50 



LECTURE VI. 



ON THE MOTIONS OF SIMPLE MASSES. 



JfXlTHERTO we have considered the motions of one or more single points 

 or atoms only, without any regard to the bulk or mass of a moveable body : 

 but it now becomes necessary to attend also to the difference of the masses of 

 bodies in motion. This may however be done, without considering the actual 

 magnitude or extent of the body. We may easily conceive different masses 

 or bulks to be concentrated in a mathematical point ; and it is most conveni- 

 ent to define a moveable body, as a moveable point or particle, composed of 

 other elementary particles, differing only in number, and thus constituting the 

 proportionally different mass or bulk of the body. 



Although, in our experiments on motion, Ave are obliged to have recourse to 

 material bodies, and although such bodies differ considerably from this defini- 

 tion of a single moveable body, yet they serve sufficiently well to represent 

 such bodies, especially when they are small, and regularly formed ; and we 

 are here considering the doctrine of motion rather in a mathematical than in a 

 physical sense, so that we are able to neglect all such properties of matter as 

 are not immediately necessary to our purpose. Indeed though the general 

 properties of matter are usually placed at the entrance of elementary works on 

 mechanics, it has yet been found necessary to omit the consideration ^f their 

 effects, in examining the laws and affections of motion. The forces of cohe- 

 sion and repulsion, for example, act, in general, in a very complicated manner, in 

 almost all cases of the communication of motion; but to consider these opera- 

 tions minutely in treating of collision, would be to involve the subject in very 

 great and veiy unnecessary difficulties ; and the complete investigation of 

 these properties of matter would require the employment of various branches 

 of mechanical and hydrodynamical science. We may therefore take a much 

 simpler course, by deferring entirely all theoretical consideration of actual 



