156 GENERAL SCIENCE 



the bodies involved, and by their distances apart. With the 

 mass of the earth considered constant, and the value of its 

 attraction directly proportional to the amount of matter in 

 the body attracted, it becomes easily possible by means of the 

 spring balance to determine the relative pull of the earth 

 for different bodies. The weight of a body is the value of the 

 earth's attraction for it. Weighing it is finding out how 

 many times greater this attraction is than that for some other 

 body whose mass is taken as a unit. The close association 

 of our ideas of matter and of force is noted in the fact that 

 the name pound is in use both for the standard unit of mass 

 carefully safe-guarded at Washington, D. C., and for the 



FIG. 53. -Weighing is a comparison of earth pulls. 



value of the earth-pull upon this mass, or any duplicate of it, 

 anywhere in the world. A like confusion exists in the use 

 of the terms gram of force and gram of mass. 



One thing must be noted in this connection, however, 

 marking a sharp distinction between a pound of matter and 

 a pound of force. The pound mass is an unchangeable 

 quantity regardless of the place to which it may be taken. 

 'On the other hand, the earth's attraction for this pound mass 

 varies in value, and this variation depends largely upon 

 altitude (distance above or below sea-level), and upon 

 latitude. This dependence upon latitude is due to the fact 



