96 



THE POPULAR EDUCATOR. 



tuning-fork, but from the sounding body at the other end of 

 the rod Every vibration of the former is conveyed along the 

 rod, and accurately reproduced at the other end. It is heard 

 much more distinctly there because it is distributed over the 

 surface of a large sounding body, and thus the waves of sound 

 produced are much more distinct. If two forks sounding dif- 

 ferent notes be struck and placed together at the end, both 

 Bounds will be conveyed along the rod, the vibrations of the one 

 appearing not to interfere with those caused by the second. 



A very interesting modification of this experiment was intro- 

 duced by Professor "Wheatstone at the London Polytechnic In- 

 stitution some years ago, and has been many times repeated 

 since. It was an arrangement known as the Telephonic Concert. 



Long deal rods were made to pass up from the basement of 

 the building through the different ceilings to the floor of the 

 lecture hall, above which they 

 projected a little distance. The 

 lower ends of these were made 

 to rest upon various musical 

 instruments ; the end of one 

 being pointed and made to 

 rest upon the sounding-board 

 of a piano, while another was 

 in contact with a violin, and 

 so on. On the upper ends of 

 these harps were placed, so 

 that the rods were in contact 

 with their sounding-boards. 

 They were, however, so ar- 

 ranged that they could very 

 easily be removed from the 

 Tods when necessary. A gentle 

 tap conveyed to the performer 

 below intimation that all was 

 ready, and every sound ema- 

 nating from the instruments 

 was faithfully conveyed along 

 the rods, and appeared to 

 issue from the harps resting 

 upon them. It can easily be 

 understood what a strange 

 effect was produced by the 

 sound of a piano, violin, or 

 other musical instrument ap- 

 pearing in this way to issue 

 from a harp, especially as no 

 performers could be seen. If 

 the harp were moved at all, 

 so as to break the contact 

 between it and the rod, every 

 sound at once ceased, though 

 the performers still continued 

 to play upon the various in- 

 struments. On renewing con- 

 tact, the sound continued as 

 before. The experiment is a 

 very remarkable one, all the 



different vibrations produced by the various wires of the piano 

 being conveyed along the one rod without interfering at all with 

 one another. 



This experiment has been carried even further than this. 

 The attempt was made to convey the music of the human voice 

 in the same way. The performers were placed with their 

 mouths very close to a sounding-board connected with the rod, 

 and, as they sung, the music was conveyed along the rod, and 

 produced the remarkable phenomenon of a singing harp. The 

 success of this experiment was even more complete than could 

 have been anticipated. The performers were obliged to be so 

 close together, and to remain in such a ludicrous and confined 

 position, that often they bumped their heads together, and 

 the music ended in a peal of laughter, which was, of course, 

 reproduced by the harp, to the no small astonishment of the 

 audience. 



This experiment is rather difficult and costly to repeat in a 

 private house. A very similar one, showing fully the principle 

 on which it depends, and creating much astonishment, may, 

 however, easily be tried with a small musical box. Let a box 

 be procured laxgo enough to contain the musical box, with 



plenty of room all round it. In the top of this make a small 

 hole through which the rod may pass. If two boxes can be 

 procured, one of which can go inside the other, the musical-box 

 being placed in the inner one, the result will be still more satis- 

 factory. Now line or pack them carefully with wadding or 

 baize, so as completely to drown the sound of the music, taking 

 care, however, to leave room for the rod to pass quite down to 

 the box, and also to arrange for winding it up, when required. 

 No sound will now be heard, the vibrations being completely 

 muffled by the non-conducting materials employed. If, however, 

 we insert the rod, and place on it a thin piece of board, tha 

 music will become distinctly audible. A spiral spring placed OK 

 the board, as shown in Fig, 4, will increase the sound consider- 

 ably. A violin, being specially constructed for the purpose o* 

 spreading sound, answers the purpose still better, and may be 



used in place of the thin wood. 

 Another point illustrated by 

 these experiments is the effect 

 of sounding-boards in increas- 

 ing the volume of sound pro- 

 duced by musical instruments. 

 It is well known that the 

 sound of a tuning-fork will be 

 heard much more distinctly if 

 its end be placed upon a thin 

 box or piece of wood. In 

 stringed instruments this is 

 especially important. If the 

 cord be merely fixed to firm 

 supports, and set in vibration, 

 the note will be faint and indis- 

 tinct ; but if a thin piece of 

 board be connected with it, or, 

 better still, if the cord be 

 stretched on one side cf a 

 hollow wooden case, as is done 

 in a violin, the volume of sound 

 is immensely Increased. For 

 this reason a sounding-board 

 is placed in the harp, the piano, 

 and most other stringed instru- 

 m ents. 



We may now collect and re- 

 view the main causes which 

 influence the intensity of any 

 sound. 



The first, as has already been 

 explained, is the distance of the 

 sounding body from the ear, the 

 sound being found to diminish 

 in intensity inversely as the 

 square of the distance ; that 

 is, a sound when heard at 

 double the distance has only 

 one-fourth the intensity. 



Another cause is the density 

 of the air in which the sound 



is produced. This is shown by the bell under the exhausted 

 receiver. As air is gradually admitted, the sound becomes 

 more and more distinct ; and if the receiver be filled with car- 

 bonic acid gas, the density of which is half as great again as 

 that of air, the sound will be rendered much more intense. 



The intensity of any sound is further dependent upon the 

 amplitude or extent of the vibrations of the sonorous body. 

 When a stretched cord is first plucked or struck, its vibrations 

 are much more extensive than they are when the sound grows 

 fainter. So, too, if a tuning-fork (Fig. 5) be violently struck 

 the sound will gradually become feebler as the vibrations cf tlio 

 limbs become more and more limited. 



The next cause to which we must refer is the motion of the 

 atmosphere and the direction of the wind. On a calm day 

 sound is always conveyed better than when the air is disturbed. 

 A gentle wind, too, causes the sound to be more intensely heard 

 in the direction in which it is blowing. 



The proximity of a sonorous body also serves tc increase the 

 power of sound. Illustrations of this have been given in the 

 case of musical instruments, and we shall meet with several 

 others as we proceed. 



