THE Mechanical Powers are certain simple arrangements of machinery, by means of which weights may be raised, or resistance overcome, with the exertion of less power, or strength, than is necessary without them. In a mechanic power, the weight, or resistance, to be acted upon, and the power, or strength, which acts upon it, should both move at the same time; and any thing constitutes a mechanic power, in which the motion of the power produces a simultaneous motion in the resistance, provided less power is necessary than is due to the weight, or strength, of such resistance. From this general definition, it might appear that XIII.-8

every machine capable of generating force would be a mechanic power; but simplicity is likewise essential, and hence the mechanical powers may be said to be the elements of machinery; and they are, in fact, so elementary as to admit of no simplification or alteration. They are but six in number; and the names by which they are distinguished are, the LEVER, the WHEEL AND AXLE, the PULLEY, the INCLINED PLANE, the WEDGE, and the SCREW. Out of the whole, or a part, of these, it will be found that every mechanical engine, or piece of machinery, is constructed.

THE LEVER. This is the simplest of all the mechanical powers, and is generally considered the first. It is an inflexible bar, or rod, of any kind or shape, so disposed as to turn on a pivot, or prop, which is always called its fulcrum. It has the weight, or resistance, to be overcome, attached to some one part of its length, and the power which is to overcome that resistance applied to another; and as the power, resistance, and fulcrum admit of various positions with regard to each other, so the lever is divided into three modifications, distinguished as the first, second, and third kinds of lever - that portion of it which is contained between the fulcrum and the power being called the acting part, or arm, of the lever; and that part which is between the fulcrum and the resistance, its resisting part, or

arm.

A beam, or rod, of any kind, resting at one part on a prop, or axis, which becomes its centre of motion, is a lever; and it has been so called, probably, because such a contrivance was first employed for lifting weights. This figure represents a lever used to move

a block of stone: a is the end to which the power, or force, is applied; b is the prop, or fulcrum; and c is the weight, or resistance: this is a simple crowbar, or handspike. According to a fundamental principle of dynamics, the power may be as much less intense than the resistance as it is farther from the fulcrum, or moving through a greater space. A man at a, therefore, twice as far from the prop as the centre of gravity of the weight, b,—will be able to lift a weight twice as heavy as himself; but he will lift it only one inch for every two that he descends; for it is also a principle of this science that what is gained in power is lost in time.

There is no limit to the difference of intensity in forces which may be placed in opposition to each other by the lever, except the length and strength of the material of which the levers must be formed. Every one has heard of the boast of Archimedes, "Give me a lever long enough, and a prop strong enough, and with my own weight I will move the world! But he must have moved with the velocity of a cannon-ball for millions of years, to alter the position of the earth

half an inch. In mathematical truth, this feat of Archimedes is performed by every man who leaps from the ground, for he kicks the world away from him when he rises, and attracts it again when he falls back.

The common claw-hammer for drawing nails is a striking example of the power of a lever of this description. A boy who cannot exert a direct force of fifty pounds may, by means of this kind of hammer, extract a nail to which half a ton may be suspended, because his hand moves eight inches, perhaps, to make the nail rise one quarter of an inch. The claw-hammer also proves that it is of no consequence whether the lever be straight or crooked, provided it produces the required difference of velocity between power and resistance. The part of the hammer resting on the plank is the fulcrum. Pincers, or forceps, are double levers, and so are common scissors. The steelyard is a lever with unequal arms.

The second kind of lever possesses the same degree of power with the first, and operates with the same results. The third kind cannot be called a mechanical power, for, since its resisting arm is longer than the acting arm, it must lose power, though it gains time. The most familiar examples of the occurrence of this kind of lever, are in the use of common fire-tongs, and in rearing a tall ladder against a wall. But the circumstance that principally gives importance to it, is, that the limbs of men and all animals are formed of it; for the bones are levers, the joints are the fulcra, while the muscles which give motion to the limbs, or

produce the power, are inserted and act close to the joints, causing action at the extremities.

To calculate the effect of a lever in practice, we must always take into account the weight of the lever itself, and its bending. But in speaking of the theory of the lever, we usually leave these out of the question considering it as a rod without weight or flexibility. THE WHEEL AND AXLE. This power consists of two parallel wheels, pulleys, cylinders, or circles, having one axis in common.

The letter d here marks the

wheel, and can axle affixed to it. We see that, in turn ing together, the wheel would take up, or throw off, as much more rope than the axle as the circumference of the wheel is greater than that of the axle. If the proportions were as four to one, one pound, at b, hanging from the circumference of the wheel, would balance four pounds at a, hanging from the opposite side of the axle. A common crane for raising weights consists of an axle, to wind up, or receive, the rope which carries the weight, and of a large wheel, at the circumference of which the power is applied. The power may be animal effort on the outside of the wheel, or