ACOUSTICS. 



cfistance is seven times 1142 feet. In like 

 manner, if you observe the number of se- 

 conds between the lightning and the re- 

 port of the thunder, you know the distance 

 of the cloud from whence it proceeds. 

 But, according to another philosopher,Dr. 

 Thomas Young, the velocity of sound is 

 not quite so great. " It has been demon- 

 strated," he observes, " by M. De La 

 Grange and others, that any impression 

 whatever,communicatedto one particle of 

 an elastic fluid,will be transmitted through 

 that fluid with an uniform velocity, de- 

 pending on the constitution of the fluid, 

 without reference to any supposed laws 

 of the continuation of that impression. 

 Their theorem for ascertaining this velo- 

 city is the same as Newton has deduced 

 from the hypothesis of a particular law of 

 continuation : but it must be confessed, 

 that the result differs somewhat too wide- 

 ly from experiment, to give us full con- 

 fidence in the perfection of the theory. 

 Corrected by the experiments of various 

 observers, the velocity of any impression 

 transmitted by the common air may, at 

 an average, be reckoned 1130 feet in a 

 second." Phil. Trans, vol. xc. 



Dr. Derham has proved by experiment, 

 that all sounds whatever travel at the same 

 rate. The sound of a gun, and the strik- 

 ing of a hammer, are equally swift in their 

 motions ; the softest whisper flies as swift- 

 ly, as far as it goes, as the loudest thun- 

 der. To these we may add, that smooth 

 and clear sounds proceed from bodies 

 that are homogeneous, and of an uniform 

 figure ; and harsh or obtuse sounds, from 

 such as are of a mixed matter and irregu- 

 lar figure. The velocity of sounds is to 

 that of a brisk wind as fifty to one. The 

 strength of sounds is greastest in cold and 

 dense air, and least in that which is warm 

 and rarefied. Every point, against which 

 the pulses of sound strike, becomes a cen- 

 tre, from which a new series of pulses are 

 propagated in every direction. Sound 

 describes equal spaces in equal times. 



There is probably no substance which 

 is not in some measure a conductor of 

 sound ; but sound is much enfeebled by 

 passing from one medium to another. If 

 a man, stopping one of his ears with his 

 finger, stops the other also by pressing 1 it 

 against the end of a long stick, and a watch 

 be applied to the opposite end of the stick 

 or of a piece of timber, be it ever so long, 

 the beating of the watch will be distinctly 

 heard ; whereas, in the usual way, it can 

 scarcely be heard at the distance o 1 ' 15 or 

 18 feet. The same effect will take place 

 if he stops both his ears with his hands, 



and rests his teeth, his temple, or the car- 

 tilaginous part of one of his ears, against 

 the end of the slick. Instead of a watch, 

 a gentle scratch may be made at one end 

 of a pole or rod, and the person who keeps 

 the ear in close contact with the other 

 end of the pole will hear it very plainly. 

 Thus, persons who are dull of hearing 

 may, by applying their teeth to some part 

 of an harpsichord, or other sounding body, 

 hear the sound much better than other- 

 wise. 



If a person tie a poker or any other piece 

 of metal on to the middle of a strip of flan- 

 nel about a yard long, then press with his 

 thumbs or fingers the ends of the flannel 

 into his ears, while he swings the poker 

 against any obstacle, as an iron or steel 

 fender, he will hear a sound very like that 

 of a large church bell. 



Sound, like light, after it has been re- 

 flected from several places, may be col- 

 lected in one point, as into a focus ; and 

 it will be there more audible than in any 

 other part, even that at the place from 

 whence it proceeded. On this principle 

 it is that a whispering gallery is construct- 

 ed. The form of a whispering gallery 

 must be that of a concave hemisphere, a"s 

 ABC, plate Acoustics, fig. 2. ; and if a low 

 sound or whisper be uttered at A, the vi- 

 brations expanding themselves everyway 

 will impinge on the points D, D,D, &c. 

 and from thence be reflected to E, E, E, 

 and from thence to the points F and G, tilt 

 at last they all meet in C } where the sound 

 will be the most distinctly heard. The 

 augmentation of sound, by means of speak- 

 ing-trumpets, is usually illustrated in the 

 following manner: Let ABC, fig. 3. be the 

 tube, BD the axis, and B the mouth-piece 

 for conveying the voice to the tube. Then 

 it is evident, when a person speaks atB in 

 the trumpet, the whole force of his voice 

 is spent upon the air contained in the tube, 

 which will be agitated through its whole 

 length, and, by various reflections from the 

 side of the tube to the axis, the air along- 

 the middle part of the tube will be greatly 

 condensed, and its momentum propor- 

 tionably increased, so that when it comes 

 to agitate the air at the orifice of the tube 

 AC, its force will be as much greater than 

 what it would have been without the tube, 

 as the surface of a sphere, whose radius 

 is equal to the length of the tube, is great- 

 er than the surface of the segment of such 

 sphere, whose base is the orifice of the 

 tube. For a person speaking at B, with- 

 out the tube, will have the force of his 

 voice spent in exciting concentric super- 

 ficies of air all round the point B; and when 



