298 M. Savart on the Acoustic Figures 



the membrane be equally thick and uniformly stretched. The 

 first of these conditions may be easily fulfilled by using the 

 finest paper, particularly what is called vegetable paper, which 

 is the most homogeneous that can be employed. 



Some of the finest figures that are obtained by the effect of 

 distant vibrations on the membrane are represented in Plate 

 VI. Fiff. 1 — 13. When the membrane is ill stretched, it of- 

 ten happens that the lines traced by the sand are very nu- 

 merous, and that they form kinds of chains, regularly ar- 

 ranged, and apparently the result of concentric lines cut by a 

 great number of diametral lines. See Fig. 14. 



From these experiments it follows, that, when the plate and 

 the membrane are parallel, the motion is communicated by the 

 air exactly as it would have been if the two bodies had been 

 separated by a common rod perpendicular to their faces ; for 

 the number of vibrations is the same in both cases; since, 

 for each sound produced, the membrane affects a particular 

 mode of division, and the direction of its motion is also the 

 same, since it is perpendicular in the plate and in the mem- 

 brane. If the vibrating circular plate is held with one of its 

 diameters in a vertical line, the grains of sand have then a 

 tangential motion, and the system of lines in repose have in 

 general the character of parallelism. By gradually inclining 

 the plate, the figures on the membrane change. 



When figures composed of concentric circular lines are ob- 

 tained, there is often formed between two of these a circular 

 line, composed of the finer particles of the sand. M. Savart 

 is of opinion that this line belongs to a kind of vibration high- 

 er than that which is produced, but which co-exists along with 

 the principal vibration. It sometimes happens, also, that the 

 centre of the membrane presents an immoveable point, which 

 probably belongs likewise to a higher mode of vibration, so 

 that the membranes appear to produce with facility several 

 kinds of motion at once. 



The preceding experiments may be varied in a great num- 

 ber of ways, by making use of membranes whose dimensions, 

 nature, tension, and contour, are different ; but they all pre- 

 sent analogous results. The figures produced by a rectangu- 



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