OK THE PROPAGATION OF SOUND. 373 



the fluid rose in the tube when the pressure was removed, and subsided 

 when it was increased. A shght correction is, however, required on ac- 

 count of the expansion and contraction of the glass, which must have tended 

 to make the elasticity of the fluids appear somewhat greater than it really 

 was. 



It is also well known that solid bodies in general are good conductors of 

 sound: thus, any agitation communicated to one end of a beam is readily 

 conveyed to the ear applied to the other end of it. The motion of a troop 

 of cavalry is said to be perceived at a greater distance by listening with the 

 head in contact with the ground, than by attending to the sound conveyed 

 through the air; and we may frequently observe that some parts of the furniture 

 of a house are a little agitated by the approach of a wagon, before we hear the 

 noise which it immediately occasions. The velocity, with which impulses 

 are transmitted by solids, is in general considerably greater than that with 

 which they are conveyed by the air: Mr. Wunsch has ascertained this by 

 direct observations on a series of deal rods closely united together, which ap- 

 peared to transmit a sound instantaneously, while a sensible interval was re- 

 quired for its passing through the air: I have also found that the blow of a- 

 hammer on a wall, at the upper part of a high house, is heard as if double by a 

 person standing near it on the ground, the first sound descending through tlie 

 wall, the second through the air. It appears from experiments on the flexure 

 of solid bodies of all kinds, that their elasticity, compared with their density, 

 is much greater than that of the air: thus, the height of the modulus of elasti- 

 city of fir M'ood, is found, by means of such experiments, to be about 

 9 500 000 feet, whence the velocity of an impulse co-nveyed through it must be 

 17 4-00 feet, or more than three miles, in a secoml. It is obvious, therefore, 

 that in all common experiments such a transmission must appear perfectly 

 instantaneous. There are various methods of ascertaining tliis velocity from 

 the sounds produced under different circumstances by the substances to be 

 examined, and Professor C-hladni has in this manner compared the proper- 

 ties of a variety of natural and artificial productions. 



We have hitherto considered the propagation of sound in a single right 

 line, or in parallel lines only; but it usually happens, at least when a sound 

 is transmitted through a fluid, that th? impulse spreads ia every direction, so- 



