26 CORRELATION OF PHYSICAL FORCES. 



nomena of motion are not made evident by the ordinary sen- 

 suous perception, as for instance the motion of a visibly mov- 

 ing projectile would be, but by an inverse deduction from 

 known relations of motion to time and space : as all observa- 

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

 to another occupy time, we conclude that, wherever a con- 

 tinuing phenomenon is rendered evident in two different 

 points of space at different times, there is motion, though we 

 cannot see the progression. A similar deduction convinces 

 us of the motion of electricity. 



As we in common parlance speak of sound moving, 

 although sound is motion, it requires no great stretch of 

 imagination to conceive light and electricity as motions, and 

 not as things moving. If one end of a long bar of metal be 

 struck, a sound is soon perceptible at the other end. This 

 we now know to be a vibration of the bar ; sound is but a word 

 expressive of the mode of motion impressed on the bar ; so 

 one end of a column of air or glass subjected to a luminous im- 

 pulse gives a perceptible effect of light at the other end : this 

 can equally be conceived to be a vibration or transmitted 

 motion of particles in the transparent column : this question 

 will, however, be further discussed hereafter ; for the present 

 we will confine ourselves to motion within the limits to which 

 the term is usually restricted. 



With the perceptible phenomena of motion the mental 

 conception has been invariably associated to which I have 

 before alluded, and to which the term force is given 

 the which conception, when we analyse it, refers us to 

 some antecedent motion. If we except the production of 

 motion by heat, light, &c., which will be considered in the 

 sequel, when we see a body moving we look to motion hav- 

 ing been communicated to it by matter which has previously 

 moved. 



Of absolute rest Nature gives us no evidence : all matter, 

 as far as we can ascertain, is ever in movement, not merely 



