ON THE SOURCES AND EFFECTS OF SOUND. 385 



vibrations of rods of different kinds as in those of chords ; it can only hap- 

 pen in particular cases that the rod will return after a complete vibration 

 to its original state, and these cases are probably such as seldom occur in 

 nature. 



The vibrations of plates diifer from those of rods in the same manner as 

 the vibrations of membranes differ from those of chords, the vibrations which 

 cause the plate to bend in different directions being combined with each other, 

 and sometimes occasioning singular modifications. These vibrations may be 

 traced through wonderful varieties by Professor Chladni's method of strewing 

 dry sand on the plates, which, when they are caused to vibrate by the operation 

 of a bow, is collected into such lines as indicate those parts, which remain 

 either perfectly or very nearly at rest during the vibrations. Dr. Hooke had 

 employed a similar method, for showing the nature of the vibrations of a bell, 

 and it has sometimes been usual, in military mining, to strew sand on a drum, 

 and to judge, by the form in which it arranges itself, of the quarter from 

 which the tremors produced by countermining proceed. (Plate XXV. Fig. 

 346 . . 348.) 



The vibrations of rings and of vessels are nearly connected with those of 

 plates, but they are modified in a manner which has not yet been sufficiently 

 investigated. A glass, or a bell, divides in general into four portions 

 vibrating separately, and sometimes into six or eight; they may readily be 

 distinguished by means of the agitations excited by them in a fluid contained in 

 the glass. It is almost unnecessary to observe, that the fluid thus applied, 

 by adding to the mass of matter to be moved, makes the vibration slower, 

 and the sound more grave. 



In some cases the vibrations of fluids and solids are jointly concerned in 

 the production of sound: thus, in most of the pipes of an organ denominated 

 reed pipes, the length of a tongue of metal is so adjusted, as to be capable of 

 vibrating in tiie same time with the air contained in the pipe. Sometimes, 

 however, the air only serves to excite the motion of the solid, as jn some 

 other organ pipes, which are usually much shorter than Avould be required 

 for producing the proper note alone, and. pvobably in the glottis, or organ 

 of the voice, of animals. On the other hand, the alternate opening and shut- 



VOL. 1. So 



