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Communication of Vibrations through Liquids.

motions, and to assume nodal figures according to the laws of that species of motion.

To show more clearly the nature of the communication, he threw out the water, and supported the wooden disc by a small solid stem perpendicular to its surface, and the bottom of the vessel, and attached to the centres of both, when it was found that the disc was affected precisely in the same way as before.

b. On a vessel of water, whose rim is maintained in a state of normal vibration by a bow drawn perpendicularly across it at any point, let a thin rectangular lamina of wood be floated, having its length parallel to the bow, and its extremity opposite to the point of the circumference excited. The lamina will be seen (as usual by sand strewed on its upper face) to execute longitudino-tangential vibrations, and will be crossed by nodal lines at right angles to its length.

But if, instead of directing the axis or longer edges of the lamina perpendicularly towards the vibrating point of the side of the vessel, we incline it obliquely to the direction of the vibrations, still the sand on its upper face will continue to glide in the same direction as before, that is, parallel to the vibrations of the side of the vessel, so that, if the floating lamina be made to revolve slowly in a horizontal plane, the direction of the creeping motion of the sand on its surface will continually vary with respect to the position of its edges, though constant with regard to the sides of the vessel.

c. Not only are the vibrations thus faithfully transferred through the water to bodies floating on its surface, but even to such as are totally immersed in it.

The experiment is easily made by suspending in such a vessel as above described under the water, and not in contact with the sides or bottom, a disc of glass, by means of fine silk threads, and strewing sand on the surface of the water which sinks and spreads evenly on the disc. This will be observed to be agitated with very decided normal, or tangential motions, according as the former or the latter of the modes of excitement used in the preceding experiments is employed; and to arrange itself in nodal figures accordingly.

Currents in a Fluid in Contact with a Vibrating Surface.




160. WHEN a surface is vibrating in the air, or any other fluid, the fluid immediately in contact with the surface is formed into currents, which proceed from the nodal points to the points of greatest excursion.

As any particular part of the surface moves upwards in the course of its vibration, it propels the fluid 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 fluid so projected, and the latter consequently tends to return into the partial vacuum thus formed. But as, of two neighbouring portions of fluid, that over the part of the surface nearest to the point of greatest excursion has had more projectile force communicated to it than the other, because the part of the surface urging it was moving with greater velocity, and through a greater space, so it is in a more unfavorable condition for its immediate return, and the other, i. e. the portion next to it towards the nodal line, presses into its place.

This effect is still further favored, because the portion of fluid, thus displaced, is urged from similar causes at the same moment into the place left vacant by the fluid still nearer the point of greatest excursion; so that each time the surface recedes from the fluid, an advance of the fluid immediately above it is made from the quiescent to the vibrating parts of the surface.

161. The existence of these currents may be rendered evident by strewing the vibrating surface with

Collection of Fine Powder at the Points of Greatest Excursion.

some substance sufficiently light to be carried forward by the fluid to the points of greatest excursion, in opposition to the tendency which there otherwise is to the nodal lines, as in art. 127.

a. M. Chladni first observed that shavings from the hair of the exciting violin bow did not proceed to the nodal lines, but were gathered together on those parts of the vibrating plate the most violently agitated. Thus, when a square plate of glass held horizontally was nipped above and below at the centre, and made to vibrate by the application of a violin bow to the middle of one edge, so as to produce the lowest possible sound, sand sprinkled on the plate assumed the form of a diagonal cross; but the light shavings were gathered together at those parts towards the middle of the four portions, where the vibrations were most powerful and the excursions of the plate the greatest.

Many other substances exhibited the same appearance. Lycopodium, which was used as a light powder by Oersted, produced the effect very well; and these motions must not be confounded with those described in art. 146, arising from tangential vibrations.

