84 THE PROGRESS OF SCIENCE n 



in motion, so long as no external cause of change 

 acts upon it. The other is that the effect which 

 the impact of a body in motion produces upon the 

 body on which it impinges depends, other things 

 being alike, on the relation of a certain quality of 

 each which is called "mass." Given a cause of 

 motion of a certain value, the amount of motion, 

 measured by distance travelled in a certain time, 

 which it will produce in a given quantity of 

 matter, say a cubic inch, is not always the same, 

 but depends on what that matter is a cubic inch 

 of iron will go faster than a cubic inch of gold. 

 Hence, it appears, that since equal amounts <>t' 

 motion have, ex hypothesi, been produced, the 

 amount of motion in a body does not depend on its 

 speed alone, but on some property of the body. 

 To this the name of " mass " has been given. And, 

 since it seems reasonable to suppose that a large 

 quantity of matter, moving slowly, possesses as 

 much motion as a small quantity moving faster, 

 " mass " has been held to express " quantity of 

 matter." It is further demonstrable that, at any 

 given time and place, the relative mass of any two 

 bodies is expressed by the ratio of their weights. 



When all these great truths respecting molar 

 motion, or the movements of visible and tangible 

 masses, had been shown to hold good not only of I 

 terrestrial bodies, but of all those which constitute I 

 the visible universe ; and the movements of the 

 macrocosm had thus been expressed by a gem-mi 



