262 



Essay on Motion and Force. 



change of motion is itself motion, and the antecedent 

 conditions which are the cause of this change, are matter 

 and change of that matter. In other words, when a change 

 takes place, another change supervenes, itself an antecedent 

 condition of another change, and so on forever. 



In order that matter may move in any particular man- 

 ner, a corresponding opposite change must take place in 

 other matter ; that is, if one body moves more, some other 

 body must move less. All of the aggerose motions of a 

 body may perhaps be theoretically imparted to another 

 body in this way, but we can adduce no instance of their 

 being thus imparted. If we should project a perfectly 

 elastic body in the plane of the earth's equator in the direc- 

 tion of the earth's rotation, it would possess two motions ; 

 the one imparted to it by projection, the other by the earth's 

 rotation. ISTow, if it should approach sufficiently near 

 another elastic body, at rest with regard to the earth's 

 rotary motion, it would impart all the motion it retained 

 from its projection, while its motion derived from 

 the earth's rotation would remain undiminished. If a 

 similar body should be projected in an opposite direc- 

 tion it might be considered as possessing two motions, 

 one in the direction of the earth's rotation, and another 

 derived from its proj ection in an opposite direction. Should 

 it approach sufficiently near another body it would impart 

 motion to it, but only that which it retained from its pro- 

 jection, while it would as before, retain all the motion 

 acquired from the earth's rotation. The true doctrine 

 with regard to the imparting of motion, appears to me to 

 be this, that as no body possesses, so far as we can discern, 

 a solitary motion, and as the conditions for the imparting 

 of these motions differ with the motions themselves, and 

 as these conditions oppose each other, it is practically 

 impossible that all of the motions which belong to any 

 particular mass of matter, should be imparted to another 

 mass. Suppose a body to be dropped from an elevation 

 toward the earth. From the time it is let fall, until some 



