the name given to this force of at- 

 traction is gravitation. Gravitation is always acting upon all 

 bodies, and their conduct is constantly affected by it. It 

 keeps the heavenly bodies from wandering away from one 

 another, as the rubber band kept the weight from flying 

 away from the hand. 



Newton also discovered that the force of attraction be- 

 tween two bodies varies as the masses of the bodies; that 

 is, the more matter two bodies contain, the more they attract 

 each other. But this attraction becomes less as the dis- 

 tance between the bodies increases. The lessening of the 

 force of gravitation on account of the increase of distance 

 is proportional not to the distance but to the square of the 

 distance. This means that if the distance between two 

 bodies is doubled, the attraction between them is only one- 

 fourth as great. Moved three times as far apart, the bodies 

 have only one-ninth the attraction for each other; and so 

 on. 



When this attraction is considered in relation to the earth 

 and bodies near its surface the term gravity is used. We are 

 constantly measuring the pull of gravity and calling it 

 weight. It is the force which causes us to lie down when we 

 wish to sleep comfortably, and which makes all unsupported 

 bodies fall to the earth. 



If two forces act upon a body free to move, each will in- 



