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This he fupports from the Inftance of a Chord, which being ftruck, and the Sound, and fenfible Undulations at Reft again, if you approach the Chord foftly with the Finger, you'll find a fmall tremulous Motion, which is the Remains of the Vibrations of the whole Chord, and the Parts. Now, the Parts vibrate without any Seumi but no fboner is the Vibration felt by the Finger, than the Sound is heard again ; which he aicribes to rhis, That the Motion of the Parts being infufficient to move the Particles, whole Motion is the firft that ceafes, requires fome Afftftance from darning againft the Finger, whereby to become enabled to give the Particles the Motion necefTary for the producing of Sound* He finifhes his Proof, by the Inftance of Flutes ; which when made of different Matters, as Wood, Metal, £5?c. whofe Parts are very different, but their Particles nearly the fame, if their Lengths and Bores be the fame ; there is very little fenfible Difference in their Sounds.
The fonorous Body having made its Impreffion on the contiguous Air, that Impreffion is propagated, from one Particle to another, according to the Laws of Pneumaticks.
A few Particles, for inftance, driven from the Surface of the Body, drive the neighbouring Particles into a lefs Space ; and the Medium, as it is thus rarified in one Place, becomes condenfed in the other: But the Air thus comprels'd in the fecond Place, is, by its Elafticiry, returned back again, both to its former Place, and its former State ; and the Air, con- tiguous to that, is comprefs'd : and the like obtains, when the ATr lefs comprefs'd expanding itlelf, a new Compreffion is generated. From each Agitation of the Air, therefore, there arifes a Motion of the Air, analogous to the Motion of a Wave on the Surface of Water ; which we call a Wave or Undulation of Air. See Undulation.
In each Wave, the Particles go and return back again, through very fhort, but equal Spaces ; the Motion of each Particle is analogous to the Motion of a vibrating Pendulum, while it performs two Ofcillations ; and molt of the Laws of the Pendulum, with very little Alteration, are applicable there- to. See Pendulum.
Sounds are as various, as are the Means that concur to their Production.
The principal Varieties are reducible to the Figure, Con- ftitution, Quantity, &c. of the lonorous Body, the Manner of PercufTion, with the Velocity, &c. of the Vibrations con- fequent thereon 5 the State and Conftitution of the Medium ; the Difpofition, Diftance, &c. of the Organ; the Obftacles between the Organ and the fonorous Objecf , and the adjacent Bodies : The mofb notable Diminutions of Sounds, arifing from the various Degrees and Combinations of the Condi- tions mentioned, are into loud and Iciv (or ftrong or weak) intograve and acute (or fharp and flar, or high and low) and into long and port. The Management whereof, makes the Office of Mufic. See Sound in Mufic.
The Velocity of Soundis the fame,,with that of the Waves ; which does not differ much, whether it go with the Wind or againft it. By the Wind, indeed, a certain Quantity of Air is carried from one Place to another ; and the Sound is accele- rated, while its Waves move through that Part of the Air, if their Direction be the fame as that of the Wind. But as Sound moves vaftly fwifter than Wind, the Acceleration it will hereby receive, is inconfiderable. In effect, the mod: violent Winds we know of, have their Celerity to that of Sound, only as 1 to 33 ; and all the Effect we can perceive from the Wind, is, that it increafes and diminifhes the Space of the Waves ; fb that the Sound may be heard to a greater Diftar.ce than otherwife it would.
That the Air is the ordinary Medium of Sound, appears from various Experiments, in rarified and condenfed Air.
In an exhaufted Receiver, a fmall Bell may be heard fbme Diftance ; but when exhaufted, it can fcarce be heard at thefmalleft Diftance. If the Air be condenfed, the Sound will be louder, proportionably to the Condenfation, or Quan- tity of Air croudedin: Of which we have many Inftances in Mr. Ffauksbee's Experiments ; and this does not only fuc- ceed in forced Rarefactions, {$c. but in fiich alio, as are Natural ; as is evident from Fredlicius y s Story of his Journey- to the Top of Mount Carpathus in Hungary.
But 'tis not the Air alone, that is capable of the ImprefTions of Sound ; but Water alfo : as is manifeft, by ft riking a Bell under Water, the Sound of which may plainly enough be heard, only not fo loud, and alfb a Fourth deeper, by the Ear of fome good Judges in nmfical Notes. Indeed Merfenne fays, a Sound made under Water, is of the fame Tone or Note, as if made in Air, and heard under Water.
The Velocity of Sound is varioufly reported by various Authors.
