produced by the Vibration of Elastic Membranes. 297 



through the air to the paper membrane, and the sand on its 

 upper surface is thrown into figures which have sometimes the 

 most perfect regularity, and are often formed with such cele- 

 rity, that the eye has scarcely time to perceive the circum- 

 stances which accompany the formation of the figures. 



This experiment succeeds in general, whatever be the vi- 

 brating body which we employ, though thin plates of glass or 

 metal are the best ; and it is always preferable to make the 

 circular plate of glass vibrate in the mode in which there are 

 concentric lines of repose. It appears from the experiments 

 of Chladni, that, in order to obtain this kind of vibration, we 

 must render immoveable several points in the surface of 

 the plate, or at least two points of the circumference and 

 one point of the surface. It is in this way, therefore, 

 that M. Savart makes the experiment. He at first renders 

 immoveable two diametrically opposite points of the cir< 'm- 

 ference of the plate by seizing it between the middle finger 

 and the thumb. He then places lightly the tip of the index 

 finger at a point whose distance from the centre of the plate is 

 about the fifth part of its circumference. The plate, thus 

 held, is made to vibrate by drawing the bow of a fiddle across 

 its circumference. By employing successively circular plates 

 of different dimensions, and which, consequently, give differ- 

 ent sounds, it is easy to prove, that, for every number of vi- 

 brations, the membrane affects a particular mode of division. 

 When the vibrating plate is parallel to the membrane, the lat- 

 ter performs normal vibrations, or in a line perpendicular to 

 its surface. The sand sometimes springs to a great height ; 

 and, by making use of an apparatus which allows us to ob- 

 serve 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 considerable. These circular lines 

 are often cut by diametral lines, which form stars, whose 

 number of points increases with the acuteness of the sound. 

 Sometimes figures are obtained which are composed solely of 

 these diametral lines. Perfect regularity and symmetry, how- 

 ever, can only be obtained by taking the greatest care that 



