ACOUSTICS. 



377 



sot in vibration, and HIM different Bound* aro prodaood, partly 

 by the M..M of those vocal a partly by the 



intliii-i!'-,- of tli" configuration of tho mouth. Tho tones of 

 lillW.-nt voices aro apparently produced by diffi-runcus in tho 

 htrurturo of those membranes or vocal cords. I3y arranging 

 un apparatus on a simitar principle, various sounds of the human 



have boon closely iinit 



Tho column of air cnnUim-d in a gloss tube may easily bo 

 thrown into vibration l.y igniting a small jot of hydrogen gas* 

 t ho tubo, as shown in Fig. 23. Tho jot should bo ignitod, 

 and sovem.1 tubes of rather largo diameter should bo taken and 

 lirM (.-,.! it, moving them up or down till a dear sound is 

 1. This sound is in reality caused by a series of ox- 

 ^ produced by tho combination of the hydrogen with tho 

 air. By employing a very largo motal tubo, and a suitable 

 burner, very powerful sounds may bo obtained, the pitch of 

 which varies with tho length of tho tube. By moving the tubo 

 up and down, it will bo found that there is a certain point in it 

 at which tho jot should be, in order to produce a sound. If 

 it bo a little above or below this, tho name will burn steadily 

 and quietly ; but by sounding tho note which tho tube utters, 

 or raising tho pitch of the syren till it comes into unison with 

 it, the vibration in tho air will cause the tube to burst into 

 sound. If wo havo a series of tubes capable of uttering 

 different notes with the flames thus burning steadily in them, 

 wo can at once make any one sing by 

 uttering its note, and it will usually 

 continue to sound when it has once 

 been started. Tho flame appears to be 

 sensitive to its own noto, and at onco 

 to commence to quiver when that is 

 uttered. 



Under some conditions ordinary gas 

 flames are found to bo more or less 

 sensitive to certain special sounds. 

 This may occasionally be seen with 

 an ordinary fish-tail or bat's-wing 

 burner. The burner must bo so ad- 

 justed that tho flame is just on the 

 point of roaring, and by trying a va- 

 riety of sounds, some will not improba- 

 bly be found which will cause the flame 

 to alter in shape, and send forth tongue- 

 like jets. Some burners respond to a 

 whistle, others to the rattle of keys, 

 while others again respond to certain 

 musical notes. 



These effects are best seen by using 

 a small circular orifice as a jet. Tho 



flame from this may, by properly regulating the pressure, 

 be made as much as eighteen or twenty inches high. This 

 may be done by filling a gas-bag with hydrogen, and regu- 

 lating the pressure by adjusting the weights on it. When 

 the flame is just commencing to roar, it will bo found sensitive 

 to almost any sound. Rattling a bunch of keys at the other 

 side of a room will cause it suddenly to shorten to about one- 

 third of its length, bulging out laterally at the same time. It 

 will frequently answer to every tick of a watch held near it, and 

 by means of two concave mirrors we havo made it answer in 

 this way to a watch held at a distance of several feet. 



Some burners will answer more readily to certain sounds. 

 One which we possess will always alter in shape at the sound of 

 an s, and, if a person is speaking near it, will indicate every 

 such sound by a bow. If a musical box be played close to it, 

 tho flame will assume different shapes at almost every note. 

 Experiments of this kind are very interesting and instructive, 

 and can easily be tried by the student. 



In musical sounds there are three distinct qualities which wo 

 lave to observe ; these are : 



1. The pilch of the note. 



2. Its intensity. 



3. Its quality ; or, as it is technically termed, its timbre. 

 The first of these has already been explained to depend upon 



the number of vibrations made by tho sounding body in any 

 given time. 



The intensity of the sound depends, not on tho number of 

 vibrations, but on their amplitude. Tho harder we strike a 

 sounding body, or tho more vigorously wo pluok string, the 



greater will bo the extent of the vibration* produced, and thsjp 

 fore the greater the intensity of the sound, 



The third quality of aooand namely, its timbre it very 

 difficult to explain ; indeed, it U M yet but imperfectly under* 

 stood. If wo strike any note as, for instance, middle o on a 

 piano, and then sound the same note on a Ante or an organ, or 

 utter it with the voice, we shall in an instant notice a great 

 difference between the sounds. They all utter the very Mine 

 note, producing the same number of vibration* per second j 

 perhaps all had exactly the same intensity, bnt yet there is > 

 difference, which at once renders itself manifest. This is known 

 as the timbre. Tho quality and shape of the sounding body, 

 and many other points, which ore only practically important to 

 musical instrument makers, influence this greatly. Tyndall, in 

 his admirable " Lectures on Sound," to which we are indebted 

 for several facts, and which we recommend all our readers to 

 study, employs tho word "Clang-tint" (the equivalent of the 

 Gorman Klangfarbe) to represent this quality. It seems mainly 

 to depend upon tho production of various other tones, in addition 

 to the fundamental one. These over- tones unite with and modify 

 the vibrations produced. 



If we take our monochord, and pluck the wire, first at the 

 central division, then at a distance of one-third from one end, 

 and again at a quarter, we shall obtain the same note each time, 

 but a different timbre will be recognised. When the cord is 

 struck at the centre, all over-tones which require that point as 

 a node vanish. The cord cannot, for instance, vibrate in two 

 segments, so as to produce the octave as well as the funda- 

 mental note, nor can it vibrato in any even number of parta, 

 since, in any of these cases, the centre point would be a uode. 

 This may bo shown by damping tho cord lightly at the centre, 

 which disturbs the fundamental note, bnt would not disturb the 

 tones if they existed. If we now strike the cord at any other 

 part, and damp tho centre in the same way, the fundamental 

 noto will cease, bnt the octave produced by the vibrations of 

 the two separate portions will be clearly perceived. Advantage 

 is taken of this fact by pionoforto-makers, who make the hammer 

 strike the wire at about one-seventh of its length from one end. 

 Experience taught that this produced the most pleasing sound, 

 before science discovered that, by striking at this point, those 

 over-tones which do not form chords with the fundamental note 

 were destroyed. 



Two musical notes are said to bo in unison when both produce 

 the same number of vibrations ; it is quite immaterial by what 

 instruments they aro produced. Sounds may be produced by 

 any number of vibrations ; it is found, however, that there is a 

 series of notes arranged at certain fixed intervals which produce 

 the most pleasing music. This series seems to depend on the 

 natural constitution of the human ear, and is known as tho 

 Musical Scale, or Gamut. 



It consists of a series of seven notes, designated in England 

 by the letters C, D, E, F, G, A, B, and on the Continent by the 

 names ut or do, re, mi, fa, sol, la, si. The same series is then 

 reproduced, each note being produced by double the number of 

 vibrations. The annexed table will show clearly the relative 

 number of vibrations and length of string required to produce 

 these notes. A second C is inserted to complete the octave : 



Tho first C given hero is that known as middle C, and the 

 number of vibrations for any higher or lower note can easily be 

 found by multiplying or dividing by 2, 4, 8, etc., according to 

 the number of octaves intervening. The length of the sound- 

 waves, corresponding to each note, may easily be found by 

 dividing 1,120 foot by tho number of vibrations. 



By comparing tho fractions above given we shall find that tho 

 intervals between the notes are nearly, bnt not quite, uniform- 

 There are three different intervals represented by the fractio-n 



