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LESSON 80.

MECHANICAL POWERS.-PART III.

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THE INCLINED PLANE, THE SCREW, AND THE WEDGE.

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.

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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.

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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 old days, that he 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.

LESSON 81.

MECHANICAL POWERS.PART IV.

ENDLESS STRAPS, TOOTHED WHEELS, AND CRANKS.

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.

This is a strap of leather or gutta percha, with the ends joined together, which passes tightly round the

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two rollers. A common window-blind is drawn up

in this way.

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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. Another

way

in which one wheel can be made to

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move another, is hy 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 turning one of the wheels, the other moves round with it. If a large and small wheel are used together, the small wheel, having fewer teeth, will go round more often than the large. Thus if a wheel with six teeth plays upon a wheel with sixty teeth, the former will move round ten times, while the latter moves round once. The cogs of a wheel

can be so arranged that a horizontal wheel will move a vertical one.

A crank is composed of two rods (one shorter than

the other,) joined together by a pivot or hinge, the shorter rod turning round a fixed centre. The shorter rod is turned round the crank by pushing the longer

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