1831.] on Vibrating Elastic Surfaces. 347 



inspection, that the heaps were not stationary, but rose and 

 fell ; and also that there were two sets regularly and alternately 

 arranged, the one set rising as the other descended. 



99. Sand gave no indications of arrangement with these large 

 heaps (86) ; but when some coarse sawdust was soaked, so as 

 to sink in water, and then distributed in the fluid, its motions 

 were beautifully illustrative of the whole philosophy of the 

 phenomena. It was immediately washed away from under the 

 rising and falling heaps, and collected in the places equidistant 

 between these spots, as the sand did in the former experiments 

 (86), and by its vibratory motion to and fro, it showed distinctly 

 how the water oscillated from one heap towards another, as the 

 heaps sunk and rose. 



100. When milk (75) was used instead of water for these 

 large arrangements in a dark room, and a candle was placed 

 beneath, the appearances also were very beautiful, resembling 

 in character those described (97). 



101. Each heap (identified by its locality) recurs or is re- 

 formed in two complete vibrations of the sustaining surface*; 

 but as there are two sets of heaps, a set occurs for each vibra- 

 tion. The maximum and minimum of height for the heaps 

 appears to be alternately, almost immediately after the sup- 

 porting plate has begun to descend in one complete vibration. 



102. Many of these results are beautifully confirmed by the 

 appearances produced, when regular crispations have been 

 sustained for a short time with mercury, on which a certain 

 degree of film has been allowed to form (77). On examining 

 the film afterwards in one light, lines could be seen on it, coin- 

 ciding with the intervals of the heaps in one direction ; in an- 

 other light, lines coinciding with the other direction came into 

 sight, whilst the first disappeared ; and in a third light, both 

 sets of lines could be seen cutting out the square places where 

 the heaps had existed : in these spaces the film was minutely 

 wrinkled and bagged, as if it had there been distended ; at 

 the lines it was only a little wrinkled, giving the appearance of 

 texture ; and at the crossing of the lines themselves, it was 



* A vibration is here considered as the motion of the plate, from the time 

 that it leaves its extreme position until it returns to it, and not the time of its 

 return to the intermediate position. 



