gained weight. But we can no more increase matter than we can destroy it, so this additional weight is caused by the air added to, or, as we say, combined with, the carbon in burning.

FORCE is also indestructible. We can neither create force nor destroy it. When a blacksmith hammers a piece of iron and turns it into a horse-shoe or a flat knife, or when a railway navvy lifts a huge piece of iron with a great crowbar which he could not raise without, neither of them creates any force: he only expends the force in his own body; and we find that we cannot get force without expending labour. So it is impossible to invent perpetual motion, which some people have tried to do, for there must be something to move any mechanical contrivance, anu that something can be traced to one source, viz., heat.

to

Now, let us see what some of the most common mechanical contrivances are, by which we seem get a force which we cannot obtain without them.

1. An inclined plane. I want to get a cask of sugar or a bale of wool into a warehouse, which is three steps above the street.

I cannot lift either

of them; but I find I can roll them till they reach the bottom step, which

bars them, and I cannot either roll them up its edge, or lift them on to its tread; but if I get a strong board, A, and lodge one end on the top step and the other on the ground, I can then roll either of them up it, but I shall have to spend more labour and take more time than when I was rolling them on the level street. So this inclined plane, A, has only helped me to use greater force. If I let go, when the cask is half-way up, it will not go on to the top, but will roll down to the bottom.

2. Lever. I might raise the cask one step at a time by means of a crowbar, or a beam of wood, by putting

E

one end under it, thus (see fig. 2), and then raising the other end of the bar until I could fix a stone, E, under the bar a few inches from

Fig. 2. the cask. By pulling down the end, C, to the ground, D, I should raise the cask up one step; but you will see that I have need to expend labour enough to move the bar from C to D, in order to move the cask the little distance from B to A. So you could not lift a boy your

own size very easily, but by means of a lever you do it often in play, at what is called rantipole. The plank on which two boys are seated is a lever;

and it is just the same as the bar of a pair of scales where the weight in the plate hung to one end raises the sugar put into a plate hung at the other end.

3. Wheel and axle. Again, I could raise the cask or bale of wool, not only up three steps, but up three stories of a warehouse, by a wheel and axle, which is a more powerful kind of lever (but it is only a lever). The lever is a bar resting on what we call a fulcrum, with power or force at one end, and a weight to be raised at the other. In the rantipole the fulcrum is the stone in the middle, the weight to be raised is a boy at one end, and the power, a boy at the other. In raising the cask up the step, the stone on which I rest my bar is the fulcrum; the cask is the weight to be raised; and I am the power. Now, if I wanted to raise the cask up three stories, I

a

Fig. 4.

might string it to one end of a rope, pass the rope over a wheel, and pull at the other end of the rope. Then my pulling at P (fig. 4) would be the power, the cask would be the weight, W, and the centre of w the wheel, a, the fulcrum. But I should have to use very great force to raise it by this means. But if I were to wrap the

rope round the axle (as in fig. 5), and pull at an endless rope running over the circumference of a large wheel, F, I could then raise it more easily; but if 1 lb. of my weight would raise 10 lbs. of sugar in the cask for 1 inch, I must pull the rope down 10 inches to raise it that 1 inch, so that I have not really gained any force.

If we multiply the number of wheels or pulleys, we can lift a greater weight with smaller power. For instance, if we have four in fig. 6) we can raise the weight with one fourth of the power, but we shall only raise it one fourth of the distance; so what we gain in power we lose in distance.

It is on the same principle that we raise water from a well, or a large stone in a quarry. The wheel and axle in both cases are only different applications of the lever (fig. 7).

I said the ultimate source of all force was heat; but we do not create heat, we' only transfer it from one place to another, call it out and use it.

When a blacksmith hammers a piece of

d

a

W

Fig. 5. pulleys (as

Fig. 6.

W

cold iron (as a good blacksmith can) till he has made it

the source of all this force and rapid motion. That heat is latent heat sucked up by the coal from the sun ages ago, when what is now coal was living vegetable matter growing upon the earth. It has lain concealed for

centuries, and now we call it out into activity. Supposing the grease in the box over the wheels runs short, you may see sparks flying out from the axle, and the carriage above might be set on fire, that is only the expenditure of some of the heat of the coal on the iron axle instead of its being used in drawing the train, and the speed of the train is relaxed by just so much as the waste of the heat on the axle of the wheel.

This will give you a few general ideas about force, heat, and mechanical contrivances. I shall supply you with a few more in the following lessons; but the particular details of machinery you must learn from special books on these subjects.-Rev. J. Ridgway.

CENTRE OF GRAVITY.

WHEN We have determined the exact spot where the centre of gravity is situated in any solid, a perpendicular line drawn from such centre to the centre of the earth is called the line of direction; and along this line every unsupported body endeavours to fall: if the line fall within the base of a body, such body will remain at rest; if otherwise, it will fall.

This will explain to us, why it is that a body stands firmly and steadily in proportion to the breadth of its base; and the difficulty of supporting a tall body upon its narrow base. It is not easy to balance a peg-top upon its peg; nor a hoop upon its edge; while, on the contrary, the cone and the pyramid stand firm and immovable, since the line of direction falls within the middle of the base, and the centre of gravity in such bodies is necessarily low down near the base.

All the art of a rope-dancer consists in altering his centre of gravity upon every variation of the position of his body, so as to preserve the line of direction within the base. He is assisted in this by means of a long pole, the ends of which are loaded with lead; this pole he holds across the rope, and fixes his eyes steadily upon some object near the rope, so as to detect instantly

the deviation of his centre of gravity to one side or the other.

If this centre deviates for an instant to one side, he would be liable to fall off the rope on that side; but he preserves his position by lowering the end of the pole on the opposite side, and thus constantly maintains the line of direction within the very narrow base on which he stands. We frequently use our arms in the same manner as the rope-dancer uses his balancing pole. If we stumble with one foot, we extend the opposite arm. In walking along a very narrow ledge, we balance our bodies by means of our arms; a man carrying a pail of water, therefore, curves his body away from the pail, and extends the opposite arm, and thus maintains his centre of gravity in its proper position. A man carrying a sack of wheat on his back, leans forward, and thus prevents the weight from throwing the line of direction beyond the base behind him. Numerous other examples of a similar kind will readily occur to the intelligent reader. We now proceed to supply instances which are not so obvious.

In fig. 1 a weight, G, is attached to a bent wire F, and the latter is

rests at its edge upon the table. Now, nothing more is necessary in order that the weight should fall to the ground, than that the small piece

F

H

G

Fig. 1.

of wood should tilt over; but a careful attention to the figure will shew that, in order to overturn the board, the weight, G, must rise towards the inner part of the table; and as almost the entire weight (and subsequently the centre of gravity) of the whole, resides in the weight G, it is contrary to the law of gravitation for G to