Sir Ifaac Ne-zvton makes its Progrefs, in a Second of Time, to be 968 Feet ; the Honourable Mr. Francis Roberts 1300 Feet; Mr. Boyle 1200 Feet ; Dr. Walker 1338 Feet ; Mer- fenne 1474 Feet ; Mr. Flamjlead and Dr. Balky j 142 Feet ; the Florentine Academy 1 148 Feet; the French Obfervers, Hi(l. Acad. Reg. 1172- The Reafon of which Variety, Mr. Deibam aicribes partly to thofe Gentlemen ufing Strings
and Plummets inftead of regular Pendulums ; partly to ther not being Diftance enough between the fonorous Body and the Place of Oblervation ; and partly, to there being n Regard had to the Winds. s °
Some of the moil considerable Queries, relating to t ue Laws of Sounds, the fame Author propofes ; and anf" Wers leveral of them accurately, from Experiments made fo r tnat Purpofe by himfelf, as follows ;
How far a Sound moves in a Second of Time? &mhl moves 1 142 Seconds in a Second, which is juft an FvglUh Mile in 9 ^ or 9.25 half Seconds ; two Miles in 18 \ ; three Miles in 27 \, £Sc.
Does the Report of a Gun, difcharged with its Mouth to- wards us, comelboner than when the Muzzle is from theOb- ferver? By repeated Experiments, it appears, there is no Difference in the Sound y from this different Direction.
Do Sounds move in the fame Time, the fame Spaces, in all States of the Atmofphere, and Heights of the Barometer by Day and by Night, in Summer and in Winter, in fnowy and in clear Weather, in this or that Climate ? By repeated Experiments, it does not appear there arifes any Difference from any of thefe different Circumftances.
Do the Winds affect the Motion of Sounds ? By repeated Experiments, it appears, there is fome, though a very fmall Difference in the Velocity of Sounds, with or againft the Wind; which is alfo augmented, or diminifhed, by the Strength or Weaknefs of the Wind.
Do a great and intenfe Sound, and a fmall or languid one, move with the fame Velocity ? It appears that they do.
Does the Sound of a Gun move equally fwift at all Eleva- tions of the Gun ? They do.
Do diffcrent.Quantuies or Strengths of Gun-powder, oc- cafion any Difference, as to the Velocity of the Sound*. None.
Does Sound move in a right Line, the neareft Way ; or does it fweep along the Earth's Surface ? And is there any Difference in the Time, if the Piece be difcharged in an ac- clive and a declive Pofition ? Sound moves the neareft Way ; and the Velocity appears to be the fame in Acclivities and Declivities.
Have all kinds of Sounds, as thofe of Guns, Bells, l§c. the fame Velocity ? And are Sounds equally fwift in the Beginning of their Motion, and in the End ? There appears no Inequality in either of thefe refpects.
For the Refletlion o/Sound, } g For the Refratlion of Sound, 5
c Eccho.
I Refraction.
Sound, in Mufic, the Quality and Diftinction of the fe- veral Agitations of the Air, confider'd as their Difpofition, Meafure, f$c. may make Mufic. See Music.
Sound is the Object of Mufic, which is nothing but the Art of applying Sounds, under fuch Circumftances of Tone and Tune, as to raife agreeable Senfations.
The principal Affection of Sound, whereby it becomes fitted to have rhis End; is that, whereby it is diftinguiilied into Acute and Grave. See Gravity, &c.
This Difference depends on the Nature of the fonorous Body, the particular Figure and Quantity thereof; and even, in fome Caies, on the Part of the Body where it is ftruck ; and is that which conflitutes what we call different Tones. See Tone.
The Caufe of this Difference appears to be no other than the different Velocities of the Vibrations of the founding Body. In effect, the Tone of a Sound, is found, by abundance of Experiments, to depend on the Nature of thofe Vibrations, whofeDifferences we can conceive no otherwife, than as having different Velocities : And fince 'tis proved, That the fmall ',
Vibrations of the fame Chord, are all performed in equal Time ; and that the Tone of a Sound, which continues for fome Time after the Stroak, is the fame from firft to laft : It follows, that the Tone is neceffarily connected with a cer- tain Quantity of Time in making each Vibration, or each Wave ; or that a certain Number of Vibrations or Waves, accomplifhed in a given Time, conftitute a certain and de- terminate Tone.
From this Principle, are all the Phenomena of Tune de- duced. See Tune.
From the fame Principle, arife what we call Concords, iyC- which are nothing but the Refults of frequent Unions and Co- incidences of the Vibrations of two (bnorous Bodies, and con- fequently of the Waves and undulating Motions of the Air, occafioned thereby. See Concord. r
On the contrary, tlie Refiilt of lefs frequent Coincidences or thofe Vibrations, is what we call a ^Tijcord. See Discord-
Another confiderable Diftinction of Sounds, with regard to Mufic, is that, whereby they are denominated long and fto r fi not with regard to \he fonorous Body's retaining a ^ otl ] l f n
once received, a longer or a lefs Time, though grad. ua \ growing weaker ; but to the Continuation of the Imp 1
ulfe ot the efficient Caufe of the fonorous Body, for_ a/(W|«" or *
mads
poner Time, as in the Notes of a Violin, &* whic^a£
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