REQUISITES OF LANGUAGE. 405 



is, how the conception shall be unJorstood and defined in order that 

 the proposition may be true." 



" To unfold our conceptions by means of definitions has never been 

 serviceable to science, except when it has been associated with an 

 immediate Tise of the definitions. The endeavor to define a Uniform 

 Force was combined with the assertion that gravity is a uniform force : 

 the attempt to define Accelerating Force was immediately followed by 

 the doctruie that accelerating forces maybe compounded: the process 

 of defining Momentum was connected with the principle that momenta 

 gained and lost are ecpial : naturalists would Imve given in vain the 

 definition of Species which we have quoted, if they had not also given 



the characters of species so sejiarated Dcifinitiou may be the best 



mode of explaining our conception, but that which alone makes it 

 worth while to explain it in any mode, is the opportunity of using it in 

 the expression of truth. When a definition is jiropounded to us as a 

 useful ^tep in knowledge, we are always entitled to ask what principle 

 it serves to enunciate." 



In giving an exact connotation to tlie phrase, " an uniform force," 

 philosophers (as JNIr. Wliewell 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 e<jual veloci- 

 ties in equal times. So, again, in defining momentum. It was already 

 a received 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 scientific men. But it was felt to contain a truth : 

 even a superficial obserA'ation of the phenomena left no doubt that in 

 the propagation of motion from one body to another, there was some- 

 thing of which the one body gained precisely what the other lost; and 

 the word momentum had been invented to express this unknown some- 

 thing. In the settlement, therefore, of the definition of momentum, was 

 contained th(; 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 showni that this something 

 was the ))roduct of the velocity of the body by its mass, or quantity of 

 matter, this became the defijiition of momentum. 



Mr. Wliewell very justly adds,* ''The business of definition'is part 



of the business of discovery To define, so that our definition 



shall have any scientific value, requires no small portion of that saga- 

 city by which truth is detected. '. When it has been clearly seen 



what ought to be our definition, it must bo pretty well known what 

 truth we bav(> to state. The definition, as well as the discovery, sup- 

 poses a decided step in our knowledge to have been made. The 

 writers on Logic, in the middle ages, made Definition the last stage in 



♦ Phil, of the Ind. Sc, ii.. 181-2. 



