194 



KNOWLEDGE. 



[Septembeii 1, 1896. 



A projectile being bodily immersed in air, the wavo of 

 compressed aii' travels out in all directions from the pro- 

 jectile, and in this the phenomenon dill'ers from that of 

 the surface waves given by a floating body. The wavo 

 front is really a cone-shaped shell. The photoLiraphs, 

 however, do not show this, for it is only where the light 

 meets the layer of compressed air at a grazing angle that 

 the refraction is suilicient to give a dark line ; through all 

 other parts of the cone the light passes priictically un- 

 diminished. The fact that the wavo front has three 

 dimensions instead of being a surface phenomenon, shows 

 that the displacement of the air is not a hillock or billow, 

 as in the case of water waves. The work both of a ship 

 and a projectile is so far alike that they both drive the 



pressure.' 

 densitv of 



Fio. 2.— riiotograph of Lcc-Mctford Bullet in Flight. 



fluid forward from the stem, but in the case of a ship the 

 level of the water is at the same time raised, whereas in 

 the case of a projectile the displaced particles of air pene- 

 trate among those in front of them, thus producing a 

 greater density. This action goes on continuously in 



##^^^ft^:^tft. 



Enlarged Draiving of aTiore, sliowing Form of Air Waves. 



front of the bullet where the wave is created, and on 

 either side where the wave is travelling freely the com- 

 pressed air acts similarly, transmitting its energy to the 

 neighbouring layer of air, which, being thus compressed, 

 becomes in its turn the wave front. The particles of air 

 in the first layer then recoil promptly to their original 

 places, the layer returning at once to the ordinary pressure 

 of the air. Thus behind the sound pulse there is no noisy 

 turbulence of the air similar to the persistent heaving of 

 the sea. The motion of the air particles is wholly forwards 

 and back to rest along the same path, the whole of the 

 energy of motion being transmitted during this single 

 oscillation, as in a " long wave.'' Were it otherwise, the 

 air would be filled always and everywhere with a jangle of 

 sounds. 



The velocity with which a free sound wave propagates 

 itself through any gas depends upon the elasticity with 

 which the gas recovers its state when pressure is released. 

 This force of elastic recovery depends dincdy upon the 



The velocity also depends imrrsfly upon the 

 the gas, which is itself proportional to the 

 pressure, so that, taking both factors into account, the 

 velocity of propagation of a sound does not depend upon 

 the violence of the compression — that is to say, upon the 

 loudness of the sound. Were it otherwise an orchestra 

 could not keep time for an audience. 



This law only holds, however, for reasonable pressures. 

 With excessively violent compression the speed is some- 

 what greater. Thus, in Fig. 2, close to the V-shaped 

 reflector on either side, the wave front is in advance of its 

 proper position, where the darkness of shadow (at least in 

 the original photograph) shows that the intensity of the 

 sound is great. The V-shaped reflector was arranged so 

 that the sound met it at a grazing incidence. In this case 

 the wave is not reflected, as it is from the plate near the 

 bottom of the figure, but, instead, it gathers force by what 

 Scott Russell called " lateral accumulation." Something of 

 the same kind happens with the "solitary" canal wave 

 when it meets a wall at a grazing angle. The nioreased 

 velocity is in this case connected with the increased height 

 resulting from lateral accumulation, which causes the wave 

 to travel faster by increasing the effective depth of the 

 canal. The wave ultimately forms a breaker. Perhaps 

 the extra swiftness of very loud sounds may be due to 

 breaking, or rather spraying, jets of air shooting forward 

 from the wave front and accelerating the compression of 

 the next layer. Lateral accumulation is. Prof. Boys 

 thinks, the secret of whispering galleries, the sound 

 running round the wall without any true reflection. 

 Perhaps it may also be a reason why sound often carries 

 so wonderfully over water. 



The movement required to make an audible wave must 

 be sharp and quick, but may be very small. Lord Eayleigh 

 has shown experimentally that a displacement of air 

 particles of less than one ten-millionth of a centimetre in 

 amount is distinctly audible. On the other hand, the 

 motion of a large mass does not give rise to an audible 

 wave if the motion be so slow that the 

 air can escape compression by sliding 

 round its sides and in behind. 



Solid bodies, when struck, tremble 

 with a rapid movement, and this is 

 the usual source of sounds. A solid 

 in regular vibration sends out a suc- 

 cession of pulses, each of which is an 

 elastic air wave independent of those 

 which precede and follow it. As, 

 however, many bodies, especially 

 those which are uniform in material 

 and regular in shape, vibrate persistently at a con- 

 stant rate, the sound pulses thus produced succeed one 

 another in a perfectly regular manner — a definite, short, 

 lapse of time and a definite distance separating each 

 pulse from that which precedes it and from that which 

 follows. Such a regular succession of pulses has a 

 notr depending upon the interval of time between them, 

 or, which comes to the same thing, upon the distance 

 from crest to crest. This distance is called the wave- 



* jUlowanee nmst be made for tlie fact that in the sharj) and 

 sudden eouipression of the air whicl; takes place when sound is made 

 the air becomes heated. Conversely, sudden heating, as in the 

 ex])losiTe combination of gasps, develops jiressure, whicli heats and 

 assists in firing the next layer, detonation proceeding as a wave which 

 is very much like a violent sound wave. It seems as if jets of heated 

 gas are projected from the front of the " explosion wave," helping to 

 fire the mixture in front. Such jets correspond to the spray sliot 

 forward from a breaking wave. — {Tide Dixon on "The Kate of 

 Explosion in Gases '" : Phil. Trans., 1893.) 



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