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0
ACOUSTICS.
l'eet per Sicond. i
Koval academy at Part.?.1' 72 i
Sir Isaac Newton, Flamstead, Halley, ami Durham ..... 11-12As no man ever had a better opportuniiy, so none could improveit with greater diligence, assiduity, and accuracy, in determiningand settling the various phenomena of sounds, Ilian the celebratedphilosopher last mentioned. He proved by experiments madewith the strokes of a hammer, and the explosion of a gun at thesame time, at the distance of a mile, that the velocity of soundsproduced from- different bodies was the same, or came to his earin the same time. That the motion of sound was equable anduniform, or that it passed through spaces proportional to the times,lie found by various experiments made by the explosion of guns,at different distances, as appears by the following table which hehas given us, where the first column shews the places at which theguns were fired; the second, the number of vibrations of an half-second pendulum; the third, the distance of places in miles anddecimal parts, as measured by trigonometry; the fourth, the dis-tances measured by the velocitv of sound", admitting it to be atflie rate of one. mile every 91 half-seconds.
At Hornchurch, .
.9.
..0,9875...
North Okenden Church,...
.
..2,004....
.2,000
Upminster-mill,.
»
o X
j 2,4
23 C '
" <2,43
Little Warlev Church,.
... o-i .
..3.0.
.2,97
Kainham Church,.
....33*.
,..3,5S
.3,59
Alvel-mill.
.3,57
Dagenham-mill,.
.,..35 ..
, ..3,85.
. 3,78
Fouthweal Church, .
....45 .
...4,59 .
. 4,S(i
East Thornden Church,....
...,4fj i....
...5,09 .
. 5,03
Barking church,.
....70* .
.
. 7,62
Guns at Blackheath .
..116 .
:.12,5 .
.... 12,55
The great exactness of measuring distances by sounds appears fromthe above table, as well as the equability of the motion; hut torender this matter still more certain and indisputable, the Dr.took a journey to Foiihie*s-sands, on the coast of lvssex, whichform a smooth" large plain for some miles. On this plain he measuredsix miles in a right line, and, causing a gun to lie tired at the end ofeach mile, lie found that his former observations were very justand true, and that sound passed the first mile in 9* half-seconds,two miles in ISt, three miles in 27$, and so on to the end of thesix. The Academia del Cimento made experiments of this sort,from which they concluded, that the velocity of sounds was sofar equable, as not to lie accelerated or retarded by conspiring or, adverse winds; lnitin this they led themselves and many others intoa verv great mistake, which was owing to their tiring off guns at toonear "a distance; for in great distances the deference is sensible.Mr. Young of Trinitv college, Dublin, lias treated this difficultsullied in a satisfactory manner, and given mathematical demon-strations to the mosL useful and important propositions: to Ills workwe refer the reader. The above calculations and experiments fur-• nidi us with a method of finding, in mans cases, the distances ofplaces liv sound, when we can see the stroke causing the sound,-or a Hash of light attending it, we have only to count by a natch,or a pendulum, the seconds between this moment and that of hear-ing the report; then (lie number of seconds multiplied bv 114'-’•rjves the distance in feet; or if the seconds he multiplied by 3and the product divided by 14 we shall have tiie distance in miles;and thus may the distance of clouds he found, or of gun» whichare tired, See. The time may also he observed by the pulse,counting oil an average 70 pulsations to a minute, or 3* to a mile.
Sound, like li fid, after it lias been reflected from several places,may he collected into one point as into a focus; and it will hethere more audible than in any other part, even than at the placefrom whence it proceeded. On this principle the whispering gal-lery is constructed. The torm of this gallery must be llull of aconcave hemisphere, as A15 C, Plate 1. Fig. 5, and if a lowsound or whisper lie uttered at A, the vibrations expanding them-selves every way will impinge on the points D D D, Ac. and fromthence lie reflected to F, K K, and from thence to the points F and(i, till at last they all meet in C, w here the sound will be the mostoi-!;netlv heard.
