THE RHYTHM OF MOTION. 261 



be seen by the unaided eye as the velocity diminishes. 

 However smooth the rails, and however perfectly built the 

 carriages, a railway-train inevitably gets into oscillations, 

 both lateral and vertical. Even where moving matter is 

 suddenly arrested by collision, the law is still illustrated ; for 

 both the body striking and the body struck are made to trem- 

 ble; and trembling is rhythmical movement. Little as we 

 habitually observe it, it is yet certain that the impulses our 

 actions impress from moment to moment on surrounding 

 objects, are propagated through them in vibrations. It 

 needs but to look through a telescope of high power, to be 

 convinced that each pulsation of the heart gives a jar to 

 the whole room. If we pass to motions of another 



order — those namely which take place in the etherial me- 

 dium — we still find the same thing. Every fresh discovery 

 confirms the hypothesis that light consists of undulations. 

 The rays of heat, too, are now found to have a like funda- 

 mental nature; their undulations differing from those of 

 light only in their comparative lengths. Nor do the move- 

 ments of electricity fail to furnish us with an illustration; 

 though one of a different order. The northern aurora may 

 often be observed to pulsate with waves of greater bright- 

 ness ; and the electric discharge through a vacuum shows us 

 by its stratified appearance that the current is not uni- 

 form, but comes in gushes of greater and lesser inten- 

 sity. Should it be said that at any rate there are 

 some motions, as those of projectiles, which are not rhyth- 

 mical, the reply is, that the exception is apparent only ; and 

 that these motions would be rhythmical if they were not in- 

 terrupted. It is common to assert that the trajectory of a 

 cannon ball is a parabola; and it is true that (omitting 

 atmospheric resistance) the curve described differs so slight- 

 ly from a parabola that it may practically be regarded as 

 one. But, strictly speaking, it is a portion of an extremely 

 eccentric ellipse, having the Earth's centre of gravity for 

 its remoter focus; and but for its arrest by the substance of 



