Sound travels 11 20 feet in a second, in air of ordinary temperatnie, and therefore 112 feet in the tenth of a second. Therefore the journey of 56 feet beyond you to reach the wall and 56 feet to return, will occupy the sound-wave one-tenih of a second and separate the two gounds.

back rebound from mountain to mountain and are driven backwards and forwards, becoming fainter and fainter till they die away ; these echoes are very beautiful.

If you are now able to picture to yourselves one set of waves going to the wall, and another set returning and crossing them, you will be ready to understand something of that very difficult question, How is it that we can hear many different sounds at one time and tell them apart ?

Have you ever watched the sea when its surface is much ruffled, and noticed how, besides the big waves of the tide, there are numberless smaller ripples made by the wind blowing the surface of the water, or the oars of a boat dipping in it, or even rain-drops falling ? If you have done this you will have seen that all these waves and ripples cross each other, and you can follow any one ripple with your eye as it goes on its way undisturbed by the rest. Or you may make beautiful crossing and recrossing ripples on a pond by throwing in two stones at a little distance from each other, and here too you can follow any one wave on to the edge of the pond.

Now just in this way the waves of sound, in their manner of moving, cross and recross each other. You will remember too, that different sounds make waves of different lengths, just as the tide makes a longwave and the rain-drops tiny ones. Therefore each sound falls with its own peculiar wave upon your ear, and you can listen to that particular wave just as you look at one particular ripple, and then the sound becomes clear to you.

All this is what is going on outside your ear, but what is happening in your ear itself ? How do these blows of the air speak to your brain ? By means oi the following diagram, Fig. 33, we will try to understand roughly our beautiful hearing instrument, th^ ear. Fig. 33.

«, Concha, or shell of the ear. be. Auditory canal. ^, Tympanic membrane stretched across the drum of the ear. E, Eustachian tube.

e, /, ear-bones : d, the hammer, malleus ; <f, the anvil, incus ; /, the stirrup, stapes. L, Labyrinth, gy Cochlea, or internal spiral shell. h. One of the little windows ; the other is covered by the stirrup.

First, I want you to notice how beautifully the outside shell, or conclia as it is called {a), is curved round so that any movement of the air coming to it from the front is caught in it and reflected into the hole of the ear. Put your finger round your ear and feel how the gristly part is curved towards the* front of your

head. This concha makes a curve much like the curve a deaf man makes with his hand behind his ear to catch the sound. Animals often have to raise their ears to catch the sound well, but ours stand always ready. When the air-waves have passed in at the hole of your ear, they move all the air in the passage, be, which is called the auditory, or hearing, canal. This canal is lined with little hairs to keep out insects and dust, and the wax which collects in it serves the same purpose. But if too much wax collects, it prevents the air from playing well upon the drum, and therefore makes you deaf. Across the end of this canal, at a membrane or skin called the tympanum is stretched, like the parchment over the head of a drum, and it is this membrane which moves to and fro as the air-waves strike on it A violent box on the ear will sometimes break this delicate membrane, or injure it, and therefore it is very wrong to hit a person violently on the ear.

On the other side of this membrane, inside the ear, there is air, which fills the whole of the inner chamber and the tube E, which runs down into the throat behind the nose, and is called the Eustachian tube after the man who discovered it. This tube is closed at the end by a valve which opens and shuts. If you breathe out strongly, and then shut your mouth and swallow, you will hear a little click " in your ear. This is because in swallowing you draw the air out of the Eustachian tube and so draw in the membranes, which clicks as it goes back again. But unless you do this the tube and the whole chamber cavity behind the membrane remains full of air.

Now, as this membrane is driven to and fro by the sound-waves, it naturally shakes the air in the cavity behind it, and it also sets moving three most curious little bones. The first of these bones d is fastened to the middle of the drumhead so that it moves to and fro every time this membrane quivers. The head of this bone fits into a hole in the next bone the anvil, and is fastened to it by muscles, so as to drag it along with it ; but, the muscles being elastic, it can draw back a little from the anvil, and so give it a blow each time it comes back. This anvil, e is in its turn very firmly fixed to the little bone /, shaped like a stirrup, which you see at the end of the chain.

This stirrup rests upon a curious body L, which looks in the diagram like a snail-shell with tubes coming out of it. This body, which is called the labyrinth, is made of bone, but it has two little w^indows in it, one h covered only by a membrane, while the other has the head of the stirrup / resting upon it.

