STEWART'S EXTRA SUBJECTS. MECHANICS, BY H. MAJOR, B.A. (HONOURS), F.R.G.S., "Science Author of "Scripture Readers for Day and Sunday Schools," FIRST YEAR. BIBLIOTHECA DEC 80 BODLEIANA LONDON: STEWART & Co., 30, New Bridge Street, E.C. GLASGOW: MENZIES & Co. NOTTINGHAM: H. MAJOR, Sherwood House. Stewart's Extra Subjects. MECHANICS. FIRST YEAR. ELEMENTARY KNOWLEDGE OF THE DIFFERENT STATES OF MATTER, SOLID, LIQUID, AND GASEOUS; WITH ILLUSTRATIONS OF COMPRESSIBILITY, ELASTICITY, AND RESISTANCE. MEASURES OF SPACE: TIME AND VELOCITY. CHAPTER I. Matter. All that we can see, feel, hear, taste, smell, or touch around us; and all that we can see, feel, and touch in ourselves, make up a part of what we call matter. It is a very hard thing to define exactly what matter is, though we have to deal with it from the first to the last moment of our lives, and are ourselves made of it. But, as just said, everything that can be recognised by any of the "Five Senses" is matter, and there is much besides which cannot be so recognised readily. Thus it is matter that makes up the universe, both in our earth and the air around it; as well as the moon, the sun, the comets and stars, and all the heavenly bodies. So far as the matter of the earth is concerned we have proof of its existence by its weight, which is a measure of it. Thus 2 lbs. of sugar contain twice as much matter as 1 lb., and this without respect to size. For instance, 1 lb. of wool would occupy much larger bulk than 1 lb. of lead, but there would be no more matter in it. Attraction of gravitation and weight. The reason why matter on the earth has weight is a curious one, though it seems so plain to us from constantly seeing its effects. If you let a marble drop from your fingers it falls rapidly to the ground. But what is the reason of this ? The answer was not found out for thousands of years by the wisest men. We know now it is because all matter attracts all other matter. You have a picture of this in a particular substance called a loadstone or magnet, which will attract iron, steel, and a few other things. This it will do at a distance, and the steel and iron will jump up to the loadstone. Now this effect which you can see in a particular instance on a particular substance represents what is true of all substances, that is all kinds of matter attract each other. This process is called attraction from its effects, but as there are many kinds of attraction it is known as attraction of gravity, or simply gravity, which means weight. When, therefore, you put 1 lb. of sugar on a scale pan of a balance, the matter in the earth is attracting it towards itself, and thus makes the scale pan fall. The sugar at the same time attracts the earth, but it is so small that the work of attraction of the earth prevails against that of the sugar. This brings us to one great law of attraction, namely, that it varies with or depends on the mass, that is the amount of matter. The earth is the largest mass with which we have to deal, so the attraction of that for all things in the air above it is greater than the attraction of these things on the earth, so the earth pulls all things with which we deal to itself, or they are said to fall. Exercise. (1) What makes any unsupported body fall to the earth? (2) What makes a pen cling to a magnet? (3) When a loadstone draws up a pen from the table how many forces are there at work on the pen at the same time? (4) How can you show any one that matter does really exist? The other great law of attraction of gravity refers to its force at different distances. You may picture the attraction by the action of light. If you take a candle in a large dark room, this is a centre as it were from which rays of light are proceeding in all directions, but the further you get from the centre of force, as we might call the light, the weaker becomes that force. Now, most of us would think at first, I dare say, that at double the distance the force, or the light, would only be half as great, and at three times the distance that it would be only as great, at four times the distance as great, and so on. Or, if this were the case, we might put down the different distances and the different strengths, or intensities of the light at those different distances in two lines, thus: If this had been the case we should have said the intensities varied inversely as the distances—that is, get smaller and smaller in the same proportion as the distances get greater and greater. But a little experiment will show that this is not the proportion between the distance and the intensity. Take a piece of cardboard, an inch square, and hold it at any distance from the light, say at a distance of 1 foot, then there is a certain number of rays of light falling on the cardboard, and the light on the square part of the cardboard has a certain |