368 LECTURE XXXI. 



ing; and if they are equal among themselves in duration, they produce a 

 musical or equable sound, as that of a vibrating chord or string, or of the 

 voice in singing. Thus, a quill striking against a piece of wood causes a 

 noise, but, striking against the teeth of a wheel or of a comb, a continued 

 sound ; and if the teeth of the wlieel are at equal distances, and the ve- 

 locity of the motion is constant, a musical note. 



Sounds of all kinds are most usually conveyed through the medium of the 

 air; and the necessity of the presence of this or of some other material 

 substance for its transmission is easily shown by means of the air pump; for 

 the sound of a bell struck in an exhausted receiver is scarcely perceptible. 

 The experiment is most conveniently performed in a moveable receiver or 

 transferrer, which may be shaken at pleasure, the frame which suspends the bell 

 being supported by some very soft substance, such as cork or wool. As the air 

 is gradually admitted, the sound becomes stronger and stronger, although it 

 is still much weakened by the interposition of the glass : not that glass is in 

 itself a bad conductor of sound, but the change of the medium of communi- 

 cation from air to glass, and again from glass to air, occasions a great di- 

 minution of its intensity. It is perhaps on account of the apparent facility 

 with which sound is transmitted by air, that the doctrine of acustics has 

 been usually considered as immediately dependent on pneumatics, although 

 it belongs as much to the theory of the mechanics of solid bodies as to that 

 of hydrodynamics. 



A certain time is always required for the transmission of an impulse through 

 a material substance, even through such substances as appear to be the hardest 

 and the least compressible. It is demonstrable that all minute impulses are con- 

 veyed through any homogeneous elastic medium, whether solid or fluid, with a 

 uniform velocity, which is always equal to that which a heavy body would 

 acquire by falling througli half the height of the modulus of elasticity, that 

 is, in the case of the air, half the height of the atmosphere, supposed to be 

 of equal density; so that the velocity of sound passing through an atmo- 

 sphere of a uniform elastic fluid must be the same with that of a wave moving 

 on its surface. In order to form a distinct idea of the manner in which 

 sound is propagated through an elastic substance, we must first consider the 

 motion. of a single particle, which, in the case of a noise, is pushed for- 



