32 Dr. Charles J. B. Williams on the Production 



sion constitute a large class of common noises, including 

 grating, filing, planing, creaking of hinges, and all sounds of 

 friction. The whistling caused by drawing the finger-nail 

 quickly over a silk fabric is of the same kind, and owes its 

 more musical character to the regularity of the threads. The 

 highest audible sounds may be excited in this way. Dr. Wol- 

 laston noticed that the shrill notes of the bat and of some 

 Grylli are inaudible to many ears. It is well remarked by 

 Sir John Herschel, that one reason of this may be their very low 

 force; and that if by mechanism we could strike on an anvil 

 a hundred thousand blows in a second, there would be heard 

 a most deafening shriek of still higher pitch. M. Savart pro- 

 duced an audible sound, by a wheel which gave in a second 

 24,000 strokes, which he counts as 48,000 vibrations, which 

 are much higher than the limits of audibility assigned by Dr. 

 WoUaston*. I believe that a higher sound is also elicited in 

 the action of the wheel for the combustion of steel exhibited 

 at the Gallery of Practical Science in Adelaide Street. This 

 wheel is 11 inches in diameter, and revolves 8500 times in a 

 minute, and when the steel touches it, some sounds are heard 

 far too high to be named on any musical scale, yet super- 

 abundantly audible to all ears. Solid conductors would pro- 

 bably convey sounds too acute to be transmitted through the 

 air (8.). 



III. On some Modifications of Echoes, 



14. The prolonged note produced by a succession of echoes 

 from a series of palisades, and between two parallel walls, has 

 been noticed by Dr. Young, Sir John Herschel, and others; but 

 there are some points with respect to the latter instance that 

 merit further consideration. Between two parallel surfaces, 

 as two stone walls, or the ceiling and paved floor of a low 

 room, the echo will take the character of a tone, prolonged iu 

 proportion to the reflecting power of the surfaces, and high 

 in pitch in proportion to their nearness. The latter may be 

 calculated roughly by dividing the velocity of sound through 

 air by the distance of the reflecting surfaces. Each reflection 

 constitutes a sound ; and if these sounds reflected to the ear 

 exceed in rate the vibrations of the original sound, the pitch 

 of the echo will be raised in proportion (10.). But the pulses 

 which fall obliquely, having a longer course, would necessarily 

 be fewer, and reach the ear somewhat later; hence these 

 echoes terminate in a lower key. This law of the modifica- 

 tion of sound by repeated reflection explains a great many 



♦ Annalcs de Chimie, 1830. [Dr. Wollaston's paper on this subject will 

 be found in the Phil. Mag., vol. Ivii. p. 187 — Edit.] 



