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to balance the weight in the scale, is fixed by an unvarying rule. If the distance of the hook from which the scale hangs he one inch from the fulcrum, and the weight in the scale he 6 lbs. it will be balanced by the weight of 1 lb. at 6 inches' distance from the fulcrum, on the other side, or by that of 2 lbs. at 3 inches. The distance of the balancingweight, or, in other words, the power required to be exerted on the lever, may always be found by multiplying the weight used in balancing or weighing by the number of inches or feet, as the case may be, which with this lesser weight will make the same amount as has to be raised.

Thus 1 lb. X 6 inches are equal to the weight in the scale, 6 lbs. and 2 lbs. X 3 inches are, in the same way equal to G. Thus, also, a 3-lb. weight placed at the distance of 2 inches on the other side of the fulcrum, would balance 6 lbs. in the scale, for 3 x 2 are equal to 6.

The common scales (Fig. 3) aro also a lever of the first kind, with the fulcrum between the weight to be raised, and the power used

to raise it. This lever is tho beam A B. C is the fulcrum or pivot of motion, at the centre, so that the two arms A C and C B are equal. But it would not do to have a straight bar only for a balance. The centre of gravity must be so arranged that the beam shall always return to a horizontal position when the two sides balance each other. The centre of gravity in the illustration is g, and the point from which the beam is suspended in weighing (m) must be perpendicular to this. The tongue of the balance is a slender pointed needle rising in the line of m, and shows, by resting at the perpendicular, or by moving an equal distance on each side of it, that the two scales are balanced.

Balances for coin or for scientific purposes are made of such extreme sensibility that they turn with the thousandth part of a grain. M. Stas

in his researches on the weight of the atoms of some elements used a balance which turned with the tenmillionth part of the weight in each pan! Another balance used by the 6ame philosopher turned with the twothousandth of a grain.

(2.) Levers of the second Hud are

also common (Fig. 4). They have

the power used to move the weight, P, and the weight itself, W, on the

same side of the fulcrum, F. A familiar example is shown in tho wood

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carver's or carpenter's Paring Knife (Fig. 5). The hook at the end is the fulcrum ; the wood to be cut, which is placed under the blade, is the resisting substance on which the power of the lever is to be exerted, and the handle is the point at which the workman's hand applies that power.

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Fig. 5.

A wheelbarrow is another example of this lever. The axle of the wheel is the fulcrum, the load in the body of the barrow is the weight to be moved, and the strength used to wheel it is the power (Fig. 6).

When a man thrusts a handspike or crowbar under a heavy box or block of stone, and moves it forward, he uses a lever of the second kind: the fulcrum is the ground, at tho point where the handspike or crowbar presses, the box or block is the weight to be moved, and tho man's force is the power exerted—the weight and the power that moves it being both on the one side of the fulcrum.

A pair of nutcrackers are an example of a lever of this kind with two arms. Tho fulcrum is the joint which holds tho two arms together, the

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nut put between them is the thing to be affected by the levers, and the hand which closes the nutcracker is the power by which the two levers which its arms form, act.

An oar with which a boat is rowed is still another illustration. Tho hand of the rower is the power, the pin on which tho oar turns to propel the boat is the weight to be moved, and the water against which the oar is pressed is the fulcrum. (Fig. 7.)

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Fig. 7.

When two men carry a load on a pole between them (Fig. 8) it is

still another example of this form of lever. The power of each man is

used in lifting the weight, but at the same time each man's shoulder is

a fulcrum through which the power of the other man act8. While the

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load is exactly in the middle, each bears just half the weight; but if it slip one way or other, he towards whom it slips, having the shorter end of the pole resting on him, bears a heavier weight than the other man,

the pressure on his shoulder, which

I is the fulcrum, being increased.

(3.) A lever of the third kind i (Fig. 9) is one in which the power p s and the weight to be moved both act on the same side of the fulcrum, but the power P is between the fulcrum F

Fig. 9.

and the weight to be moved W.

An angler, when using his fishing-rod, employs it as a lever of IM3 kind. The fish is the weight to bo raised, his outer hand is the power, and the hand nearer his body is the fulcrum. (Fig. 10.)

When a gardener lifts earth on a spade this form of lever is illustrated. The weight is of course the earth, the power is the hand on the shank of the spade, and that which holds the handle is the fulcrum.

The foot-board of a turninglathe is another example. The foot on the ground is the fulcrum, the foot on the foot-hoard is the power, and the wheel to be set in motion is the weight. The footboard of a street knife-grinder's grindstone is an illustration of the same principle.

5. Common levers are used, for the most part, to raise weights to a small height, and this is done by a succession of efforts, after each of which the lever is applied afresh. But this, of course, involves a great deal of trouble, and hence contrivances have beoa long since invented by which weights

may be lifted with a continuous motion, by an ingenious application of lever power. This application forms The Second Mechanical Power

THE WHEEl AND AXLE.

The simplest form of this improvement is called the rack and pinion, and is shown in the cut of the rack-work on a Scuew-jack in Fig. 11. A screw-jack, I may say, is a contrivance much in use for lifting timber or heavy weights to short heights, and hence all the parts of it are made very strong. They are enclosed in a strong wooden frame, which is not represented here. A B is the rack, which should, however, be at least four times as long in proportion to the wheel. O is the wheel, which should have four times more teeth than in the cut. H P is the handle. The three-leaved wheel in the middle, R, is called the pinion.

6. Now when the handle is turned it moves the large wheel round a proportionately large space, but the rack at its centre, being so much smaller, must move round many times for each revolution of the large wheel; and the result is, without troubling you with difficult explanations, that the turning of the handle raises a weight 220 times greater than the power

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applied to it by the labourer who turns it; so that if his hands, when thus turning it, exert a force equal to 50 lbs., he is able by this jack to sustain a weight of 11,000 lbs., or about 5 tons—that is of 220 times the 50 lbs. of force he is exerting.

The power in this machine is the man's hand at the handle, the fulcrum is the successive teeth against which the power he exerts presses, and the weight is the rack and what may be fixed to it. The screw-jack acts, therefore, as a lever of the first kind.

7. A further advance in the securing a continuous motion by the power of levers is effected very simply. You can see that, if the spaces between the three teeth of the pinion were filled up, it would be round, and would thus form a cylinder or barrel on which a rope might be put so that the weight to be raised might be hung from one end of it. The rope would then take the place of the rack A B, and the weight hung to it would be moved in the same way, by power applied to a handle or handles.

Now if such a cylinder or barrel be placed upright, and the rope wound round it, to be moved by handles placed so as to increase the power on it, you have a ship's windlass (Fig. 12).

"When the sailors wish to heave the anchor, a cable from it is passed round the upright barrel or spindle of the windlass, and long handles, called handspikes, having been put into the holes in the drumhead D, the sailors force the drumhead round by them, and this turns the spindle and pulls up the weight attached to the rope wound round it. The teeth you see at the bottom of the windlass pass over notches in the frame, so placed that the teeth catch in them the moment the force is withdrawn from the handspikes, and prevent the weight from again rushing to the point from which it has been raised. The power of the windlass is

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Fig. 12. D the drum-head; W the whelps; C the cheek; P the paul-head; p the paulreins. The cylinder above C is that round which the rope is wound.

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