262 THE RHYTHM OF MOTION. 



the Earth, the cannon ball would travel round that focus 

 and return to the point whence it started; again to repeat 

 this slow rhythm. Indeed, while seeming at first sight to 

 do the reverse, the discharge of a cannon furnishes one of 

 the best illustrations of the principle enunciated. The ex- 

 plosion produces violent undulations in the surrounding air. 

 The whizz of the shot, as it flies towards its mark, is due to 

 another series of atmospheric undulations. And the move- 

 ment to and from the Earth's centre, which the cannon 

 ball is beginning to perform, being checked by solid matter, 

 is transformed into a rhythm of another order; namely, 

 the vibration which the blow sends through neighbouring 

 bodies.* 



Rhythm is very generally not simple but compound. 

 There are usually at work various forces, causing undula- 

 tions differing in rapidity ; and hence it continually happens 

 that besides the primary rhythms there are secondary 

 rhythms, produced by the periodic coincidence and antago- 

 nism of the primary ones. Double, triple, and even quad- 

 ruple rhythms, are thus generated. One of the simplest in- 

 stances is afforded by what in acoustics are known as 

 " beats: " recurring intervals of sound and silence which 

 are perceived when two notes of nearly the same pitch are 

 struck together; and which are due to the alternate corre- 

 spondence and antagonism of the atmospheric waves. In 

 like manner the various phenomena due to what is called 

 interference of light, severally result from the periodic 

 agreement and disagreement of etherial undulations un- 

 dulations which, by alternately intensifying and neutraliz- 

 ing each other, produce intervals of increased and dimin- 

 ished light. On the sea-shore may be noted sundry instances 

 of compound rhythm. We have that of the tides, in which 

 the daily rise and fall undergoes a fortnightly increase and 



* After having for some years supposed myself alone in the belief that all 

 motion is rhythmical I discovered that my friend Professor Tyndall also held 

 this doctrine. 



