II. MOTION. 



MOTION which has been taken as the main exponent 

 of force in the above examples is &quot;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 at 

 tempt to define it would be to render it more obscure ; but 

 with motion, as with all physical appearances, there are cer 

 tain vanishing gradations or undefined limits, at which the 

 obvious mode of action fades away ; to detect the continu 

 ing existence of the phenomena we are obliged to have re 

 course 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 pe 

 riods 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 

 Bound immediately 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 phe- 

 2 



