208 



NATURE 



{Jan. 15, 1874 



spring to another ball, and so on, until at last the shock 

 reaches the last ball, which is projected against the 

 india-rubber pad at the end (D), placed there to represent 

 in a rude mechanical way the drum of the ear. I press 

 the stem (A), with a sudden motion of my hand, and you 

 see that though the ball (C) only moves to and fro, yet it 

 sends forward a kind of pulse (/*• ef), which travels 

 along the line, and ultimately causes the last ball to give 

 a smart stroke against the pad (D). 



If you could creep into the tube of the ear you 

 would find, a little way in, a beautiful fine membrane 

 called the tympanum, or tympanic membrane. The 

 shock of the pulses of air falling on this membrane causes 

 it to shiver ; its tremors are transmitted to the auditory 

 nerves, and by them are conveyed to the brain and cause 

 you to have the sensation which we call sound. 



You ought to be able now to figure the way in which 

 the explosion of this pop-gun is transmitted through the 

 air. I place a ramrod in the tube, there is a cork in the 

 other end, and pushing the rod towards the cork, I cause 

 a crowding together of the particles of air, this they resist, 

 as I can feel by the force I am compelled to exert, and at 

 last their combined resistance takes effect by blowing out 

 the cork at the other end with a sort of explosion. 



The suddenly expanding air communicates its motion 

 to the air adjacent to it ; this again to the air farther 

 off ; finally the condensed pulse strikes the tympanum of 

 your ears, and you hear the noise. 



I can show you the passage of a pulse through air in 

 another way. We have here a tube 1 1 feet long, and about 

 4 inches wide, its two ends are closed by thin sheet india- 

 rubber. Against the india-rubber surface at one end 

 a cork gently presses (as in Fig. 2, a), to the cork a 

 slender stem is attached having a little hammer at its 

 upper end {b), kept from striking the bell [c), against 

 which it abuts by a slender wire spring {d). If now 

 a pulse be sent from the other end of the tube the 

 india-rubber will drive away the cork, and will drive the 

 hammer against the bell. A dull pusti will not ring the 

 bell at the farther end. The particles of air are very mobile 

 aTd readily slip round one another, so that it requires a shaip 

 shock to generate a soundwave in the tube and make the 

 bell ring outside the tube. I tap sharply with my fingers on 

 the india-rubber and the sound of my tap and the blow of 

 the hamrr.er upon the bell at the other end of the tube are 

 audible at one and the same time. This tube is 1 1 feet 

 long, sound travels through air of the temperature of this 

 room at about the rate of 1,100 ft. per second ; the time 

 therefore taken by the sound wave in traversing this tube 

 is ijo*'^ °f t^ second, an interval of time far too minute to 

 be measured by our ears. 



Air is therefore a carrier or tr.insmitter of sound. 

 Suppose we remove the air from about a sounding body, 

 will it then be heard ? This experiment was made by 

 Mr. Hawkibee a gre.it many years ago (1705). A bell 

 with a hammer worked by clock-work is placed under 

 a glass globe. Frcm the globe we will pump as much 

 of the air as we can. At present you hear the sound 

 with perfect distinctness, the pumping has at first app.a- 

 rently little effect upon the sound, but very soon it dies 

 away, and now you see the hammer thumping away upon 

 the bell, without producing any noise. It is doing its 

 work in perfect silence. I allow the air to re-enter the 

 glass globe, the tinkling sound of the bell is soon heard, 

 and quickly grows up into the usual musical ring. 



We have therefore proved that when the air is removed 

 we have no sound, and when the air returns the sound 

 returns also. 



We will now follow the matter up a little further, Prof 

 Leslie found that when a little air was in the chamber sur- 

 rounding the bell, and you could hear a little sound, that 

 if the space from which the air had been taken was filled 

 up with hydrogen, that the hydrogen quenched the sound. 

 Now Prof. Stokes has shown us that to create a sound- 



wave in hydrogen a sharper tap is necessary than in air, 

 so that the shock that produces a sound-wave in air does 

 not suffice to produce a sound-wave in hydrogen (which 

 is a much lighter and less dense gas). 



My assistant, Mr. Cottrell, has devised the experiment 

 I am about to show you to demonstrate this effect. 



I have a long tin tube (Fig. 2) narrower than the one I 

 used just now, but having like it a piece of india-rubber 

 stretched over each open end, with a hammer and bell 

 arranged against one of them, as before ; at the other is 

 a cork hammer fixed to a thin strip of steel, which can be 

 drawn back to any given distance (measured ony graduated 

 card). I have thus the means of sending a pulse along the 

 tube as before and making the bell at the other end sound, 

 but 1 now do it by a stroke of measured force. I now let 

 hydrogen into the tube at the end adjacent to the striking 

 cork (by the tube H), which is a little lower than the 

 other end, and while the hydrogen is entering I continue to 

 send pulses of measured strength along the tube ; the bell 

 continues to sound for a little while, but in one minute a 

 sufficient amount of air has been displaced to cause the 

 bell to cease ringing. When we remove the hydrogen 

 you again hear the bell, showing that the pulse can again 

 be carried from end to end of the tube. 



Up to this point our illustrations have been audible ; 

 I now wish to render visible to you the action of a tube in 

 preventing the dissipation of the sound. The test that 

 I propose to use is a flame. I have 

 behind the table a good-sized gas- 

 holder, by which gas can be forced 

 through a steatite burner. I light it, 

 and we have that long pointed flame 

 (Fig. 3 (7), and we shall find that that 

 flame is very sensitive. Chirrup to 

 it, and see how rapidly it answers ; a 

 great part of the length of the flame 

 is abolished instantly when the sound 

 wave reaches it (Fig. 3 b and c). I 

 rattle money, tap two keys, and this 

 flame jumps in response to each jingle 

 that 1 m.ike. The current of air in the 

 room, owing to our care for your com- 

 fort in the matter of fresh air, prevents 

 these phenomena showing themselves 

 as well as they do when the theatre is 

 empty ; but they are perfectly manifest. 

 No one in this room can hear my 

 watch ticking ; but if I hold it near 

 the flame you can distinctly hear the 

 flame give a little roar, and see it sud- 

 denly shorten for each tick of the 

 watch. The regularity with which it 

 jumps indicates the regularity with 

 which my watch is ticking. 



And now observe the action of a 

 tube in preventing the dissipation of 

 sound. Using a less sensitive flame as 

 the sound-test, I take oft' the india- 

 rubber ends from our i i-foot tube, and 

 place the flame at the end farthest from 

 myself The tapping of these two keys 

 together does not affect the flame ; but 

 now, my distance from the flame being 

 as great as before, I tap them opposite 

 the open end of the tube, and each tap 

 is rendered, by means of the flame, as 

 visible to your eyes as it is audible to 

 your ears. ^ 



Through the unconfined air this \\ ,1.1 



small bell does not affect the flame ^% '\ 



when set ringing ; but when I place 

 it at the extremity of the tube the 

 flame dances to each stroke. Speaking-pipes possess 

 their value solely from their preventing the dissipation of 