These effects were also observed by M. Savart, and particularly investigated by him in circular, rectangular, triangular, and other plates; and in rods, rings, and membranes. Figs. 121-136 exhibit some of the arrangements of the powder which he obtained.

b. M. Savart, however, attributed this disposition of the powder to a wrong cause, namely, to a secondary mode of vibration; and Mr. Faraday was the first to propose the true explanation, and establish it beyond a doubt by a series of most ingenious experi


c. The motions of the powder, as observed by Mr. Faraday, are as follows. Let the plate just mentioned, which may be three or four inches square, be nipped and held in a horizontal position by a pair of pincers of the proper form, and terminated, at the part touching the glass, by two pieces of cork; let lycopodium

Collection of Fine Powder at the Points of Greatest Excursion.

powder be sprinkled over the plate, and a violin bow be drawn downwards against the middle of one edge, so as to produce a clear full tone. Immediately the powder on those four parts of the plate towards the four edges will be agitated, whilst that toward the two diagonal cross lines will remain nearly or quite at rest.

On repeating the application of the bow several times, a little of the loose powder, especially that in small masses, will collect upon the diagonal lines, and thus show the position of the nodal lines.

Most of the powder which remains upon the plate will, however, be collected in four parcels; one placed near to each edge. of the plate, and evidently towards the place of greatest agitation. Whilst the plate is vibrating (and consequently sounding) strongly, these parcels will each form a rather diffuse cloud, moving rapidly within itself; but as the vibration diminishes, these clouds will first considerably contract in bulk, and then settle down into four groups, each consisting of one, two, or more hemispherical parcels, which are in an extraordinary condition; for the powder of each parcel continues to rise up at the centre, and flow down on every side to the bottom, where it enters the mass to ascend at the centre again, until the plate has nearly ceased to vibrate. The form of these heaps, and the involved motion they acquire, are, however, no part of the phenomenon under consideration at present, but will be hereafter explained.

If the plate be made to vibrate strongly, the heaps are immediately broken up, being thrown into the air, and form clouds, which settle down as before; but if the plate be made to vibrate in a smaller degree, by a more moderate application of the bow, the little hemispherical parcels are thrown into commotion without being sensibly separated from the plate, and often slowly travel towards the nodal lines.

When one or more of them have thus receded from the place over which the clouds are always formed, and a powerful application of the bow is made sufficient to raise the clouds, it will be seen that these heaps rapidly diminish, the particles of which they are composed being swept away from them, and passing back in a current over the glass to the cloud under formation, which ultimately settles as before into the same four groups of heaps.

Collection of Fine Powder at the Points of Greatest Excursion.

d. So powerful are the currents of air, that, when the vibrations are energetic, the plate may be inclined 50, 60, or 8° to the horizon, and yet the gathering clouds retain their places. As the vibrations diminish in force, the little heaps formed from the clouds will descend the inclined plane; but on strengthening the vibrations, the heaps will melt away, their particles will ascend the plane and pass again to the cloud. This will take place when the inclination of the plate is so great, that neither sand nor filings can rest on the nodal lines.

e. A piece of gold-leaf being laid upon the plate, so that it does not overlap the edge, as in fig. 137, the current of air towards the points of greatest excursion is beautifully shown; for, by its force, the air creeps in under the gold-leaf on all sides, and raises it up into the form of a blister; that part of the gold-leaf corresponding to the centre of locality of the cloud, when light powder was used, being frequently a sixteenth or twelfth of an inch from the glass.

Lycopodium, or other fine powder, sprinkled round the edge of the gold-leaf, is carried in by the entering air, and accumulated underneath.

f. When fine powder is placed on the edge of another glass plate, or upon a book, or block of wood, and the edge of the vibrating plate brought as nearly as possible to the edge of the former, (fig. 138,) part of the powder is always driven on to the vibrating plate, and collected in the usual place; showing that, in the midst of all the agitation of the air in the neighbourhood of the two edges, there is a current towards the point of greatest excursion even from bodies not themselves vibrating.

g. In the exhausted receiver of an air-pump the phenomena do not occur as in air; for, as the force of the currents is excessively weakened, the light powders assume the part of heavier grains in the air.

To observe this difference in the phenomena, Mr. Faraday made use of a circular stretched membrane, so excited that the powder collected in air at the centre, as in fig. 121. Upon exhausting the

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