MOTION. 



MOTION which has been taken as the main exponent of 

 force in the above examples is the most obvious, the most 

 distinctly conceived of all the affections of matter. Visible 

 motion, or relative change of position in space, is a phenomenon 

 so obvious to simple apprehension, that to attempt to define 

 it would be to render it more obscure ; but with motion, as 

 with all physical appearances, there are certain vanishing 

 gradations or undefined limits, at which the obvious mode of 

 action fades away ; to detect the continuing existence of the 

 phenomena we are obliged to have recourse to other than 

 ordinary methods of investigation, and we frequently apply 

 other and different names to the effects so recognised. 



Thus sound is motion ; and although in the earlier periods 

 of philosophy the identity of sound and motion was not traced 

 out, and they were considered distinct affections of matter 

 indeed, at the close of the last century a theory was advanced 

 that sound was transmitted by the vibrations of an ether we 

 now so readily resolve sound into motion, that to those who 

 are familiar with acoustics, the phenomena of sound imme- 

 diately present to the mind the idea of motion, i.e. motion of 

 ordinary matter. 



Again, with regard to light : no doubt now exists that 

 light moves or is accompanied by motion. Here the pheno- 

 mena of motion are not made evident by the ordinary sensuous 

 perception, as, for instance, the motion of a visibly moving 

 projectile would be, but by an inverse deduction from known 

 relations of motion to time and space : as all observation 

 teaches us that bodies in moving from one point in space to 

 another occupy time, we conclude that, wherever a continuing 

 phenomenon is rendered evident in two different points of space 

 at different times, there is motion, though we cannot see the 



