22 



The Realm of Nature 



CHAP. 



doubt ( 19, 20), yet no one understands what gravitation 

 is nor how it produces its remarkable effects. The greater 

 the mass of two bodies, the more strongly do they attract ; 

 if the total mass is doubled the attraction is doubled. The 

 nearer they are the more strongly do they attract in the 



proportion that halving the 

 distance increases the at- 

 traction fourfold, reducing 

 the distance to one-third in- 

 creases the attraction nine- 

 fold. Fig. 4 illustrates the 

 law of inverse squares as 

 applied to central forces. 



37. The meaning of 

 "Down." If two distant 

 bodies equal in mass could 



FIG. 4. Inverse Squares. The gravita- 

 tional force of O acting on the square 

 at i, is spread over four times the area 

 at 2, and nine times the area at 3, so 



be left free to follow the 

 attraction of gravitation, they would approach each other and 

 meet midway. But if one of the distant bodies had a much 

 larger mass than the other it would move a shorter distance, 

 because the result of attraction is to give the same amount 

 of motion or momentum to each ( 50). If one body is 

 very large and the other very small, the small body seems 

 to fall to the larger, while the latter does not apparently 

 leave its place. This is the case of a stone outside the 

 Earth's surface. It falls directly toward the centre, and the 

 word " down " is used to designate this direction. The 

 movement of the Earth to meet the stone is so slight that it 

 cannot be detected, nor very easily expressed by figures. 

 Still the attraction of gravitation is equal and opposite, the 

 stone attracting the Earth as much as the Earth attracts the 

 stone. 



38. Weight. The attraction of the Earth would draw 

 an external body down to the centre, but the rigidity of the 

 Earth's crust resists distortion. Those parts of the surface 

 which possess no rigidity (the oceans) allow any body 

 denser than water to pass through, or sink in obedience to 

 the pull of gravity until it reaches the solid crust below. 

 The pull of gravity which is counteracted by the push of 



