OF FLUIDS ON VIBRATING ELASTIC SURFACES. 
333 
this paper) are in form, quality, and motion of their parts, the same with what 
are called stationary undulations ; and if the mercury in a small circular basin 
be tapped at the middle, stationary undulations, resembling- the ring-like heaps 
(83. 110), will be obtained; or if a rectangular frame be made to beat at equal 
intervals of time on mercury or water, heaps like those of the erispations, ar- 
ranged quadrangularly at angles of 45° to the frame, will be produced. These 
effects are in fact the same with those described, but are produced by a cause 
differing altogether. The first are the result of two progressing and opposed 
undulations, the second of four: but the heaps of erispations are produced by 
the power impressed on the fluid by the vibrating plate ; are due to vibrations 
of that fluid occurring in twice the time of the vibrations of the plate ; and 
have no dependence on progressive undulations, originating laterally, as many 
of the phenomena described prove. Thus, when the edges were bevelled (/2. 
110), or covered with cloth, or wet saw-dust, so that waves reaching the side 
should be destroyed, or when the limits of the water or plates were round (91) 
or irregular, still the heaps were produced, and their arrangement square. 
When the round plate (93) was used, regular erispations were still produced, 
though, as the water extended over the nodal line, and was there perfectly 
undisturbed, no progressing and opposed undulations could originate to pro- 
duce them. Vellum stretched over a ring, and rendered concave by the pressure 
of the exciting rod, produced the same effect. 
109. When a plate of tin, rendered very slightly concave, was attached to a 
lath (69), so as to have equality of vibratory motion in all its parts, and a 
little dilute alkali (which would wet the surface) put into it, the erispations 
formed in the middle, but ceased towards the sides, where, though well wetted*, 
there was not depth enough of water, and from whence also no waves could 
be reflected to produce stationary undulations in the ordinary manner. 
110. When a similar arrangement was made with mercury on a concave tin 
plate, the effects were still more beautiful and convincing. The centre portion 
was covered with one regular group of quadrangular erispations ; at some 
distance from the centre, and where the mercury was less in depth, these passed 
into concentric, ring-like heaps, of which there were a great many; and outside 
of these there was a part wet with mercury, but with too little fluid to give 
either lines or heaps. Here there could be no reflected waves ; or, if that were 
thought possible, those waves could not have formed both the circular rings and 
