1831.] On a Peculiar Class of Acoustical Figures. 329 



by horse-hairs drawn between the fingers. The space beneath 

 the membrane could be retained, filled with air, whilst the upper 

 surface was covered two or three inches deep with water ; or 

 the space below could also be filled with water, or the force 

 applied to the membrane by the horse-hair could be upwards 

 or downwards at pleasure. In all these experiments the sand or 

 filings could be made to pass with the utmost facility to the 

 most powerfully vibrating part, that being either at the centre 

 only, or in addition, in circular lines, according to the mode in 

 which the membrane vibrated. The edge of the funnel was 

 always a line of rest ; but circular nodal lines were also formed, 

 which were indicated, not by the accumulation of filings upon 

 them, but by the tranquil state of those filings which happened 

 to be there, and also by being between those parts where the 

 filings, by their accumulation and violent agitation, indicated 

 the parts in the most powerful vibratory state. 



40. Even when by the relaxation of the parchment from 

 moisture, and the force upwards applied by the horse-hair, the 

 central part of the membrane was raised the eighth of an inch 

 or more above the edges, the circle not being four inches in 

 diameter, still the filings would collect there. 



41. When in place of parchment common linen was used, as 

 becoming tighter rather than looser when wetted, the same 

 effects were obtained. 



42. Both the reasoning adopted and the effects described 

 were such as to lead to the expectation,'that if the plate vibrating 

 in air was covered with a layer of liquid instead of sand or ly co- 

 podium, that liquid ought to be determined from the quiescent 

 to the vibrating parts and be accumulated there. A square 

 plate was therefore covered with water, and vibrated as in the 

 former experiments (2. 6) ; but all endeavours to ascertain 

 whether accumulation occurred at the centres of oscillation, 

 either by direct observation, or the reflexion from its surface 

 of right-lined figures, or by looking through the parts, as 

 through a lens, at small print and other objects, failed. 



43. As, however, when the plate was strongly vibrated, the 

 well-known and peculiar crispations which form on water at the 

 centres of vibration, occurred and prevented any possible de- 

 cision as to accumulation, it was only when these were absent 

 and the vibration weak, and the accumulation therefore small, 



