and Propagation of Sound, 29 



The mass is thus, by the superiority of the impulse over the com- 

 bined normal resistance oi'the molecules of its diameter (6.)j 

 brought under a new law, from which it derives its altered tones. 

 If the impulse be infinite, this law will find its limit in masses 

 of the thickness of a vibration or wave of sound in the matter, 

 which of course varies with its compressibility: in greater 

 lengths, the impulse being efficiently reacted on by the proper 

 elasticity of the material, and not continuing long enough to 

 increase the sphere of the vibration, passes on as a wave, alter- 

 nated with a counter-wave of reaction (3.). Such is the case 

 with the longitudinal vibrations of rods, as illustrated by 

 Chladni; and the law is fundamental to all the various simple 

 sounds of solid bars, balls, plates, bells, and even wires and 

 cords, the molecular elasticity being brought in these last into 

 uniform force by extraneous tension. It would be endless to 

 follow it through its extensive relations ; but it may be noticed 

 that one of the applications of the preceding view is to explain 

 how (by increasing its excursions (T.)?) thinning or beating out 

 a mass of metal augments its power of impressing the air to a 

 degree far greater than the reason usually assigned, increased 

 contact, could account for. 



8. Air is a bad conductor of the vibrations of solids, and 

 solids are much worse conductors of the vibrations of air than 

 air itself. The very different molecular elasticity (2. and 4.) of 

 these two classes of matter is the obvious reason of this. Thus, 

 a pulse of air coming against a hard surface, instead of over- 

 coming the inertia of its molecules, so as to extend the vibra- 

 tion through the solid mass (4.), is first condensed, and then 

 recoiling back by its own elasticity, constitutes an echo. The 

 vibrations of a dense solid, on the other hand, as they are 

 strong by the sturdy elasticity of the molecules (2. and 4.), so 

 for the same reason, unless the impulse be very powerful in pro- 

 portion to the mass, they are limited in their excursions. They, 

 therefore, produce but minute vibrations in air, which being 

 much less strongly elastic, requires longer pulses for a similar 

 effect ; and many direct vibrations are thus lost hy the noise- 

 less yielding (3.) of the air; oblique ones, by irregular refrac- 

 tion in passing into a medium of different density ; and the 

 few that are transferred are too weak to extend far. 



9. The best mode of overcoming the difficulty of the trans- 

 fer of vibrations from one medium to another is an interesting 

 point, as it includes the principle of sounding-boards of musi- 

 cal instruments. We have already noticed that thinning a 

 sonorous solid increases the sphere of its vibrations (?.)> and 

 therefore their power of affecting the air (8.) ; and provided that 

 attention be paid to the direction of the vibrations, a similar 



