SECTION V. 



ACOUSTICS, AND ITS APPLICATIONS, 



CHAPTER I. 

 THE PRODUCTION OF SOUND. 



ALTHOUGH we can scarcely consider the phenomena 

 which are usually ranged under the term "Acoustics," 

 as resulting from a purely undulatory force the 

 analogy which exists between the production of sound, 

 an.1 light and heat, is so great, as to permit of our in- 

 cluding its laws and applications under that head, if we 

 > only by way of analogy. 



We have previously observed, that the vibrations of 



losphere are really typical of those of the suppo- 



1 1 ether to whose undulations we assign the pro- 



i of light ; and our readers who have perused our 



us remarks, will therefore bo prepared to enter 



on a careful examination of tho various conditions to 



which sound ia subject. 



In the experiments and remarks we suggested in 

 connection with the subjects of Heat and Light, we 

 had occasion to discuss the effects of reflection, radia- 

 tion, and conduction on those agents ; and, to a large 

 extent, we shall find sound obedient to the same laws. 

 Whenever motion is conveyed to a body, it partakes 

 of the force imparted to it ; and the result is, that heat, 

 light, electricity, magnetism, and, almost universally, 

 sound, is individually produced. If we take, for in- 

 stance, tho common process of striking a lucifer-match, 

 we find sound first perceptible to the senses ; then heat 

 and light are observed ; and, lastly, electricity may be 

 discovered as having been set free, if the electroscope 

 be employed to detect its presence.* Our present sub- 

 ject will therefore form a kind of corollary on all that 

 have preceded it; and as such we shall treat it, in- 

 rlii'ling, however, many applications specially incident 

 thereto. 



The production of sound is so familiar to all our 

 readers, that wo need scarcely draw their attention to 

 it. We cannot, however, fail to notice, that there exists 

 a vast diversity in the sovmds wo hear. Thus, if a 

 piece of wood or stone be struck, we simply notice a 

 dull, heavy sound, entirely devoid of musical intona- 

 tion, providing the bodies be struck in large irregular 

 masses. If, however, they have certain definite shapes, 

 and their lengths and thicknesses increase and diminish 

 in definite ratios, then we hear graduated sounds. These 

 may be produced by percussion as in the case of the 

 pianoforte, the strings of which afford "music" on being 

 struck by the hammer or by simple vibration; aa we 

 observe in tho cornet, tho flute, and other wind in- 

 btiuments. 



Now, in any case where sound is produced, such re- 

 sults simply from the vibration of the particles of the 

 body which gives it ; and the common i tuning-fork, 

 perhaps, affords us the best illustration of this fact. If 

 the fork be struck against any hard body, then its prong 

 at once vibrates, and, in so doing, it causes the air next 

 to it to vibrate also. We hence have a successive series 

 of waves afforded, just as we observe the same effect 

 produced on casting a stone into still water. Now, just 

 as a succession of circles passes from the spot at which 

 the stone arrived on the surface of the water, so the 



Sec ante, p. 176 j Fig. 16. 



same result occurs when a bounding body is put 

 into motion. This is illustrated, so far Fig:, l. 

 as sound is concerned, in the annexed 

 engraving, which shows, by the dotted 

 lines, the vibrations which the metal of 

 the tuning-fork undergoes after percus- 

 sion. The body, however, must be elastic 

 to produce sound: thus a piece of clay 

 scarcely affords sonorous waves, because 

 it has but a very slight amount of elasti- 

 city. If, however, a glass vessel of any 

 kind, wluch is elastic, bo struck, it at once 

 produces and propagates in the air a series 

 of waves, or vibrations ; and these reaching the ear, so far 

 act on that organ as to make it sensible to the sounds 

 produced. 



That an elastic body really vibrates when it is put 

 into sufficient motion to produce waves of sound, is most 

 readily proved : and the following engraving shows a 

 Fig.S. 



very neat method of proving this fact. For this pur- 

 pose, place a thin glass tumbler on any firm support, 

 and hang a small wooden ball, by means of a thread, 

 from any support, or by the hand, so that the ball may 

 just touch the rim of the glass ; then draw a violin bow 

 against the rim of the glass. The vessel will thus be set 

 into vibration ; and the ball, resting against the rim, will 

 be repelled, and oscillate for some considerable time. 

 The mode of carrying out this interesting experiment is 

 illustrated in Fig. 2 ; in which the straight line repre- 

 sents the cord holding the bull as suspended from any 

 support, and the dotted line shows its vibration. The 

 success of the experiment will depend on the glass rim 

 being quite clean ; and the bow should be well rubbed 

 with resin, to afford as much friction as possible. Con- 

 fining our attention for tho present to tho vibration 

 of solid bodies, we may suggest a very instructive and 

 pleasing mode of illustrating the production of musical 

 sounds by means of glasses of various sizes. If eight 

 finger-glasses, or tumblers, are placed on a hollow square 

 box, and they be chosen so that they may have a relative 

 size (of which we shall speak more fully hereafter), a series 

 of musical sounds may be readily produced, which accord 

 with an octave, or eight successive notes of any musical 

 instrument. The sounds are given off on the edges 

 of the glasses being rubbed by the finger, which has 



