produced by the Vibration of Elastic Membranes. 299 



lar * membrane are shewn in Plate VI. Fig. 15 — 21, and those 

 produced by a triangular one in Fig. 22- — 29. 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. 



The figures which have now been described vary with the 

 tension of the membrane. In those made of paper, which 

 changes its hygrometric state, and consequently its tension, 

 continually, M. Savart observed that the figures changed at 

 every instant. When the same figure is represented several 

 times, it was necessary only to breathe upon the paper to 

 create a new one, which in a short time disappeared, and re- 

 turned to its former state through a great number of interme- 

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

 thod of detecting small hygrometrical variations in the- air. 

 In order to protect the paper membranes 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 unceasingly with the sound produced. But, 

 what appears more astonishing, the voice produces an ana- 

 logous effect, which is extremely well marked, even under 

 the influence of a sound which is neither strong nor sustained. 

 By whatever method, in short, the air is agitated, it is capa- 

 ble of communicating to thin membranes the motion which it 

 has received, and that without any alteration. 



These experiments succeed also equally well when the mem- 

 branes are wetted, or when they have imbibed an oily sub- 

 stance. . In this last case, in place of sand, we must cover the 

 membrane with a thin stratum of oil, which is agitated in rip- 

 ples, which increase in number with the acuteness of the 

 sound. 



• Almost all the figures given by square membranes are analogous to 

 tbe figure of a square plate, and are almost always of the kind which Mr 

 Chladni calls distortions. 



