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in which there is an arrangement of several parts. There are a few simple means of applying force which are called the mechanical powers, and machines are constructed by making use of these in various ways.

1st. The Lever.-If you wish to move a large block of stone, you may take an iron bar five or six feet long. Then laying a block of wood near the stone, place the bar so that it rests against the wooden block, with one end under the stone. You will find that by bearing your weight on the other end of the bar, you can raise a stone, which you could not stir with your hands alone. This contrivance is called a

lever. The block of wood which serves as a rest is called a fulcrum, which is the Latin word for prop. The parts of the bar on either side the fulcrum are called the arms of the lever. The nearer the fulcrum is to the stone, the more easily will it be raised ; and if the fulcrum remains in the same place, a long bar will move the stone with less effort than a shorter one.

If you had a very long bar, strong enough not to bend, and could arrange the fulcrum properly, you might in this way move enormous weights, though it would be rery slowly.

We are constantly in the habit of using levers. If a big and a little boy are playing at see-saw on a plank over the trunk of a tree, the big boy places himself nearer the trunk and the little boy farther off. In this way a small boy can raise up a great one. This plank and trunk formi a lever, the part of the trunk on which the plank rests is the fulcrum, and the parts of the plank on either side the trunk are the arms of the lever.

A common pump is moved by a lever; the pivot or pin on which the handle turns is the fulcrum; the sucker is at the end of the short arm, and the handle is the long arm of the lever.

A steel-yard is a lever. This is a bar of iron used to weigh meat, and is hung up from a point in the bar much nearer one end than the other. At the end of the short arm is a hook, and on the long arm there

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is a sliding weight. If you attach to the hook a piece of meat weighing six pounds, you will by sliding the weight upon the long arm, find some point at which the weight will exactly balance the meat on the other side. The point from which the steel-yard is hung is the fulcrum, the two parts of the bar on either side are the arms of the lever, and here a less weight supports a greater by being placed further from the fulcrum.

A lever sometimes has only one arm. If a rod be fixed to a wall by means of a binge, and held out straight

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with a weight hung to it, it will be found that the nearer the weight is to the hinge, the easier it will be to support it. Here the hinge is the fulcrum.

Levers are not always used to raise weights. A pair of scissors is composed of two separate levers; turning round a pivot which forms a fulcrum to each lever. A pair of tongs is also a double lever with a hinge for a fulcrum. Here each lever has but one arm, and the result produced is to press the two ends together, so as to hold a coal between them. The hands which act upon the levers are nearer the fulcrum than the ends which hold the coal; but this does not signify, we do not want to increase our power, but to make it act at a distance.

LESSON 79.

MECHANICAL POWERS.-Part II.

THE WHEEL AND AXLE, AND THE PULLEY.

2nd. The Wheel and Axle.—Suppose a wheel and an axle fixed in a frame, as in the following figure.

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The wheel is moved by a cord passing round it, and a weight raised by another cord wound round the

axle. The larger the wheel is in proportion to the axle the easier it is to raise the weight.

A common windlass for drawing water from a well is an example of this machine. The bucket of

water is 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

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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 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. It is not that you have less weight to pull, but you are able to use your strength in the most effective way.

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You can

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pull downwards with greater force than you can upwards, because the weight of your own body helps you. This is the use of the fixed pulley.

Now, take another pulley not fixed, and fasten the

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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 movable 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 movable pulley, because it moves up and down with the weight.

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