GRAVITY 



2579 



GRAVITY 



But when he tried to apply them he used fig- 

 ures for the various dimensions of the earth 

 which were afterwards found to be wrong, and 

 so, at first, he could not prove his law. 



The general law of gravitation formulated 

 by Newton is that every particle of matter in 

 the universe attracts every other particle of 

 matter in the universe with a force that is 

 directly proportional to its bulk, or mass, and 

 inversely proportional to the square of the 

 distance between them. C.R.M. 



Related Subjects. The reader is referred to 

 the following articles in these volumes : 

 Earth Mass 



Falling Bodies Newton, Sir Isaac 



Gravity, Center of Weight 



GRAVITY, grav'iti, CENTER OF. If you 

 stick a pin through a point near the edge of 

 a blotter and allow the latter to swing freely, 

 it will come to rest at a position in which 

 most of the blotter is below the point of sus- 



pension; if you move it, it will return always 

 to this same position. There is one point in 

 the blotter, however, called the center of grav- 

 ity, or center of weight; if you stick the pin 

 through this, the blotter will remain in what- 

 ever position you place it. How to find this 

 center of gravity is shown at (1) in the illus- 

 tration. 



When the force of gravity acts on an object 

 it does so as if its power were all applied at 

 the center of gravity. When a cone is in the 

 position shown at (2), so that a line directly 

 downward from the center falls outside of the 

 object, the pull of gravity will upset the cone 

 by drawing the center toward the earth. But 

 in the position at (3), where a similar line 

 falls within the base of the cone, gravity 

 merely holds the article still, because to tip it 

 in either direction the center must be raised. 

 In the first case it is said to be in unstable 

 equilibrium, in the second in stable equilibrium. 

 Everyone knows that a board standing on end 



will fall, that one on its edge may be bal- 

 anced but is easily overturned, but that a 

 board resting on its broad side will stay in 

 that position. From this it may be seen that 

 the higher the center of gravity of an object, 

 the more liable the object is to tip. 



A center of gravity may be outside of its 

 object, as in the case of anything hollow, like 

 a boat or a dish. The point at which the 

 attraction of the sun influences the earth is 

 not the center of the earth, but the center of 

 gravity of the earth and the moon combined, 

 for although they do not form a single object, 

 their mutual attraction makes it impossible for 

 the sun to pull on one without pulling on the 

 other and they are like a dumb-bell with one 

 large and one small end. 



GRAVITY, SPECIFIC. There was consterna- 

 tion at the court of Syracuse one day many 

 hundreds of years ago when Hiero, the king, on 

 receiving a new crown, decided that it was not 

 made of solid gold, as he had ordered, but 



AN EXPERIMENT IN FINDING SPECIFIC 

 GRAVITY 



Figure 1 is a stone. If it is Jmmersed In watt-r 

 in Figure 2 it will raise the water in the tumbler 

 as much as the space'Tt occupies ; that is. speak- 

 ing scientifically, it will displace its own bulk of 

 water. Figure 3 indicates the increase in the 

 height of water. Figure 4 represents the water 

 which equals the bulk of the stone. When that 

 quantity of water and the stone are weighed It can 

 be determined how many times heavier than water 

 is the stone. The resulting quotient is the apcciflr 

 gravity of the stone. 



was alloyed with silver. He was furious! A 

 king's crown has to be of purest gold. Be- 

 sides, he did not intend to be cheated. But 

 his problem was simple; he merely sent the 

 crown to Archimedes with the command that 

 he settle the matter. 



Archimedes was one of the wisest men in 

 Greece in the period about 200 years B.C. 

 In the article under his name you will find an 

 account of his life and the marvelous things 

 he did. But Archimedes, for all his wisdom, 

 was much puzzled by the problem presented 

 to him. For a long time he was unable to 

 devise a way of finding out whether the 

 king's crown was actually made of pure gold, 

 without destroying it. Then, one day, when 

 he was getting into the bath, he noticed the 

 spilling over of the water and it gave him his 



