SCIENCE. 



PLATE 

 CCCCLXXXIII 

 Fig. 5. 



Fig. 6. 

 Fig. 7. 



Fig. 8. 

 Fig. 9. 

 Fig. 10. 



Fig. 11. 

 Fig. 18. 



Fig. 13. 

 Fig, 14. 



Fig. 15. 



Fig. 16, 



Fig, 17. 



ed by pressing it with the thumb against a fixed point, 

 such as a piece of cork, and two points at its circum- 

 ference are darnped by the application of two fingers. 



] n Fig. 3. the point e is damped by bringing the 

 elliptical plate against an upright fixed obstacle a b, 

 and the two points c, d are darnped by the two fingers. 



In place of using the fingers alone, particular places 

 may be damped by means of a wooden vice, Fig. 4. 

 furnished with small wooden screws, or it would be 

 easy to construct a more complicated one in which the 

 arms moved round joints, so that a number of points 

 could be easily pinched at the same time. 



1. If we take a square plate, Fig. 5. and pinch it in 

 the centre, while it is put into vibration by drawing 

 the bow near one of the angles, the sand will accumu- 

 late as in the figure, and the sound will be the gravest 

 of all. Sometimes the figure changes into four curves, 

 which join the middle of the sides of the plate. 



2. If the square plate pinched at the centre is made 

 to vibrate by drawing the bow along the middle of one 

 of its sides, the sand will accumulate as in Fig. 6. and 

 the sound will be less grave than in the preceding case. 



3. If the plate Fig. 7. is pinched at N, and the bow 

 applied at F in a line perpendicular to the side AB, 

 the sand will arrange itself in three parallel lines. 



4. But if it is pinched a little farther from the edge, 

 as at N, Fig. 8. the lines will change into curves as in 

 the figure. 



5. If the square plate is held at N, and the bar ap- 

 plied at F, Fig. 9. two parallel lines, and one perpen- 

 dicular line will be produced as in the figure. 



6. If a circular plate, Fig. 10. is pinched at its centre, 

 and also at another point of its circumference, and if 

 the bow is applied 45 from this last point, a cross will 

 be produced as in the square plate, Fig. 5. The tone is 

 here the gravest that the plate can produce. Fig. 10. 

 may be produced merely by pinching the plate at its 

 centre. 



7. Fig. 11. may be produced by the very same pro- 

 cess as the last, merely by drawing the bow more gently 

 and rapidly over the point of 45. Sound less grave. 



8. The effect in Fig. 12. where the rays form angles 

 of 40 is produced by the very same process as that of 

 Fig. 10, with this difference only, that the bow is drawn 

 at a point 30 from the point in the circumference 

 that is pinched. The figures which are formed of 2, 

 4, 6, 8, and 10 diameters, often assume deformed 

 shapes, in which two of the lines unite to form a curve 

 line, which does not pass through the centre of the 

 plate. Thus the figure with eight rays, Fig. 13. some- 

 times assumes the form shown in Fig. 14. in which 

 there is one nodal line left, the others being united 

 in pairs into three curves. 



When the centre of the circular plate is left free, a 

 new set of figures are formed, as shown from Fig. 15. 

 to 19- 



9. Of these, one that gives the gravest sound, and 

 the most simple form, is that of Fig. 15. formed of a 

 single circular line. In order to produce this, any 

 point through which the circular line is to pass is 

 pinched, and the bow is drawn on the opposite side of 

 the diameter passing through the point that is pinched ; 

 as this circular line has a determinate position, a few 

 trials are necessary, to find the proper distance from 

 the centre at which the plate is to be pinched. 



10. To produce Fig. 16. composed of a diameter and 

 a circular line, we must pinch the plate nearer its 

 edge, and apply the bow to a point 90 from that point. 

 The sound produced is more acute than the preceding. 



11. Fig. 17. is produced by pinching the plate as 



in the last case, and applying the bow at a point 45 Science, 

 from the point pinched. In proportion as the diame- Curiosities 

 tral lines increase in number, the circular line ap- "* 

 preaches the margin of the plate. 



12. Several circular nodal lines may be produced 

 as in Fig. 18. To do this, we have only to pinch the plate Fig. 19. 

 in two or more places through which the lines are to 

 pass ; the two points that are pinched being always in 

 the same radius, and their proper distance from the 

 centre being found by trial. 



13- The distortion of Fig. 18. shown in Fig. 19. is Fig. 19. 

 produced by pressing a point of the circumference 

 against a fixed obstacle, and applying the bow at a point 

 30 from the point of contact, the interior circle be- 

 ing changed into an ellipse, and the outer one bent 

 five times into itself. 



14. When a circular plate of metal is pinched as in 

 Fig. 3. against a fixed plate e, and pressed with the 

 fingers at c and d, the figure in Fig. 20. is produced. Fig. 50. 



15. In an elliptical plate, of which the ratio of the 

 diameters is as 4 to 3, and held as in Fig. 2. the sand 



is arranged as in Fig. 21. Fig- 21. 



16. With a triangular plate, Fig. 22. the form thus Fig. 22. 

 represented is produced by pinching the plate at n 



and applying the bow at r. 



17- in the rhomboidal plate, Fig 23. the form is Fig. 23. 

 produced also by pinching at n and applying the bow 

 at r. 



2. On the Acoustic Figures produced ly the vibration 

 communicated through the air to elastic membranes. 



The very curious experiments on this subject made 

 by M. Savart, have been recently (1822) read to the 

 Academy of Sciences in Paris. The following abridged 

 account of them is copied from Dr. Brewster's Edin- 

 burgh Journal of Science, vol. ii. p. 296. 



In order to perform the experiments described by M. 

 Savart, we must stretch a thin sheet of paper, about 

 four or five inches in diameter, over the mouth of a 

 vessel, such as a large glass with a foot-stalk, so that 

 the paper has an uniform degree of tension, and a ho- 

 rizontal position. A thin layer of fine and dry sand 

 or Lycopodium powder being then scattered over the 

 paper, a plate of glass, in a state of vibration, is brought 

 within a few inches of the membrane. The vibrations 

 of the glass plate are conveyed through the air to the 

 paper membrane, and the powder on its upper surface 

 is thrown into figures which have sometimes the most 

 perfect regularity, and are often formed with such ce- 

 lerity, that the eye has scarcely time to perceive the 

 circumstances which accompany their formation. 



This experiment succeeds in general, whatever be 

 the vibrating 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 im- 

 moveable two diametrically opposite points of the cir- 

 cumference of the plate, by seizing it between the mid- 

 dle 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 cir- 

 cumference. The plate thus held is made to vibrate, 

 by drawing the bow of a fiddle across its circumfe- 

 rence. By employing successively circular plates of 



On the ac- 

 coustic fi- 

 gures pro- 

 duced by 

 the vibra- 

 tion com- 

 municated 

 through 

 the air to 

 elastic 

 membrane: 



