II. 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 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 ef 

 sound 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- 

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