RESPECTING SOUND AND LIGHT. 



nerve the sensation of a continued sound, On 

 forcing a current of smoke through the tube, 

 the vibratory motion of the stream, as it 

 passed out at the lateral orifice, was evident 

 to the eye ; although, from various circum- 

 stances, the quantity and direction of its mo- 

 tion could not be subjected to exact mensu- 

 ration. This species of sonorous cavity seems 

 susceptible of but few harmonic sounds. It 

 was observed, that a faint blast produced a 

 much greater frequency of vibrations than 

 that which was appropriate to the cavity : a 

 circumstance similar to this obtains also in 

 large organ pipes ; but several minute ob- 

 servations of this kind, although they might 

 assist in forming a theory of the origin of vi- 

 brations, or in confirming such a theory 

 drawn from other sources, yet, as they are 

 not alone sufficient to aflPord any general 

 conclusions, are omitted at present, for the 

 sake of brevity. 



IV. Of the Vtlocily of Sound. 



It has been demonstrated, by Isl. De la 

 Grange and others, that any impression 

 whatever, communicated to one particle of an 

 elastic fluid, will be transmitted through that 

 fluid with a uniform velocity, depending on 

 the constitution of the fluid, without refer- 

 ence to any supposed laws of the continua- 

 tion of that impression. Their theorem for 

 ascertaining this velocity is the same as New- 

 ton has deduced from the hypothesis of a 

 particular law of continuation : but it must 

 be confessed, that the result differs somewhat 

 too widely from experiment, to give us full 

 confidence in the perfection of the theory. 

 Corrected by the experiments of various ob- 

 servers, the velocity of any impression trans- 

 mitted by the common air, may, at an aver- 

 age, be reckoned 1 1 30 feet in a second. 



VOL. II. 



V. Of sonorous Cavities. 



M. De la Grange has also demonstrated, 

 that all impressions are reflected by an ob- 

 stacle terminating an elastic fluid, with the 

 same velocity with which they arrived at that 

 obstacle. When the walls of a passage, or 

 of an unfurnished room, are smooth, and per- 

 fectly parallel, any explosion, or a stamping 

 with the foot, communicates an impression to 

 the air, which is reflected from one wall to 

 the other, and from the second again to- 

 wards the ear, nearly in the same direction 

 with the primitive impulse : this takes place 

 as frequently in a second, as twice the 

 breadth of the passage is contained in 1130 

 feet ; and the ear receives a perception of a 

 musical sound, thus determined in its pitch 

 by the breadth of the passage. On making 

 the experiment, the result will be found ac- 

 curately to agree with this explanation. If 

 the sound is predetermined, and the fre- 

 quency of vibrations such, that each pulse, 

 when doubly reflected, m;iy coincide with the 

 subsequent pulse proceeding directly from 

 the sounding body, the intensity of the sound 

 will be much increased by the reflection ; 

 and also, in a less degree, if the reflected 

 pulse coincides with the next but one, the 

 next but two, or more, of the direct pulses. 

 The appropriate notes of a room may readily 

 be discovered by singing the scale in it ; and 

 they will be found to depend on the pro- 

 portion of its length or breadth to 1 130 feet. 

 The sound of the stopped diapason pipes of 

 an organ is produced in a manner somewhat 

 similar to the note from an explosion in a 

 passage; and that of its reed pipes to the 

 j.esonance of the voice in a room : the 

 length of the pipe in one case determining 

 he sound, in the other, increasing itg 

 strength. The frequency of the vibrations 

 3 z 



