VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 60Q 



3. Ocular Evidence of the Nature of Sound. — A tube about the 10th of an 

 inch in diameter, with a lateral orifice half an inch from its end, filed rather deeper 

 than the axis of the tube, fig. 27, was inserted at the apex of a conical cavity 

 containing about 20 cubic inches of air, and luted perfectly tight; by blowing 

 through the tube, a sound nearly in unison with the tenor c was produced. By 

 gradually increasing the capacity of the cavity as far as several gallons, with the 

 same mouth-piece, the sound, though faint, became more and more grave, till it 

 was no longer a musical note. Even before this period a kind of trembling was 

 distinguishable; and this, as the cavity was still further increased, was changed 

 into a succession of distinct puffs, like the sound produced by an explosion of air 

 from the lips; as slow, in some instances, as 4 or 3 in a second. These were un- 

 doubtedly the single vibrations, which, when repeated with sufficient frequency, 

 impress on the auditory 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; though, from various 

 circumstances, the quantity and direction of its motion could not be subjected to 

 exact mensuration. 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 circum- 

 stance similar to this obtains also in large organ pipes; but several minute observa- 

 tions of this kind, though they might assist in forming a theory of the origin of 

 vibrations, or in confirming such a theory drawn from other sources, yet, as they 

 are not alone sufficient to afford any general conclusions, are omitted at present, 

 for the sake of brevity. 



4. On the Velocity of Sound. — It has been demonstrated, by M. 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 reference to any supposed laws of the conti- 

 nuation of that impression. Their theorem for ascertaining this velocity is the 

 same as Newton has deduced from the hypothesis of a particular law of continua- 

 tion : 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 observers, the velocity of any impression trans- 

 mitted by the common air, may, at an average, be reckoned 1 130 feet in a 

 second. 



5. Of Sonorous Cavities. — M. De la Grange has also demonstrated, that all im- 

 pressions are reflected by an obstacle 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 perfectly 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 2d again towards the ear, nearly in the 

 same direction with the primitive impulse: this takes place as frequently in a 



vol. xviii. 4 I 



