Beautiful acoustic figures may also be obtained on the surface of mercury, water, spirit, &c., contained in a glass goblet, by drawing a violin-bow against its edge. This part of the subject is fully and carefully investigated in Mr. Tomlinson's ' Student's Mammal of Natural Philo sophy,' (1838). M. Felix Savart has observed, that if a rectangular plate of glass about 27 inches long, I inch broad, and inch thick, be held horizontally between a finger and thumb at the middle of its opposite edges, and it be put in a state of longitudinal vibration, either by rub bing its under side, near one of the ends, with a wet cloth, or by striking it gently at one end, the dust on the sipper surface will arrange itself in lines perpendicular to the length of the rod. It is remarkable, that if the plate be turned with its upper face downwards, being held as before, and vibrations be produced in like manner, the places of the nodal lines will be opposite to the middles of the intervals between the lines observed in the other position of the plate—a circumstance which proves that the motions of the particles in one-half of the thickness of the lamina of glass are directly contrary to those in the other half. A like effect has been observed when cylindrical rods of glass have been made to vibrate longitudinally ; the lines of nodes then assuming a spiral form about the cylinder, and the curves consisting of portions which run alternately in contrary directions : the points of greatest and least inclination to the axis of the cylinder are at DV from each other with respect to a line drawn on the surface parallel to the axis, and the effect is such as would arise if the cylinder were divided longi tudinally into four quadrantal portions, having opposite qualities. The places of the nodes on that part of the rod which was uppermost were found, the rod being held in a horizontal position, by annular pieces of paper loosely encircling it in 1822, N. Savart read to the AcadmIrnie des Sciences at Paris, an account of some curious experiments in which acoustic figures were produced In consequence of vibrations communicated through the air to elastio membranes. e Brewater's Edinburgh Journal of Science,' vol. ii., p. 2964 A sheet of thin paper was slightly stretched over a gime vessel four or five inches in diameter, and on it was strewed light dust. A thin circular plate of glass, hi a state of vibration, was then brought within a few inches of the paper, when the vibrations were communicated to the latter, and, the paper being of uniform thickness and well stretched, the dust assumed figures which were perfectly regular. M. Savart, having rendered the glass plate immoveable at
opposite points on its circumference by holding it there between the thumb and a finger of each hand, placed the tip of another finger at a I point on the surface, at a distance from the centre equal to about one fifth of the diameter, and caused the plate to vibrato by drawing a violin-bow across its circumference. In these circumstances, on pre senting the plate to the stretched paper, the following appearances were observed. The nodal figures on square paper were analogous to those formed on a square plate of glass or metal, and on circular plates their general character was circular. The circular lines were some times cut by diametrical lines which formed nodal points, or stars, and the number of these increased with the acuteness of the sound produced.by the bow. When the plate of glass was parallel to the paper, the nodal lines were similar on both, as when two plates were connected with each other, by a rod fixed perpendicularly between them. When the plane of the glass plate was held vertically, the nodal lines on the paper became parallel to one another; and the figures on the paper changed as the glass plate was made to decline from the vertical position.
Some curious acoustic figures have been produced by M. Lissajoux by attaching a mirror to one of the prongs of a tuning fork, and allow ing a ray of light to fall somewhat obliquely on the mirror. It is evident that under such circumstances the direction of the reflected ray will change with every vibration of the fork, and on allowing this reflected ray to fall on the screen, the vibrations of the fork become visible in the rapid motions of a point of light on the screen. In this way M. Lissajoux has rendered the vibratory movement of solid bodies visible and has made evident the optical composition of vibra tory motions which take place in the same direction : he has also been able to compound two vibratory motions at right angles to each other; to show the accordance of two diapasons separated by a musical interval, and to give the means of fixing on a constant or invariable fundamental sound. The only fault of this highly suggestive method is that it does not leave a permanent trace of its indications.