THE LEVER.

OF all the mechanical powers, the lever is the most simple. It is formed of any strong substance, in the shape of a beam or rod, which rests on a prop or axis called a fulcrum," which is its centre of motion. There are three kinds of levers. The following is an exemplification of the first (fig. 4) :—

In this diagram, I is the lever, f the fulcrum, w the weight. By pressing down the end 7, the other end of the lever raises w, the weight; the centre of motion is at f, the fulcrum. In other words, the power of force resting on the prop or fulcrum overcomes the weight or resistance. Thus if the lever be under the centre of gravity of the weight, and the length of the lever from the fulcrum be twice as long as the other part, a man can raise the weight one inch for every two inches he depresses the end of the lever.

Now if the end of the lever be four times the length of the part from the fulcrum to the centre of gravity of the weight, then the power of raising the weight is in

creased four times; but the space that the end of the lever will pass through is four times greater.

It will thus be perceived, that if a weight of one stone moves through a space of ten feet, we may raise a weight of ten stones through a space of one foot ; or a weight of ten stones moving through a space of one foot will make a weight of one stone move through a space of ten feet.

Now if a man can raise the weight at the end of the lever, and then the lever be made twice as long, and a boy of half the man's strength can then raise it, the boy will be sooner worn out by fatigue than the man, because the man in the exertion of his strength only goes through half the space that the boy has to pass through. It is stated that "the force of the lever increases in proportion as the distance of the power from the fulcrum increases, and diminishes in proportion as the distance of the weight from the fulcrum increases." It was from this general law that Archimedes exclaimed, "Give me a lever long enough, and a prop strong enough, and with my own weight I will move the world."

Should it be desired to know what power will balance a certain weight at the short end of the lever, it is done by multiplying the weight by the length of lever from it to the fulcrum, and then dividing the result by the other length of lever, and the result is the power required: thus if 100 lbs. be on one end of a lever 12 inches from the fulcrum, 100 × 12 1200; then suppose the long end of the lever be 24 inches, 1200 ÷ 24 = 50 lbs., the power required.

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A spade is a lever, the earth being a fulcrum, in the operation of digging. In Ireland, and on the Continent, they make the spade much longer then that used in England; and thus a man stands nearly upright when digging. The fisher-girls, who dig for worms as bait in

the sands on our coast, also use a long-handled spade; this is to compensate for manual strength.

In moving barrels and very large weights, and principally on board ships, a handspike is the lever found best adapted to the purposes required.

Carpenters, masons, and others who have to move bulky masses of matter short distances, adopt the use of a crowbar, which is a lever made of iron, having a claw at one end.

A hammer has usually a claw for drawing out nails. Now in this the power seems great, for the nail will bear an immense weight attached to it; yet because we move the hand through several inches while the nail moves only a very short way, we can draw it out, and thus the velocity overcomes the resistance.

The fire-poker is a lever, having the bar of the grate for a fulcrum.

The simple lever has sometimes two arms; it is then called a double lever. Scissors are of this kind, having the rivet as a fulcrum for both levers. Large scissors, called shears, used in cutting cloth, pasteboard, tin, copper, and sheets of iron, are double levers.

Nippers, pincers, for

ceps, snuffers, are all of this description of levers.

The scale-beam used in weighing is a simple lever. The arms a, a (fig. 5) are made of equal length, and suspended over the centre

Fig. 5.

of gravity. The axis or pivot b, which is the point of suspension, is sharpened to a very thin edge, sometimes

equal to that of a razor, that the beam may easily turn with as little friction as possible, when weights are placed in the scales. Should the arms not be of equal length, then the scales cannot act justly, although the beam may seem fairly balanced and the weights true; but if one was half an inch longer than another in an arm of eight inches in length, the customer would lose more than an ounce in every pound. The deceit can be discovered by changing the weight and material to the opposite scales.

In some cases where the beams of scales are not accurate, the articles to be weighed are put in and balanced by shot, sand, or other things. The things, of which it is desired to know the weight, are then removed, and weights put in their place; thus the true and exact weight is known. By this mode almost any elastic substance may answer the purpose of a weighing-beam. Suppose a piece of steel, or a walking-stick that will bend, were held over a place, and a substance attached to its end; then when so attached, mark exactly the place the stick or steel bent to when the substance was on it; remove the thing to be weighed, and attach weights until the steel or stick bends again to the mark, and then the weight of the material is truly found.

Some persons use instead of the weighing-beam an instrument called a steelyard (fig. 6), which is a lever with arms of unequal length. The lever is suspended from a hook a, which is the fulcrum or pivot, and from which the steelyard must truly balance; thus it is its centre of gravity. Thus one pound weight will weigh any number of pounds in the scale that the yard is long enough to perform. In the diagram the one pound weight at c is weighing eight pounds in the scale at b, for the space, over which it is placed on the long arm of

the lever, is eight times that of the short arm. By dividing the spaces in the long arm into halves, quarters,

a

12

Fig. 6.

and sixteenths, then half pounds, quarters, and ounces can be weighed.

The second kind of lever is that where the weight and the power are on the same side of the fulcrum, the power being furthest from the fulcrum.

Thus, if a mason (fig. 7) desires to move forward a large

piece of stone, instead of bearing down upon the lever to raise it up a little, he sticks his crowbar into the ground, and pushing upward, moves the stone little by little onward, the ground being the fulcrum.