328 On a Peculiar Class of Acoustical Figures. [1831. 



37. In this form of experiment there were striking proofs of 

 the existence of a current upwards from the middle of the mem- 

 brane when vibrating in air (24), and the extent of the system 

 of currents (26) was partly indicated. The powder purposely 

 collected at the middle by vibrations, when the receiver was 

 full of air, was observed as to the height to which it was forced 

 upwards by the vibrations ; and then the receiver being ex- 

 hausted, the height to which the powder was thrown by similar 

 vibrations was again observed. In the latter cases it was nothing 

 like so great as in the former, the height not being two-thirds, 

 and barely one-half, the first height. Had the powder been 

 thrown up by mere propulsion, it should have risen far higher 

 in vacuo than in air : but the reverse took place ; and the cause 

 appears to be, that in air the current had force enough to carry 

 the fine particles up to a height far beyond what the mere 

 blow which they received from the vibrating membrane could 

 effect. 



38. For the experiments in a denser medium than air, water 

 was chosen. A circular plate of glass was supported upon four 

 feet in a horizontal position, surrounded by two or three inches 

 of water, and thrown into vibration by applying a glass rod 

 perpendicular to the middle, as in the first experiment in vacuo 

 (34) ; the feet were shifted until the arrangement gave a clear 

 sound, and the moistened brass filings sprinkled upon the plate 

 formed regular lines or figures. These lines were not however 

 lines of rest, as they would have been in the air, but were the 

 places of greatest vibration ; as was abundantly evident from 

 their being distant from that nodal line determined and indicated 

 by the contact of the feet, and also from the violent agitation 

 of the filings. In fact, the filings proceeded from the quiescent 

 to the moving parts, and there were gathered together ; not 

 only forming the cloud of particles over the places of intense 

 vibration, but also settling down, when the vibrations were 

 weaker, into the same involving groups, and in every respect 

 imitating the action of light powders in air. Sand was affected 

 exactly in the same manner ; and even grains of platina could 

 be in this way collected by the currents formed in so dense a 

 medium as water. 



39. The experiments were then made under water with the 

 membranes stretched over funnels (22) and thrown into vibration 



