GRAVITATION, AND HOW IT WORKS. 37 



no less than 32 on the Erie Canal. The finest of them are two across 

 the Mohawk River, a third at Richmond over the Seneca Rivej-, and a 

 fourth across the Genesee at Rochester. The latter is a splendid stone 

 arcade 920 feet long, having six cut-stone arches of 52 feet span. A 

 wire suspension-bridge of seven spans, each 160 feet long, conveys 

 the Pennsylvania Canal across the Alleghany River at Pittsburg. 



GRAVITATION, AND HOW IT WORKS. 



By GRANVILLE F. FOSTEE, 



" The force of gravity acts on bodies directly in proportion to the quantity of matter 

 in each." 



" The force of gravity decreases in the reciprocal proportion of the square of the dis- 

 tance." (Orr's " Circle of the Sciences," vol. vi., p. 1.) 



AMONG students of natural philosophy no facts are more fre- 

 quently misunderstood than those pertaining to the laws of 

 gravitation. It is readily admitted that if a body A exerts on B a 

 certain force of attraction, if A's mass be doubled, then will A's at- 

 tractive influence on B be doubled also, but the fact is not so apparent 

 that any two bodies, whatever their disparity of mass, or however 

 great their distance apart, will attract each other with precisely equal 

 forces ; and that if, for instance, the mass of A be doubled, not only 

 will''A's attraction for B be doubled, but at the same time B's attrac- 

 tion for A wnll be doubled also. The pen I hold in my hand attracts 

 the sun with precisely the same amount of force that the sun attracts 

 the pen, and, if either the mass of the pen or sun be doubled, the 

 mutual attraction will be doubled also. The first law of gravitation 

 most certainly teaches that the earth, so insignificantly small as com- 

 pared with the sun, both in volume and mass, attracts the sun with a 

 force exactly equal to tliat which, being by the sim exerted on itself, 

 reduces it to obedience, and compels it to make its annual revolution. 

 So, too, the moon and the earth mutually and equally attract each 

 other. 



The fact that the forces of attraction between two bodies are equal 

 may be easily explained as follows : Let there be five bodies. A, B, C, 

 D, E, and let A be so situated as to be at equal distances from the 

 other four : then it is evident that the forces which measure the mu- 

 tual attractions of (A and B), (A and C), (A and D), and (A and E), 

 are equal. Calling the force which A exerts on B, or B exerts on A, 

 one, then will the sum of the forces which B, C, D, and E exert on A 

 be equal to foicr, but the sum of A's attractions for B, C, D, and E, 

 will also be equal to four, since A's attraction for B is in no way 



