the weight, which is drawn up by a cord winding round the axle. The handle moves round in a circle, in just the same manner as if, instead of the handle, there were a large wheel with a peg in the rim to lay hold of.

3rd. The Pulley.A pulley is a block of wood, cut in the shape of a small roller, with a groove round it in which a cord runs. This is fixed in a frame by a

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pin round which it turns. If you wish to raise a weight to the top of a building, it is very convenient to fasten a pulley to the scaffolding, and pass round the pulley a rope, of which one end is attached to the weight, while you hold the other end, as you stand upon the ground.

Now, take another pulley not fixed, and fasten the weight you wish to raise to its frame; then tie one end of a cord to the scaffolding, pass the other end under the groove of the moveable pulley, and afterwards over

the groove of a fixed pulley, and you will find that you can raise the weight with less effort than by means of a fixed pulley alone. In fact, this weight would be balanced by one half as great at the end of the cord.

The lower pulley is called a moveable pulley because it moves up and down with the weight.

4th. The Inclined Plane.-This is a sloping board, along which a weight is drawn. It would be easier to draw a weight along such a board than to raise it with

no other help than a cord passing over a fixed pulley. This is a common mode of raising casks from an underground cellar.

Sometimes at railway-stations a sloping board is used, up which portmanteaus and boxes are drawn by cords. This is a simple instance of the application of the inclined plane as a mechanical power.

We find enormous stones in the upper parts of some very ancient buildings, and many have wondered how such large blocks were lifted to their places. It is probable that these stones were moved upon rollers up an inclined plane, or sloping causeway, constructed expressly for this purpose.

5th. The Screw.-The screw is used either to raise weights, or, still more commonly, to press down linen or

other things. There is a strong frame, through the top of which a hole is bored, with a groove winding round in it, and the screw has a projecting thread round it exactly fitting the groove. On the top of the screw is placed the weight which is to be raised, or

at the bottom of it is fixed the board which is to be pressed down. Through the middle of the screw, at some convenient part, a horizontal bar passes, which is used as an arm to work the screw round. The force which we can apply by the screw depends upon the length of the arm and the slope of the thread.

6th. The Wedge.-The wedge differs from the other mechanical powers, because it is acted upon by violent sudden blows, instead of continual pressure; but the way in which it produces its effect is simple enough. It is generally a piece of iron, with a thin edge gradually sloping up to a flat back. It is used for splitting wood. The edge is placed in a thin slit made in the wood, and then the back is struck violently with a hammer, so that the wedge is driven farther and farther in, and the small crack is continually widened till the tree is split in two.

A story is told of a wrestler in former days, who was so strong that he could tear an oak asunder with his

hands, and that having once partly succeeded in doing it, the parts of the tree pressed together again with so much force that they held the poor man prisoner, and being far from all help, he was kept there till he died of hunger. I do not suppose that this story is true, for I cannot think that any man was ever strong enough to attempt such a thing; but you see how, by means of the wedge, any person of ordinary strength could do more than the strongest man who ever lived could have done with his hands alone. This will give some idea of the great use of Mechanical Powers.

CHAPTER LXI.

MECHANICAL POWERS.-PART II.

SOMETIMES, when we have the means of moving a great wheel or roller fixed in one part of a building, we wish, by turning this wheel, to move another wheel or roller at some distance off.

This is done by what is called an Endless Strap.

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This is a strap of leather or gutta-percha, with the ends joined together, which passes tightly round the two rollers. A common window-blind is drawn up in this way.

If we turn one of the rollers, the strap, passing round and round, will move the other roller; and thus by means of straps we can connect together any number of rollers, so that by moving one we set all the rest in

motion.

If the first roller be greater than the second, the second will move round more than once while the other moves round once; but if the first roller be smaller than the second, the second will not have moved entirely round when the first has moved once. In this way we

can get a quick or a slow movement by altering the size of the rollers.

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Another way in which one wheel can be made to move another, is by having teeth or cogs all round the

circumferences; such wheels are called Toothed Wheels. The teeth of two wheels fit into each other, and so. by