SCIENCE. 



Science, different dimensions, and which, consequently, give 

 ( imi(iri different sounds, it is easy to prove, t! very 



number of vibrations, the membrane affects a purlieu- 

 S ^V*' lar mode of division. When the vibrating plate is pa- 

 rallel to the membrane, the latter performs normal vibra- 

 tions, or vibrations in a line perpendicular to its surface. 

 The sand sometimes springs to a great height ; and, by 

 making u -e of an apparatus which allows us to observe 

 what passes at both surfaces of the membrane, it is easy 

 to see that the distribution of the nodal lines is the 

 same. The general character of these lines is to be 

 circular, and their number is sometimes very conside- 

 rable. These circular lines are often cut by diametral 

 lines, which form stars, whose number of points in- 

 creases with the acuttncss of the sound. Sometimes 

 figures are obtained which are composed solely of these 

 diametral lines. Perfect regularity and symmetry, 

 however, can only be obtained by taking the greatest 

 care that the membrane be equally thick and uniform- 

 ly 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 ho- 

 mogeneous that can be employed. 



Some of the finest figures that are obtained by the 

 effect of distant vibrations on the membrane are repre- 

 I.ATE sented in Plate CCCCLXXXIII. Fig. 24 36'. When 



i n i. xxx in. t ne membrane is ill stretched, it often happens that the 

 ig. 24 36. ii nes traced by the sand are very numerous, and that 

 they form kinds of chains, regularly arranged, and ap- 

 parently the result of concentric lines cut by a great 

 rig. 37. number of diametral lines. See Fig. 37 t 



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 vibra- 

 tions 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 

 membrane. 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 paralle. 

 lism. By gradually inclining the plate, the figures on 

 the membrane change. 



When figures composed of concentric circular lines 

 are obtained, 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 be- 

 longs to a kind of vibration higher 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 vibra- 

 tion, so that the membranes appear to produce with 

 facility several kinds of motion at once. 



The preceding experiments may be varied in a great 

 number of ways, by making use of membranes whose 

 dimensions, nature, tension, and contour, are different ; 

 but they all present analogous results. The figures 

 produced by a rectangular* membrane are shown in 

 Plate CCCCLXXXIII. Fig. 38 45, and those produ- 

 ced by a triangular one in Fig. 4.> 51. When the dia- 

 meter of the membranes is less than from half an inch 

 to an inch, it is not easy to observe regular nodal lines, 

 unless when the sound is extremely acute. 



n described vary 



In those made of 



The figures which have now been 



with the tension of the membrane, 

 paper, which changes its hygrometric state, and con- 

 M-qm-ntly its tension, continually, M. Savart observed 

 that the figures changed at every instant. When Utt 

 same figure is represented several times, it was neces- 

 sary only to breathe upon the paper to create a new 

 one, which in a short time disappeared, and reti 

 to its former state through a great number of interme- 

 diate figures. Hence M. Savart proposes this as a sure 

 method of detecting small hygrometrical variations in 

 the air. In order to protect the paper membrane* 

 from the humidity of the air, they should be covered 

 with a thin coat of varnish made of gum lac. 



The membranous vibrations and figures which have 

 now been described may also be produced by the 

 sound of the pipe of an organ, even at the distance of 

 some feet. If we play with a slow motion an air on 

 the flute, at. about half a foot from the membrane, the 

 sand will form lines, the figure of which varies unceas- 

 ingly with the sound produced. But, what appears 

 more astonishing, the voice produces an analog., 

 feet, which is extremely well marked, even under the 

 influence of a sound which is neither strong nor sus- 

 tained. By whatever method, in short, the air is agi- 

 tated, it is capable of communicating to thin mem- 

 branes the motion which it has received, and that 

 without any alteration. 



These experiments succeed also equally well when 

 the membranes are wetted, or when they have imbibed 

 an oily substance. In this last case, in place of sand, 

 we must cover the membrane with a thin stratum of 

 oil, which is agitated in ripples, that increase in num- 

 ber with the acuteness of the sound. 



M. Savart next applies these principles to a method 

 of appreciating very small quantities of sound. He 

 stretches a piece of thin vegetable paper or goldbeater's 

 skin across the mouth of a glass about four inches in 

 diameter. He then covers this with sand, and ascer- 

 tains the intensity of different sounds by the distance 

 at which they cease to agitate the membrane; and he 

 remarks that the membrane will often be moved by an 

 augmentation of sound which the ear itself is incapable 

 of appreciating. He proposes also to use it for ascer- 

 taining the augmentations of sound which arise from the 

 coincidence of vibrations produced by numbers of vi- 

 brations not very distant from each other. 



Bodies which are neither rigid in themselves, and 

 which are not rendered rigid by tension, such as the 

 skin, a silken fabric, paper, &c. are, even when the^r 

 are not stretched, susceptible of being thrown into vi- 

 brations by the influence of a body vibrating at a dis- 

 tance ; and it appears that, under some circumstances, 

 they are even more susceptible of this kind of action 

 than most elastic membranes. This may be proved by 

 covering a horizontal portion of any of these substances 

 with sand, and sounding the pipe of an organ at the 

 distance of a foot or so. The sand will be violently 

 agitated, and will form figures composed of numerous 

 curved and bending lines interlaced with one another. 

 In the second part of his able memoir, M. Savart 

 applies these experiments to the illustration of the uses 

 of the membrane of the tympanum, and of those of the 

 external ear, both of which, as he shows by Direct ex- 

 periments on the ears of animals, are susceptible of be- 

 ing thrown into a state of vibration, by bodies vibrating 

 at a distance. As our limits will not permit us to fol- 



Almost all the figures given by square membranes are analogous to the figure of a square plate, and are almost always of the kind 

 which Mr. Chladni calls dbtortiom. 



