292 LECTURE XXXI. 



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

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

 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 appeared to transmit a sound instantaneously, while 

 a sensible interval was required 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 the 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 elasticity of fir wood, is found, 

 by means of such experiments, to be about 9,500,000 feet, whence the 

 velocity of an impulse conveyed through it must be 17,400 feet, or more 

 than three miles, in a second. It is obvious, therefore, that in all common 

 experiments such a transmission must appear perfectly instantaneous. 

 There are various methods of ascertaining this velocity from the sounds 

 produced under different circumstances by the substances to be examined, 

 and Professor Chladnit has in this manner compared the properties 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 the impulse spreads in every direction, 

 so as to occupy at any one time nearly the whole of a spherical surface. 

 But it is impossible that the whole of this surface should be affected in a 

 similar manner by any sound, originating from a vibration confined to a 

 certain direction, since the particles behind the sounding body must be 

 moving towards the centre, whenever the particles before it are retreating 

 from the centre ; so that in one half of the surface the motions may be 

 called retrograde or negative, while in the other they are direct or positive, 

 consequently at the sides, where these portions join, the motions can be nei- 

 ther positive nor negative, and the particles must remain at rest ; the mo- 

 tions must also become gradually less and less sensible as they approach to 

 the limit between the two hemispheres. And this statement may be con- 

 firmed by an experiment on the vibration of a body of which the motion is 

 limited to a certain direction, the sound being scarcely audible when the ear 

 is in a direction precisely perpendicular to that of the vibration. 



The sound thus diverging must always be spread through a part of a 

 spherical surface, because its velocity must be equal in every direction, so 

 that the impulse will always move forwards in a straight line, passing 

 through the centre of the sphere, or the vibrating body. But when a hemi- 



* Berlin Memoirs, 1788, p. 87. 



f Traite d'Acoustique, Paris, 1809, p. 319. See Herschel's remark on these 

 results, Encyc. Met. art. Sound, p. 773. 