j'i'k Augmentation of sound bv Speaking Trumpets, isillust/.J ed in the following manner; See Plate 1. Fig , (3. Let AJ3C
he tlie tube, 15 D the axis, anti 13 the mouth-piece for conveyingti e vob e to the lube, it is evident, when a person speaks at isin the tiump.t, the whole force of his voice is spent upon (lie aircontained in the tube, which will he agitated through the wholelength of the tube; anti, by various reflections from the side of itto the axis, the air along the middle will tie greatly condensed, andits momentum proportionality increased, so that when it comes toagitate the air at the orifice of the tube A C, its force will lie asmuch greater, than it would have been without the tube, as thesurface of a sphere, whose radius is equal to the length of thetube, is greater than the surface of the segment of such a sphere,whose base is the orificemf the lube. For a person speaking at 13,without the tube, will have the force of his voice spent in excitingconcentric pulses of air all around the point 13; and when the pul-ses are diffused as far as D every way, it is plain the force of thevoice will there be diffused through the whole superficies of asphere, whose radius is 13 D; hut indite trumpet it will lie so con-fined, that, at its exit, it will lie diffused through so much of thatspherical surface of air, as corresponds to the orifice of 'the tube.But since the force is given, its intensity w ill be always inverselyas the number of particles it has to move; and therefore in thetube it will he to that without, as the superficies of such a sphereto the area of the large end of the tube nearly. But it is obvious,Mr. Young observes, that the confinement of the voice can havelittle effect in increasing the strength of the sound, as this strengthdepends on the velocity with which the particles move. Werethis reasoning conclusive, the voice should issue through tiie smal-lest possible orifice; cylindrical tubes would he preferable to anythat increased in diameter; and the less the diameter, tiie areaterwould be the elfeel of tiie instrument; because tiie mass of air tobe moved w ould, in that ease, be less, and consequently the ef-fect of the voice greater; all which is contradicted by experience.
The cause of the increase of sound in these tubes must there-fore lie derived from some other principles : and amongst these weshall probably find, that what the ingenious Kircher lias suggested, ■in his Phonurgia, is the most deserving of our attention, lietells us, - that “ the augmentation of the sound depends on its re-flection from the tremulous sides of tiie tube; which reflections,conspiring in propagating tiie pulses in the same direction, mustincrease its intensity.” Newton also seems to have considered thisas the principal cause, in the scholium of prop. 50. 13. 2. Princip.when lie says, “ we lienee see why sounds are so much increasedin stentorophonic lubes; for every reciprocal motion is, in eachreturn, increased by the generating cause.” When we speak inthe open air, the died on the tympanum of a distant auditor isproduced merely hv a single pul.-e. But when we use a lube, allthe pubes propagated from the mouth, except those in the direc-tion of the axis, strike against the sides of the tube, and everypoint of impulse becoming a new centre, from whence the pulsesare propagated in all direi lions, a pulse will arrive at the ear fromeach of those points; thus, by the use of a tube, a greater num-ber of pulses are propagated to (lie ear and consequently the soundincreased. The confinement too of the voice may' have some ef-fect/ though not such a- is ascribed to it by some; for the con-densed pulses produced by the nuked voice, freely expand every-way; hut in tubes lhelater.fi exixinsion being diminished, the di-rect expansion will lie increased, and consequently the velocity ofthe particles, and tin- intensity of the sound. The substance alsoof the tube lias its effect; for it is found by experiment, that themore elastic tlie substance of the tube, and consequently the moresusceptible it is of these tremulous motions, tiie stronger is thesound, if the tube lie laid on anv non-elastic substance, it dea-dens the sound, because it prevents tiie vibratory motion of theparts. Tiie sound is increased in speaking trumpets, if the tubehe suspended in the air; because (lie agitations are then carried onwithout interruption. Tlmse tubes should increase in diameterfrom tiie mouth-piece, because the parts, 'vibrating in directionsperpendicular to the surface, will conjoin in impelling forw ard theparticles of air, and consequently, by increasing their velocity,will inefease ttie intensity of the sound: and the surface also in-creasing, the number of points of impulse and of new propaga-tions will increase proportionally'. The several causes, therefore,of the increase of sound,in these fillies, Mr. Young concludes tohe, 1. Tiie diminution of the lateral, and consequently the in-crease of the direct expansion and velocity of the included air.2. The increase of tiie number of..pulses, by increasing the points1 of