Now the first ball may be likened to the particles of air which are next the cannon, and the last ball to the particles that are next your ear, and thus you see how the blow from the air next the cannon is transmitted to the air next your ear without the necessity of the same individual particles of air moving all the distance in order to carry it.

Those of you who have played at croquet must have noticed what takes place when you croquet your adversary's ball. In this case you hold your own ball tightly under your foot while your adversary's is just touching it: you then by means of the mallet give a blow to your own ball, which does not however move, but which transmits the blow to your adversary's ball with sufficient force to send it a great way off. We have here, therefore, a result the same as in the series of balls.

45. Its rate of motion.-Again, this impulse or blow which we call sound requires time in order to pass from the cannon to our ear. No doubt it travels very fast, as fast as a rifle-ball, but yet it does not pass instantaneously from the cannon to our ear.

Most of you have no doubt seen a cannon fired a long distance off, and you then saw, first of all the flash and puff of smoke, and after a few seconds you heard the noise. Now these few seconds are the time which the sound or impulse took to travel from the cannon to your ear. You saw the flash the very moment the cannon was fired, and therefore, counting from its appearance, you know how long the sound took to travel from the cannon to you. Suppose, for instance, that the cannon was 11,000 feet away, and that you reckoned ten seconds between the flash and the report,

you therefore conclude that sound takes ten seconds to pass through 11,000 feet of air, or that it moves at the rate of 1,100 feet a second, which is pretty near the truth.

Sound will, however, pass through water much more quickly than through air, and by means of experiments made at the Lake of Geneva it has been ascertained that the rate of progress of sound through water is nearly four times as great as through air. Sound travels through wood or iron still faster-through wood, for instance, it travels from 10 to 16 times as fast as through air, so that it would pass through more than two miles' length of wooden logs in one second of time.

46. Echoes. Suppose now that I stand in the centre of a large natural amphitheatre, having rocky cliffs all round me, and from this position let me discharge a gun-the noise or impulse will spread from the gun to the rocky cliffs and strike them, but something more will happen after that. The sound when it has struck the cliffs, finding it can get no further, will come back again, and in this particular case it will come back along the very same line that it went, travelling always at the rate of about 1,100 feet per second. The result will be that a few seconds after the gun has been fired I shall hear the sound that has travelled back from the cliffs just as if another gun had been fired. Now this sound is called an echo.

You thus see that in the case of echoes we have the sound or impulse striking an obstacle and then reflected back from it, but it does not always come back in the same direction in which it goes; this depends upon the shape of the surface against which it strikes. A very curious experiment is that which is

shown in the following figure. Place two large hollow reflectors at some distance from one another, and in a point called the focus of the one put a watch, while you place your ear in the focus of the other; you will then hear the ticking of the watch very distinctly, just as if it were close to your ear. The reason of this is that the blows given by the watch to the air strike against the left-hand reflector, and are reflected from

Fig. 22.

it in directions which bring them to the other reflector, from which they are then all reflected into the ear. All this is shown in the figure. This property of sound makes a very nice experiment, but it has sometimes proved inconvenient in practice: for instance, in the Cathedral of Girgenti in Sicily, it is related that the slightest whisper is conveyed from the great western door to the cornice behind the high altar, and that unfortunately the former station was chosen as the place of the confessional. The result was that a listener placed at the other station often heard what was never intended for the public ear, until at length this came to be known, and another site was chosen. The

reflection of sound also explains what takes place in whispering galleries. In that of St. Paul's in London, for instance, a whisper at one side of the dome is conveyed to the opposite side across a very considerable distance.

47. How to find the number of vibrations in one second corresponding to any note. -I have told you that when a vibrating body gives the air a small number of blows in one second, we have a deep note, and that when it strikes the air very often in one second, we have a shrill high note: what

is called the pitch or tone of the note depends there. fore upon the number of blows which is given to the air in one second. Now we can find out by experiment how many blows in one second correspond to any particular note, and I hope by means of the above figure to make it clear to you how this is done.

You see a large wheel A to the right, which is turned by a handle. Over the circumference or rim of this wheel we have a strong tight strap which passes over the axle of another wheel B. The result is that by means of the strap the axle of the wheel B will go round a great many times for a single turn of A, and the wheel B will itself of course move with its axle-in fact, в may be made to move round very quickly. You see, too, that в is full of small teeth. Now there is a bit of card placed at E against the teeth of B, so that each tooth strikes the card as it passes.

Each time the card is struck we hear a sound, because a blow is given by the card to the air. If there are 100 teeth in the wheel B, there will be 100 blows given to the air in the time that в goes once round. If B goes round once in a second, 100 blows will be given to the air, and in consequence 100 sounds will strike our ear in one second, each single sound of which we shall not be able to distinguish, but we shall hear an apparently continuous deep note. Now by driving the handle fast enough I can make B go round 100 times in a second, and during each time it will strike the card 100 times; the card will in this case be struck 100 times 100, or 10,000 times in one second: 10,000 little blows will now strike the ear each second, and we shall hear a continuous shrill note.

Now when you wish to find the number of blows in one second corresponding to a given note, what you have to do is this. Turn the handle more and more quickly until the instrument by means of the card gives you a note precisely of the same pitch as the note you have got to measure; and when you have once