UNDULATORY FORCES. ACOUSTICS. 



[CONDUCTION or SOUND. 



previously been wetted by a little lomon-iuioo, to give it a 

 degree of roughness. The finger should be gently but 

 firmly rubbed round the rim of each, glass ; and the ar- 

 rangement of which wo speak is illustrated in the follow- 

 ing engraving the octave succession of notes being 

 also indicated. The effect produced by the action of the 

 finger, is exactly similar to that afforded by the bow in 

 our last illustration. The glasses are caused to vibrate, 

 and the production of sound is the natural result 



PI,. S. 



Wo might odd, to a great extent, various illustrations 

 of the production of sound of a musical kind, by moans 

 of vibrations of the atmosphere and solid bodies. The 

 ^Eolian harp is an instance in which the air, moving 

 through the intervals between wires of different thick- 

 nesses, produces such results. Even the wires extended 

 on the poles observed by the sides of our railways, for 

 telegraphic purposes, often afford, by their vibration, 

 what we may venture to call a wild kind of music. The 

 striking of a gong, and similar instruments, on the other 

 hand, afford instances in which sound is produced by 

 the vibration of solid bodies, subsequently propagated 

 through the air. Wind instruments of all kinds pro- 

 duce these effects on the ear, by a combination of the 

 vibrations of their solid parts with those of the surround- 

 ing atmosphere. Having thus spoken generally of the 

 production of sound, we shall now refer to its conduction 

 through various media. 



THE CONDUCTION OF SOUND. 



WE have already remarked, that whenever vibrations are 

 produced, affording sound, the waves of sound are propa- 

 gated by the surrounding bodies. But tins depends on the 

 elasticity of those bodies ; as many substances to use a 

 common phrase stop sound ; that is, they prevent its pas- 

 sage, being incapable of continuing the vibrations which 

 have arrived at them. This is taken advantage of in the 

 jrection of astronomical instruments, which, from the 

 vibrations of adjacent bodies, would become so unsteady 

 as to bo perfectly useless. But if a telescope, of high 

 magnifying power, be mounted on a clay and leaden 

 foundation, the vibrations of surrounding bodies are 

 stopped, and cease. The same materials, with many 

 others, equally oppose the propagation of soniferous 

 waves ; and may, therefore, to some extent, be termed 

 non-conductors of sound. The atmosphere permits the 

 passage of sound at the rate of about 1,140 feet per 

 so.-ond ; that is, the sonorous vibrations of any body 

 will not be perceived by the ear in less than a second 

 of time, if 1,140 feet of air interpose. We find, how- 

 ever, that water, and many solids, convoy sound 

 much more rapidly than air; and if we make the rate 

 of conduction in the air the standard of comparison, 

 the following will show the relative powers of various 



1 

 4 

 7J 

 9 

 12 



Air 



Water . . .. 



Tin 



Silver 



Copper 



Iron 



Glass 



Various kinds of woods, 11 to . 



Most of our readers must have noticed, in daily ex- 



perience, how differently bodies conduct sound. Thus, 



if a long row of iron railings be struck at one ex- 



tremity, the sound produced is heard at the other 



1 

 LIT 



. J 



end long before it would be convoyed by the atmo- 



The temperature and density of any medium, have 

 the effect of modifying its conducting power ; and this is 

 most readily observed in the atmosphere. The rate, 

 however, which wo have mentioned, is pretty nearly 

 uniform for all ordinary atmospheric changes; and, for 

 this reason, the conduction of sound may be employed 

 as a mean* of measuring distances. This depends on 

 the fact that light travels infinitely faster than sound ; 

 requiring, as it does, no more than one second of time 

 for traversing a distance of 192,000 miles. To illustrate 

 this mode of ascertaining distances, we will suppose a 

 flash of lightning to be seen, and that twelve seconds 

 elapsed before the sound of thunder was heard. Pre- 

 suming 1,140 feet per second to represent the rate at 

 which the atmosphere conveys sonorous waves, we shall 

 ascertain the distance of the place at which the flash 

 occurred, by multiplying 1140 by 12. Thus 1140 

 (the rate of sound) multiplied by 12, gives the 

 number of feet (13680) as the distance of the flash ; 

 and this sum, divided by 5280 (the numlior of fret 

 in a mile), shows that the position of the electric 

 discharge must be two miles and 1,040 yards from the 

 observer. In a similar manner, the firing of a gun at sea, 

 may indicate the distance of any vessel from the shore. 



If the atmosphere be greatly rarefied, or gases of less 

 specific gravity are employed in experiments with Bound, 

 then the resulting effects are greatly modified. Thus, 

 if a person breathe pure hydrogen, or if that gas bo 

 blown through an organ-tube, then the sound produced 

 will lose its volume, and a shrill whistle will be noti 

 NVe. may hero state, by way of anticipating our future 

 remarks, the difference which exists in sounds. Some 

 are sharp or acute ; others dull or grave ; high and low, 

 itc. : and these differences -are partly due to the source of 

 vibration, the body vibrated, and the media through 

 wliich the vibrations are propagated. It matters 

 not, however, what pitch or intensity sound h:is in 

 respect to its speed, as each kind travels at the same 

 rate. The conducting power of any material may be 

 readily illustrated by means of the following plan : 

 Place a musical box inside of a larger box covered with 

 'baize, or any woollen material, so as to prevent the 

 passage of sound from its in- 

 side. A hole should bo 

 bored in the top. Into this 

 insert a wooden rod, so that 

 it may rest on the musical 

 box. If a violin, or empty 

 box of any kind, be then 

 placed on the outer end of 

 the wooden rod, the sounds 

 produced by the box will be 

 distinctly heard. On re- 

 moving the violin from the 

 rod, the sound apparently 

 ceases. This is owing to the 

 wood conveying sonorous 

 vibrations from the musical box, and the subsequent 

 diffusion of them through the air by means of the violin. 

 The mode of carrying out tliis interesting experiment, is 

 illustrated in the above engraving. 



Tins experiment also illustrates the use of sounding- 

 boards, as seen in the piano, harp, violin, JL-c. ; for it is not 

 only essential that sound bo conducted by a sufficient mate- 

 rial, but also that an adequate diffuser bo also present. 



The stethoscope is constructed so as to convoy the 

 sounds of the beating of the heart, <tc. , to the oar of the 

 person employing it : and a very common plan of n 

 taining the regular supply of steam to each end of the 

 cylinder of a steam-engine, is that of placing a woolen 

 rod against the cylinder biting it with the teeth, and 

 closing the ears. The vibrations are thus conveyed to 

 the oar by means of the conducting- rod. If a musical 

 box be placed at a considerable distance from the ear, 

 so that ita tones con scarcely DO heard, and afterwards a 

 deal rod be placed with one end on the box and the 

 other between the teeth, the sounds previously inaudible 



Fig. l. 



A A ii the outer deal box, enclosing 

 It, tbe musical box ; C U a 

 deal rod, through the hole D 

 resting on the musical box ; 

 is a violin for diffusing sound. 



