1831.] On a Peculiar Class of Acoustical Figures. 331 



46. When the vibration of the plate ceased, the oil gradually 

 flowed back until of uniform depth. On renewing the vibration, 

 the accumulations were re-formed, the phenomena of accumula- 

 tion occurring with as much certainty and beauty as if lyco- 

 podium powder had been used. 



47. To remove every doubt of the fluid passing from the 

 quiescent to the agitated parts, centres of vibration were used, 

 nearly surrounded by nodal lines. A square plate, fig. 11, 

 being held at c, and the bow applied at x , Fig 1 1 

 gave with sand, nodal lines, resembling those 



in the figure. Then clearing off the sand, 

 putting oil in its place, and producing the same 

 mode of vibration as before, the oil accumulated 

 at a and b, forming two heaps or lenses as in 

 the former experiment (45). 



48. The experiment made with water on the under surface 

 (44) was now repeated with oil, the round plate being used (45). 

 The hanging drop of oil rose up as the water did before, but 

 the lateral diffusion was soon limited ; for lenses were formed 

 at the centres of vibration just as when the oil was upon the 

 upper surface, and, as far as could be ascertained by general 

 examination, of the same form and power. On stopping the 

 vibration, the oil gathered again into hanging drops ; and on 

 renewing it, it was again disposed in the lens-like accumulations. 



49. With white of egg the same observable accumulation at 

 the centres of vibration could be produced. 



50. Hence it is evident that when a surface vibrating normally, 

 is covered with a layer of liquid, that liquid is determined from 

 the quiescent to the vibrating parts, producing accumulation 

 at the latter places ; and that this accumulation is limited, so 

 that if purposely rendered too great by gravity or other means, 

 it will quickly be diminished by the vibrations until the depth 

 of fluid at any one part has a certain and constant relation to 

 the velocity there and to the depth elsewhere. 



51. From the accumulated evidence which these experiments 

 afford, I think there can remain no doubt of the cause of the 

 collection of fine powders at the centres or lines of vibration of 

 plates, membranes, &c. under common circumstances ; and that 

 no secondary mode of division need be assumed to account for 

 them. I have been the more desirous of accumulating experi- 



