SCIENCE PRIMERS.

PHYSICS.

INTRODUCTION.

1. Definition of Physics.-You have been told in the Chemistry Primer what sort of things we have around us. You have seen what the chemist does; how he weighs things and finds their quantity, and also how he finds that certain things are compound, and may be split up into two or more new things; while again other things are simple or elementary and cannot be so split up.

In fact you have been told about the various kinds of things we have in the world, but you have not yet learned much about the affections or moods of these things. You are yourself subject to change of moods; sometimes you appear with a smile on your face, and sometimes, perhaps, with a face full of frowns or tears; sometimes, again, you feel vigorous and active; sometimes dull and listless.

Now if you think a little you will see that the things around you are subject to moods very much like yours. To-day the face of nature looks bright and happy, and

full of smiles; to-morrow the same face is dark and lowering; the rain falls, the thunder roars, and the sea is tossed with waves and very stormy. Or again: let us take an iron ball which lies upon the floor; it is cold and heavy to the touch, but let us put it into the fire, and when it comes out the same substance is there, but the state of it is very different; if you now attempt to touch it, you will be sure to burn your fingers. Or again: if, instead of putting it into the fire, we put it into a cannon and discharge the cannon, it will come out with tremendous velocity, and will knock to pieces anything it touches.

Thus you see that a cold cannon-ball is a very different thing from a hot cannon-ball; and also that a cannon-ball at rest is a very different thing from a cannon-ball in motion.

Now if we see you crying and unhappy, we ask what is the cause of this mood, and we always find there is a cause; or if we find you listless and sleepy, and wanting energy, we inquire what is the meaning of all this, and we find that it has a meaning and a cause. So likewise when we find changes in the moods or qualities of dead matter we inquire what is the cause of these changes, and we always find they have a cause. This inquiry we shall make in the following pages, and you must attend well to the answer we get. You have already been told that this mode of questioning nature is called experiment.

2. Definition of Motion.-You must in the first place get a clear idea of motion. Motion means change of place. Some of you may have heard that this solid earth on which we dwell is in truth moving very fast round the sun, but we may, in the meantime,

put away this thought altogether from our minds, because although the earth is moving very fast it carries us all along with it, and everything goes on as smoothly and quietly as if the earth were at rest.

Well then, if I sit on a chair in a room I may say that I am at rest, but if I walk up and down the room I am in motion. Now in order to understand my movements, you must know something more than the mere fact that I am moving about; you must know the direction or line in which I am moving, and you must also know the rate or velocity with which I am moving. You must try clearly to understand the meaning of this word "velocity;" and to make you do So, let us suppose that I go out of doors and walk along a straight road for two or three hours, and always at the same pace. Well, I find that in one hour I have got four miles beyond my starting point, and that in two hours I have got eight miles beyond it, and I therefore say that I am walking on at the rate or with the velocity (for both words mean the same thing) of four miles an hour.

But what if the rate be not always the same? Suppose a railway train to be coming near a station, and just beginning to slacken its speed. The train is moving to commence with, let us say, at the rate of forty miles an hour, but presently its velocity gets less and less, until when it arrives at the station it comes quite to a standstill. Now, how can we find its rate when this is always changing? or what do we mean when we say that the train, before it began to slacken its speed, was moving at forty miles an hour? We simply mean, that if the train had been allowed to move for a whole hour at the same rate it had before it

began to slacken its speed it would have moved over forty miles. In fact, if instead of coming to rest at the station it had been an express train, and gone on, it would have been forty miles away an hour after we began to notice it.

There are different ways of expressing velocity: sometimes we speak of so many miles an hour, as we have done here, but sometimes it is better to use feet and seconds; thus if I drop a stone down a well I should say that it fell sixteen feet during the first second after it was dropped. Sixty seconds, you all know, make a minute, and sixty minutes make an hour.

In this little book, when speaking of velocity or rate, we shall use feet and seconds more frequently than miles and hours, and speak of a body as moving at the rate of ten, or twenty, or thirty feet a second, as the case may be.

3. Definition of Force.-Now what is it that sets in motion anything that was previously at rest? Or what is it that brings to rest a thing that was previously in motion? It is force that does this. It is force that sets a body in motion, and it is force (only applied in an opposite direction) that brings it again to rest. Nay, more, if it requires a strong force to set a body in motion, it requires also a strong force to bring it to rest. You can set a cricket-ball in motion by the blow of your hand, and you can also stop it by a blow, but a massive body like a railway train needs a strong force to set it in motion, and a strong force to stop it. That which is easy to start is easy to stop; that which is difficult to start is difficult to stop. You see now that force acts not only when it sets a body in motion, but as truly when it brings a body to rest

In fact that which changes the state of a body is called force, whether that state be one of rest or of motion.

EXPERIMENT I.-To prove this, take a tin pan with some peas in the bottom of it, and hold the pan in your right hand. Now quickly raise your right hand, with the pan in it, until your right arm is brought to a stop by a fixed bar of wood, which you have placed a little above it (your other arm held stiffly will do as well as the wood). Now what you have done is to make the pan with the peas rise quickly up, and then

suddenly come to a dead stop. You have first, by the force of your arm, given an upward motion to the pan, and the pan has forced the peas to mount with it, since clearly they could not remain behind. Then, again, when your right arm holding the pan was mounting quickly, you allowed it to be stopped all at once by the bar of wood; that is to say, the bar of wood forced your hand to stop, and your hand in its turn forced the pan, which you held tightly, to stop also. But this stopping force does not affect the peas which lie loosely at the bottom of the pan, so that they will