Measurement of Weight 45 



That they do not do so when placed near each other 

 is a matter of everyday experience. The mutual pull 

 between the Earth and each ball is so much greater than 

 the pull of the balls on one another that their movement 

 towards each other is entirely checked, and thus the 

 existence of the gravitational force between them is 

 completely masked. Experiments have been devised, 

 however, which clearly show that force is exerted 

 between bodies independently of the force exerted 

 upon them by the Earth, but the consideration of such 

 experiments is outside the scope of this book and the 

 student must take the universal distribution of the 

 force of gravity as proved, without requiring actual 

 demonstration. 



26. Since the pull of the Earth upon a body is so 

 much greater and more important than any other 

 force due to gravity, it is quite usual to speak of the 

 " force of gravity " as if it applied solely to the mutual 

 force exerted between the Earth and each object on 

 or near its surface. We may measure the magnitude 

 of the force of gravity in the case of any particular 

 body by balancing it against another force. The 

 spring-balance is a familiar instrument designed to 

 this end. It consists essentially of a tightly-coiled 

 metallic spring to which is attached at its upper end 

 a ring by which it may be held in a vertical position, 

 while at the lower end is a hook or a scale-pan by 

 means of which the force of gravity exerted on a body 

 attached thereto may be caused to stretch the spring. 

 The stretching is resisted by the force produced by the 

 tendency of the coil to resume its original shape, and, 

 for amounts of stretching which are too small to 

 distort the spring permanently, the increase of length 

 is proportional to the stretching force. Thus, if one 



