COMPOSITION AND RESOLUTION OF FORCE. 



207 



COMPOSITION & RESOLUTION OF FORCE. 



Motion and pressure are terms too familiar to need explanation. It may be 

 observed, generally, that definitions in the first rudiments of a science are sel- 

 dom, if ever, comprehended. The force of words is learned by their applica- 

 tion ; and it is not until a definition becomes useless, that we are taught the 

 meaning of the terms in which it is expressed. Moreover, we are perhaps jus- 

 tified in saying that, in the mathematical sciences, the fundamental notions are 

 of so uncompounded a character, that definitions, when developed and enlarged 

 upon, often draw us into metaphysical subtleties and distinctions, which, what- 

 ever be their merit or importance, would be here altogether misplaced. We 

 shall, therefore, at once take it for granted, that the words motion and pressure 

 express phenomena or effects which are the subjects of constant experience 

 and hourly observation ; and if the scientific use of these words be more pre- 

 cise than their general and popular application, that precision will soon be 

 learned by their frequent use in the present treatise. 



Force is the name given in mechanics to whatever produces motion or pres- 

 sure. This word is also often used to express the motion or pressure itself; 

 and when the cause of the motion or pressure is not known, this is the'' only 

 correct use of the word. Thus, when a piece of iron moves toward a magnet, 

 it is usual to say that the cause of the motion is " the attraction of the magnet ;" 

 but in effect we are ignorant of the cause of this phenomenon, and the name 

 attraction would be better applied to the effect, of which we have experience. 

 In like manner the attraction and repulsion of electrified bodies should be un- 

 derstood, not as names for unknown causes, but as words expressing observed 

 appearances or effects. 



When a certain phraseology has, however, gotten into general use/ it is nei- 

 ther easy nor convenient to supersede it. We shall, therefore, be compelled, 

 in speaking of motion or pressure, to use the language of causation ; but must 

 advise the student that it is effects, and not causes, which will be expressed. 



If two forces act upon the same point of a body in different directions, a sin- 



