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



with the broad surface of one leg uppermost, and a little lyco- 

 podium be sprinkled upon it, the collection of the powder in a 

 cloud along the middle, and the formation of the involving 

 heaps also in a line along the middle of the vibrating steel bar, 

 may be beautifully observed. But if a piece of paper be 

 attached by wax to the side of the limb, so as to form a fence 

 projecting above it, as in the former experiments (19), then 

 the powder will take up its place close to the paper ; and if 

 pieces of paper be attached on different parts of the same leg, 

 the powder will go to the different sides, in the different parts, 

 at the same time. 



22. The effects upder consideration are exceedingly well 

 shown and illustrated by membranes. A piece of parchment 

 was stretched and tightly tied, whilst moist, over the aperture 

 of a funnel five or six inches in diameter ; a small hole was 

 made in the middle, and a horse-hair passed through it, but with 

 a knot at the extremity that it might thereby be retained. Upon 

 fixing the funnel in an upright position, and after applying a 

 little powdered resin to the thumbs and fore-fingers, drawing 

 them upward over the horse-hair, the membrane was thrown 

 into vibration with more or less force at pleasure. By sup- 

 porting the funnel on a ring, passing the horse-hair in the op- 

 posite direction through the hole in the membrane, and 

 drawing the fingers over it downwards, the direction in which 

 the force was applied could be varied according to circum- 

 stances. 



23. When lycopodium or light powders were sprinkled upon 

 this surface, the rapidity with which they ran to the centre, 

 the cloud formed there, the involving heaps, and many other 

 circumstances, could be observed very advantageously. 



24?. The currents which I have considered as existing upon 

 the surface of the plate, membranes, &c. from the quiescent 

 parts towards the centres or lines of vibration (9), arise neces- 

 sarily from the mechanical action of that surface upon the air. 

 As any particular part of the surface moves upwards in the 

 course of its vibration, it propels the air and communicates a 

 certain degree of force to it, perpendicular or nearly so to the 

 vibrating surface ; as it returns, in the course of its vibration, 

 it recedes from the air so projected, and the latter consequently 

 tends to return into the partial vacuum thus formed. But as 



