GRAVITATION. 47 



100. Laws of Gravitation. (1.) Gravitation 

 varies directly as the mass. 



(2.) Gravitation varies inversely as the square 

 of the distance (between the centres of gravity. 107). 



For example, doubling the mass, doubles the attraction ; 

 doubling the distance, quarters the attraction ; doubling 

 both the mass and distance will halve the attraction. 

 Trebling the mass will multiply the attraction by three; 

 trebling the distance will divide the 'attraction by nine; 

 trebling both the mass and distance will divide the attrac- 



(3 1 \ 

 3*~3/' 



101. Equality of Attraction. Tl%e force 

 exerted by one body upon a second is the same 

 as that exerted by the second upon the first. 



The earth draws the falling apple with a force that gives 

 it a certain momentum; the apple draws the earth with an 

 equal force which gives to it an equal momentum. The 

 momenta are equal ; the velocities are not. Why not ? 



102. Gravity. The most familiar illustration of grav- 

 itation is the attraction between the earth and bodies 

 upon or near its surface. This particular form of 

 gravitation is commonly called gravity; its measure is 

 weight; its direction is that of the plumb-line, vertical. 



103. Weight. Weight, like gravity, the force of 

 which it is the measure, varies directly as the mass, 

 and inversely as the square of the distance. This 

 distance is to be measured between the centres of gravity 

 of the earth and of the body weighed. When we ascend 

 from the surface there is nothing to interfere with the 

 working of this law ; but when we descend from the surface 



