

H any <j\ven time vane* at the tquare root of the tenrion. It 

 is by varying tbo tension of the wires that a piano U tuned ; 

 IDS UMually yield a littln l>y the constant blows of the 

 hiimini I'M, and tl'UH tho note* beoome somewhat flat, and have 

 to be tuned up t<> tlicir former standard. 



Tho diameter of the oord likewise affects its rate of vibration. 

 If wo take two cords of the same substance, similar in tension 

 and length, but one of which has twice tho diameter of tho 

 other, we shall find that the note produced by tho thicker one 

 is an octave below tho other. The simplest way of showing this 

 is to take four thicknesses of oord twisted together for tho one 

 string, and a niiitrlo length of tho same cord for tho other. It 

 is on. account of this fact that tho wires of a piano are mnch 

 thicker in the baas than in the treble, and those for the lowest 

 notes of all aro frequently wound round with thin wire. Tho 

 density of the 



oord also affects m 



more or less tho 

 tone produced 

 by it. 



Bods of wood 

 or metal vibrate 

 in a very simi- 

 lar manner to 

 strings, and 

 have occasion- 

 ally been used 

 in the construc- 

 tion of musical 

 instruments. 

 Fig. 17 repre- 

 sents one of 

 these, in which 

 the sound is pro- 

 duced by tho vi- 

 brations of some 

 thin deal rods, 

 fixed in a firm 

 pedestal. This 

 instrument is 

 known as Mar- 

 loye's Harp, and 

 the different 

 notes in it aro 

 produced by 

 different lengths 

 in the rods ; the 

 instrument is 

 played by rub- 

 bing them longi- 

 tudinally, the 

 fingers being 

 powdered over 

 with resin. The 

 vibrations here 

 are longitudinal, 

 instead of trans- 

 verse, as in the 

 case of the wires 

 we have been 

 considering. 



In a musical box the sound is produced by a number of 

 narrow plates or tongues of metal, set in vibration by pins on a 

 revolving barrel, which strike against them. In the harmonium 

 there are somewhat similar tongues, but tho motion is imparted 

 to them by a current of air rushing past their edges. 



Plates of glass or metal may easily be thrown into vibration 

 and made to utter a musical note. When this is the case, we 

 shall find that the vibrations Interfere with one another, as in 

 the case of strings, so that in some parts wo find nodal lines, 

 or curves, where the vibration seems altogether unfelt, while 

 other parts correspond to ventral segments. A very simple 

 and beautiful mode of rendering these vibrations visible was 

 suggested by Chladni, and may very easily be carried out. 



A plate of glass or of metal may be employed for tho purpose, 

 and should be held by tho centre in a suitable clamp, or else 

 fixed to a stand, as shown in Fig. 18. A violin-bow should then 

 oe drawn across the edge, and a musical note will be produced. 



When the vibration* hay* ceased, scatter torn* ine Mod evenly 

 over the plate, and then excite it u before. Th sand will be 

 shaken off the ventral segment*, and will accumulate along the 

 nodal lines, clearly indicating the mode of vibration. 



If we damp one side near the middle by lightly touching it 

 and draw the bow aoroM the edge near one of the corner*, the 

 sand will arrange itself in two straight line* aoroee the plate, 

 dividing it into four square*. By thu* damping the plate in 

 different place*, an immense variety of beautiful form* will be 

 produced, some of them being very complicated. The more 

 rapid the vibration*, the greatei the number of the nodal \\nrn 

 which will thus bo traced upon the plate. 



Stretched membranes vibrate in a very similar manner ; the 

 smaller and more tightly stretched they are, the higher will be 

 tho note produced by them. If a thin membrane be stretched 



in a frame, and 

 held cloee to a 

 sounding body 

 (Fig. 19), the 

 vibration* of the 

 surrounding air 

 will be imparted 

 to it, and if sand 

 be sprinkled on 

 the membrane, 

 they will at once 

 bo rendered 

 visible by ito 

 accumulation at 

 the nodal lines. 

 In a drum the 

 sound is pro- 

 duced by the 

 vibrations of a 

 tense mem- 

 brane. The 

 hollow body of 

 the instrument 

 serves a* a 

 sounding-box to 

 increase tho 

 power, and the 

 note may be 

 modified by 

 altering the ten- 

 sion of the drum- 

 head by means 

 of adjusting 

 screws frequent- 

 ly provided for 

 that purpose, 

 .a Vibrating 

 plates are also 

 sometimes em- 

 ployed in the 

 * construction of 

 musical instru- 

 ments. The 

 glass harmo- 

 nicon, in which 

 the sounds are 

 produced by strips of glass of various lengths affixed to cords, 

 is an illustration of this. A similar instrument, made with 

 strips of wood, is known by the French as a claquebois, and 

 has a very agreeable tone. Both are played by striking them 

 with a small hammer. 



Bells may also come under the class of musical instruments, 

 and, by suitable variations in their size and shape, and in the 

 metal of which they are composed, an almost endless variety of 

 tone may be obtained from them. 



Finger-glasses may likewise be made to sound by rubbing the 

 edges with a damp finger. With a carefully-collected series of 

 these we have heard several airs played in a very pleasing 

 manner. Even a pane of glass in a window may be made to 

 vibrato and emit sound by rubbing it with a wetted finger ; and 

 the passage of a snail over a window-pane has been known tc 

 emit sounds of different intensity as the animal went 

 further from the edge. 



Fig. 16. 



