1831.] on Vibrating Elastic Surfaces. 353 



plate four inches by five, fixed on at the Fig. 28. 



ends in a perpendicular position to pre- 

 vent lateral disturbance at those parts, 



very regular and beautiful ridges were ! 1 1 1 1 1 1 ! f ! 1 1 ! 1 1 j 

 obtained of any desired width, fig. 28. 



These ridges, as before, formed only on the wood 5 and were 

 parallel to the direction of its vibration. They occurred on 

 each side of the vibrating plane with equal regularity, force 

 and magnitude, but seemed to have no connexion, for some- 

 times they corresponded in position, and at other times not ; 

 the one set shifting a little, without the others being displaced. 



1 16. It could now be observed that the ridges on either side 

 the vibrating plane consisted of two alternating sets ; the one 

 set rising as the other fell. For each fro and to motion of the 

 plane, or one complete vibration, one of the sets appeared, so 

 that in two complete vibrations the cycle of changes was com- 

 plete. Pieces of cork and lycopodium powder showed that 

 there was no important current setting in the direction of the 

 ridges ; towards the heads of the ridges pieces of cork oscil- 

 lated from one ridge towards its neighbour, and back again. 

 The lycopodium sometimes seemed to move on the ridges from 

 the wood, and between them to it ; but the motion was irregu- 

 lar, and there was no general current outwards or inwards. 

 There was not so much disturbance as amongst the heaps (90). 



117. A very simple arrangement exhibits these ripples beau- 

 tifully. If an oval or circular pan, fifteen or eighteen inches 

 in diameter, be filled with water, and a piece of lath (69) twelve 

 or fifteen inches long be held in it, edge upwards, so as to 

 bear against the sides of the pan as supporting points, and cut 

 the surface of the water, then on being vibrated horizontally 

 by the glass rod and wet finger, the phenomenon immediately 

 appears with ripples an inch or more in length. When the 

 upper edge of the lath was an inch below the surface, the rip- 

 ples could be produced. When the vessel had a glass bottom, 

 the luminous figures produced by a light beneath and a screen 

 above, were very beautiful (96). Glass, metal and other plates 

 could thus be easily experimented with. 



118. These ripple-like stationary undulations are perfectly 

 analogous as to cause, arrangement and action with the heaps 

 and crispations already explained, i. e. they are the results of 



2 A 