Now, with a little attention you will understand that when the air in the canal be shakes the drumhead c to and fro, this membrane must drag with it the hammer, the anvil, and the stirrup. Each time the drum goes in, the hammer will hit the anvil, and drive the stirrup against the little window ; every time it goes out it will draw the hammer, the anvil, and the stirrup out again, ready for another blow. Thus the stirrup is always playing upon this little window. Meanwhile, inside the bony labyrinth L there is a fluid like water, and along the little passages are very fine hairs, which wave to and fro like reeds ; and whenever the stirrup hits at the little window, the fluid moves these hairs to and fro, and they irritate the ends of a nerve and this nerve carries the message to your brain. There are also some curious little stones called otoliths, lying in some parts of this fluid, and they, by their rolling to and fro, probably keep up the motion and prolong the sound.

You must not imagine we have explained here the many intricacies which occur in the ear ; I can only hope to give you a rough idea of it, so that you may picture to yourselves the air-waves moving (as in Fig. 32) backwards and forward in the canal of your ear, then the tympanum vibrating to and fro, the hammer hitting the anvil, the stirrup knocking at the little window, the fluid waving the fine hairs and rolling the tiny stones, the ends of the nerve quivering, and then \ how we know not) the brain hearing the message.

Is not this wonderful, going on as it does at every sound you hear } And yet this is not all, for inside that curled part of the labyrinth which looks like a snail-shell and is called the cochlea^ there is a most wonderful apparatus of more than three thousand fine stretched filaments or threads, and these act like the strings of a harp, and make you hear different tones. If you go near to a harp or a piano, and sing any particular note very loudly, you will hear this note sounding in the instrument, because you will set just that particular string quivering, which gives the note yoj sang. The air-waves set going by your voice touch that string, because it can quiver in time with them, while none of the other strings can do so. Now, just in the same way the tiny instrument of three thousand strings in your ear, which is called Corti's organ, vibrates to the air-waves, one thread to one set of waves, and another to another, and according to the fibre that quivers, will be the sound you hear. Here then at last, we see how nature speaks to us. All the movements going on outside, however violent and varied they may be, cannot of themselves make sound. But here, in the little space behind the drum of our ear, the air-waves are sorted and sent on to our brain, where they speak to us as sound.

But why then do we not hear all sounds as music ? Why are some mere noise, and others clear musical notes ? This depends entirely upon whether the soundwaves come quickly and regularly, or by an irregular succession of shocks. For example, when a load of stones is being shot out of a cart, you hear only a long, continuous noise, because the stones fall irregularly, some quicker, some slower, here a number together, and there two or three stragglers by themselves ; each of these different shocks comes to your ear and makes a confused, noisy sound. But if you run a stick very quickly along a paling, you will hear a sound very like a musical note. This is because the rods of the paling are all at equal distances one from the other, and so the shocks fall quickly one after another at regular intervals upon your ear. Any quick and regular succession of sounds makes a note, even though it may be an ugly one. The squeak of a slate pencil along a slate, and the shriek of a railway whistle are not pleasant, but they are real notes which you could copy on a violin.

I have here a simple apparatus which I have had made to show you that rapid and regular shocks produce a natural musical note. This wheel (Fig. 34) is milled at the edge like a shilling, and when I turn it rapidly so that it strikes against the edge of the card fixed behind it, the notches strike in rapid succession, and produce a musical sound. We can also prove by this experiment that the quicker the blows are, the higher the note will be. I pull the string gently at

Fig. 34.

first, and then quicker and quicker, and you will notice that the note grows sharper and sharper, till the movement begins to slacken, when the note goes down again. This is because the more rapidly the air is hit, the shorter are the waves it makes, and short waves give a high note.

Let us examine this with two tuning-forks. I strike one, and it sounds C, the third space in the treble ; I strike the other, and it sounds the first leger line, five notes above the C. I have drawn on this

diagram (Fig. 35), an imaginary picture of these two sets of waves. You see that the G fork makes three waves, while the C fork makes only two. Why is this ? Because the prong of the G fork moves three times backwards and forwards while the prong of the C fork only moves twice ; therefore the G fork does not crowd so many atoms together before it draws back, and the waves are shorter. These two notes, C and G, are a fifth of an octave apart ; if we had two Fig. 35.

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