Royal Society. 43 



have among themselves an intestine motion of revolution, rising in 

 the centre of the heap, and rolling down again on the outer sides. 

 The powders are collected in the same situations on the vibratmg 

 plate, although the plate may be considerably inclined to the hori- 

 zon, and remain there even when the inclination is so great as to 

 prevent grains of sand from resting on the nodal lines. A piece of 

 gold leaf laid upon the plate was raised up in the form of a blister 

 at that part which corresponded with the centre of the clouds, even 

 to the height of one-twelfth of an inch. 



On attaching small pieces of card to different parts of the surface 

 of the vibrating plate, the currents of air are modified in various 

 ways, as shown by the different positions of the clouds, and the pro- 

 duction of partial accumulations of the powders. When a tuning- 

 fork is made to vibrate, and a little powder of lycopodmm is 

 sprinkled over it, the powder collects into clouds on the middle of 

 the upper surface, and also forms heaps along its sides, exhibiting 

 in a striking manner the intestine revolution of their particles. These 

 effects are also well illustrated by vibrating membranes ; for which 

 purpose a piece of parchment was stretched, and tied while moist 

 over the mouth of a funnel, and made to vibrate by means of a horse- 

 hair, having a knot at the end, passed through a hole in the centre 

 of the parchment ; the hair being drawn between the finger and 

 thumb, to which a little powdered rosin was previously applied. 

 The pheenomena were still more conspicuous when the parchment 

 was made to vibrate under a glass plate held near it. When the 

 interval between the membrane and the glass plate was very small, 

 the whole of the powder was sometimes blown out at the edge, in 

 consequence of the vibrating membrane acting as a bellows. 



Reasoning from the theory which the author had framed in ex- 

 planation of these phaenomena, he conceived that if the currents 

 were weakened by placing the apparatus in rarefied air, they would 

 no longer be capable of sustaining the hght powders, which would 

 then be collected, like the heavy powders in air, at the nodal lines. 

 In a denser medium, such as water, the reverse should happen ; the 

 heavy powders should be carried along by the more powerful cur- 

 rents then produced, and would accumulate in the vibrating parts. 

 All these conclusions were found to be fully verified by actual ex- 

 periment. 



May 19.— A paper was read, entitled, " A Table facilitating the 

 Computations relative to Suspension Bridges." By Davies Gilbert, 

 Esq. V.P. U.S. 



The table here communicated is supplementary to those accom- 

 panying the paper " On the Mathematical Theory of Suspension 

 Bridges," which was published in the Philosophical Transactions for 

 182G, and is deduced from the first of the tables there given ; but 

 admits of a far more ready application than the former to all cases 

 of practical investigation. It consists of five columns, exhibiting 

 respectively the deflections or versed sines of the curve ; the lengths 

 of the chains ; the tension at the middle points, or apices of the 

 curve ; the tensions at the extremities ; and the angles made by the 



chains with the horizon at the extremities. 



(;, <i A paper 



