260 THE RHYTHM OF MOTION. 



are thrown into a state of up and down movement that is 

 slow or quick in proportion as they are large or small; and 

 where, as in great rivers like the Mississippi, whole trees are 

 thus held, the name " sawyers," by which they are locally 

 known, sufficiently describes the rhythm produced in them. 

 Note again the effect of the antagonism between the current 

 and its channel. In shallow places, where the action of the 

 bottom on the water flowing over it is visible, we see a ripple 

 produced a series of undulations. And if we study the 

 action and re-action going on between the moving fluid and 

 its banks, we still find the principle illustrated, though in 

 a different way. For in every rivulet, as in the mapped-out 

 course of every great river, the bends of the stream from 

 side to side throughout its tortuous course constitute a lat- 

 eral undulation an undulation so inevitable that even an 

 artificially straightened channel is eventually changed into 

 a serpentine one. Analogous phenomena may be observed 

 where the water is stationary and the solid matter moving. 

 A stick drawn laterally through the water with much force, 

 proves by the throb which it communicates to the hand that 

 it is in a state of vibration. Even where the moving body is 

 massive, it only requires that great force should be applied 

 to get a sensible effect of like kind : instance the screw of a 

 screw-steamer, which instead of a smooth rotation falls into 

 a rapid rhythm that sends a tremor through the whole ves- 

 sel. The sound which results when a bow is drawn 

 over a violin-string, shows us vibrations produced by the 

 movement of a solid over a solid. In lathes and planing 

 machines, the attempt to take off a thick shaving causes a 

 violent jar of the whole apparatus, and the production of a 

 series of waves on the iron or wood that is cut. Every boy 

 in scraping his slate-pencil finds it scarcely possible to help 

 making a ridged surface. If you roll a ball along the 

 ground or over the ice, there is always more or less up and 

 down movement a movement that is visible while the 

 velocity is considerable, but becomes too small and rapid to 



