18 



LECTURE ir. 



ON MOTION. 



TL HE whole science of mechanics depends on the laws of motion, either ac- 

 tually existing, or suppressed by the opposition of the forces which tend to 

 produce it. The nature of motion requires therefore to be particularly ex- 

 amined at the entrance of the science of mechanical philosophy ; and although 

 the subject is so abstract as to demand some effort of the attention, being 

 ijeldom capable of receiving much immediate illustration from the objects of 

 sense, yet we shall find it indispensable to our progress in the investigation of 

 many particular problems of importance, to obtain, in the first place, a clear 

 conception of the properties and affections of motions of all kinds. 



One of the ancient philosophers, on being asked for a definition of motion, 

 i?. said to have walked across the room, and to have answered, you see it, but 

 what it is, 1 cannot tell you. It does not, however, appear absolutely necessary 

 to appeal to the senses for the idea of motion: for a definition is the resoliition 

 of a complex idea int,o the more simple elements which compose it; and, in the 

 present instance, these elements are, the existence of two points at a certain 

 ^stance^ and after a certain interval of time, the existence of the same points 

 at.a different distance; the difterence of the distances being supposed to be as- 

 certained according to that postulate of geometry, which has in general been 

 tacitly understood, but which I have expressly inserted iri the geometrical part 

 of my syllabus ; requiring that the length of a line be capable of being identi- 

 fied, whether by the effect of any object on the senses, or merely in ima- 

 gination. 



Motion, therefore, is the change of rectilinear distance between two points. 

 Allowing the accuracy of this definition, it appears that two points are ne- 



\ 



