REQUISITES OF LANGUAGE. 245 



In giving an exact connotation to the phrase, " an 

 uniform force," philosophers (as Mr.Whewell observes) 

 restricted themselves by the condition, that the phrase 

 should continue to denote gravity. The discussion, 

 therefore, respecting the definition, resolved itself into 

 this question, What is there of an uniform nature in the 

 motions produced by gravity ? By observations and 

 comparisons it was found, that what was uniform in 

 those motions was the ratio of the velocity required to 

 the time elapsed ; equal velocities being added in equal 

 terms. An uniform force, therefore, was defined, a 

 force which adds equal velocities in equal times. So, 

 again, in defining momentum. It was already a re- 

 ceived doctrine, that when two objects impinge upon 

 one another, the momentum lost by the one is equal 

 to that gained by the other. This proposition it was 

 deemed necessary to preserve, not however from the 

 motive (which operates in many other cases) that it 

 was firmly fixed in popular belief; for the proposition 

 in question had never been heard of by any but sci- 

 entific men. But it was felt to contain a truth: even 

 a superficial observation of the phenomena left no 

 doubt that in the propagation of motion from one 

 body to another, there was something of which the 

 one body gained precisely what the other lost; and 

 the word momentum had been invented to express 

 this unknown something. In the settlement, there- 

 fore, of the definition of momentum, was contained 

 the determination of the question, What is that of 

 which a body, when it sets another body in motion, 

 loses exactly as much as it communicates ? And when 

 experiment had shown that this something was the 

 product of the velocity of the body by its mass, or 

 quantity of matter, this became the definition of 

 momentum. 



