tank of water, but at a level lower down than that of the mill; which mill is likely to work? You will at once tell me, the one with the pond of water at a high level, because the fall of water will drive round the wheel. You see, therefore, that there is a great deal of work to be got from a pond of water high up, or a head of water, as this is called-real substantial work, such as grinding corn or threshing it, or turning wood or sawing it. On the other hand, there is no work at all to be got from a pond of water that is low down.

:

Let us now compare a water-mill driven by a head of water with a windmill driven by the wind. The wind is like the cannon-ball, although not moving so fast, its energy being that of a body which is actually moving it is in fact rushing against the sails of the windmill and driving them round; and if we throw up a feather or a straw in a strong gale, we find that it is hurried away by the wind. But a water-mill has one decided advantage over a windmill, for in a windmill we must wait for the wind; but if we have a watermill with a good head of water we may turn the water on and off whenever we choose. We can keep our stock of energy and draw upon it whenever we have a mind. In fact, the energy of a body in motion is like ready money which we are in the act of spending, but the energy of a head of water, or of any body which is high up, is like money in a bank, which we may draw out whenever we want it.

6

VIBRATING BODIES.

40. Sound.-A body that is changing its place is of course in motion, but it does not follow that every moving body changes its place as a whole; a top that spins round very quickly is in motion, but it does not change its place as a whole.

EXPERIMENT 33.-Here is a wire which you see attached by one end to a support; now if the other end be struck it goes backwards and forwards rapidly, but the wire as a whole does not change its place. When the particles of such a wire are moving backwards and forwards, they are said to be in a state of vibration. In like manner, when a bell or a drum is struck the particles of the bell or drum are in a state of vibration, or when the string of a musical instrument is pulled and let go, the string is in a state of vibration.

Fig. 20.

Now vibrating motion, just like motion. from place to place, denotes energy, and indeed the particles of a vibrating body are moving actively about from side to side; if you try to stop them, they will give you a blow. If anything is in their way, they will give it a blow-the atmospheric air is, and they consequently give it a blow. Indeed each time the top of this vibrating wire comes back it gives the air a knock in the same direction. In fact, a vibrating body gives in a short time a great number of little knocks to the air. When the air is struck, it does not receive the stroke

quietly, but strikes the air next it, and this in turn strikes the air next it, and so on, until the blow given to the air is carried over a great distance. At last this blow reaches your ear or mine, and we get a blow, which, however, does not affect us in the same way as a blow that knocks us down, and therefore we do not call it a blow; but we say that a sound has struck our ears-in fact we hear a sound.

41. What is noise and what music.-Now if the body that strikes the air just deals it one single blow, such as when a cannon is fired, the air carries this one blow to our ear, and we say that we hear a noise. If however the body that strikes the air be in vibration, and deal it a great number of little blows in one second, the air will carry these on and give our ears just as many blows in one second, and then we say that we hear a musical sound. Thus you see a noise is a single blow given to our ear, but a musical sound is caused by a series of little blows following one another at regular intervals. More than this, if the vibrating body which is the cause of this disturbance deals the air only a comparatively small number of blows in one second, then the air will of course only deal us the same number in one second, and we shall hear a deep low note; but if the vibrating body vibrates very quickly and deals the air a great number of blows in one second, the air will of course deal us just as many, and we shall hear a shrill high note. Thus you see a deep low note means a small number of blows dealt to our ears in one second, while a shrill high note means a great number of blows in the same time. A very shrill note will

be given by 20,000 blows in one second, and a very low note by 50 blows in the same time.

42. Sound can do work.-A musical note is pleasant, but a noise or single blow is disagreeable, and sometimes it hurts or even destroys the ear if it be a very violent one. Thus if a large cannon were discharged, the blow to the ear might in some cases destroy its hearing power; or if the sound struck against a pane of glass, the concussion might be so strong as to shatter the glass, and sometimes in such cases as the explosion of a powder magazine all the windows in the neighbourhood are shattered to pieces. Thus you see that a loud noise is something with energy in it, and that it can do work—more especially work of a destructive nature.

43. It requires Air to carry it. * EXPERIMENT 34. Let us try to ring a bell in a place where there is no air, such as an exhausted receiver.

There

being no air, there will be nothing which the moving particles of the bell can give a blow to, and hence no sound will reach our ears. In fact, a bell that has been struck, or any other vibrating body, has in it a quantity of energy, of which it parts with some to the air, while the air in its turn parts with some to our ear. But if there be no air, there is nothing to carry to our ear the energy of the vibrating body.

44. Its mode of motion through Air.-Let us now think a little about the nature of this thing called sound, which is given out to the air by bodies in vibration, and which is then carried to a great distance by the air itself.

In the first place, when a cannon is discharged a mile or two off, do not imagine that the same particles

of air travel all the way from the cannon to your ear. The particles near the cannon give a blow to those next them and then stop, the particles that have received the blow give in their turn a blow to those next them and then stop, and so on, till the blow reaches your ear. What really happens will be made quite plain by the following experiment.

* EXPERIMENT 35.-Let us take a series of elastic balls suspended in a row by separate threads, so as to

Fig. 21.

hang loosely together, just touching one another. Let us now pull aside endways the first ball, and allow it to give a blow to the second. What will happen? The first ball having delivered its blow to the second, will become quite still. The second will very quickly transmit the blow to the third, and become still in its turn; the third will do so likewise, until the impulse reaches the last ball of the series, which being the last will be put in motion by the blow.