A wheel-barrow affords another example. In using it, a point in the wheel of the barrow pressing on the ground is the fulcrum; the load is the weight, and the handles held by the man the power. As the person shortens or lengthens his hold on the handles, so does he move the centre of gravity to the wheel or himself.

If two men carry a load slung from a pole resting on their shoulders, and the load be in the middle between them, they have an equal share of the weight; but in proportion as it is more towards one than the other, so is the extra amount of weight to the one nearer to it. The men are the fulcra in this case; they act in that capacity the one to the other, while both supply the moving power. Should the pole be 8 feet long, and the weight 200 lbs., each man will bear 100 lbs. weight. Suppose that a man and a boy are set to carry this weight, and the man, from the boy's inability to carry his equal share, out of humanity, places the weight three times as far from the boy as from himself; that is, 6 feet distance from the boy, and only 2 feet from himself, then the boy will only have 50 lbs. weight, while the man will have 150 lbs. to bear.

A hand-barrow is on the same principle, and one man may bear less or more as the load happens to be placed, or as the handles may be held to increase or lessen the lever.

The common operation of opening a door is an illustration of this lever: the hinges are the fulcra or centre of motion, the door is the resistance or weight, and the hand the moving power. The finger is painfully nipped when caught near the hinge, from that part being near the fulcrum, acted upon by a lever passing through a larger space. In opening a box the same thing is noticed.

The oar of a boat is also a lever of this kind, the water being the fulcrum, the person who rows the power, and the boat the resistance or weight. This lever is most powerfully employed in the coal-barges on the rivers in the north of England. These vessels retain the old Saxon name of keels, which is the term that distinguished the ships containing Horsa and Hengist, and their enterprising followers, on first coming to this country. They are in the form of half a walnut-shell, huge and unwieldy, and contain upwards of twenty-one tons of coals. The keel is propelled with one immense oar, wielded by three men remarkable for their muscular powers; they pull with all their might, adding the entire weight of their bodies. They do not sit, but move backward with the motion of the oar. Thus this heavy, clumsy barge has but the yielding water for a fulcrum, and yet is skilfully managed even among the waves of the ocean.

The masts of a ship act as levers, having the cargo or ballast and the vessel as the resistance, the bottom of the vessel as the fulcrum, and the sails holding the wind as the moving power. Thus well-equipped smuggling vessels and gentlemen's yachts, the masts of which seem enormously long for the size of the vessel, lean over in a perilous manner when in full sail, in consequence of the pressure on the levers.

Nut-crackers, lemon-squeezers, &c., are also illustrations of this kind of lever. The two legs are joined by a hinge, which is the fulcrum, the article placed between is the resistance, and the hand is the power.

The rudders of boats, ships, &c., are levers acting on the same principle.

Many are the industrial purposes to which this form of the lever is applied by chemists, grocers, chaff-cutters,

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coopers, patten-makers, &c., &c. The wooden soles of the shoe called a clog, at one time almost universally worn by boys and countrymen, were formed by this cutting lever. In snowy or wet weather, or where persons' avocations compel them to work amid wet or stand on cold stones, this ancient shoe is invaluable in the preservation of health, being warm and dry.

A cutting-lever is a common appliance in the country for bending down haystacks partially cut, and other loose light bodies that might be carried away by the wind; and it is even retained in some places for pressing cheese when in course of manufacture. A pole is stuck into the wall as a fulcrum, the resistance is the object to be pressed or held in its place, and at the other end are hung weights as the power.

The third description of lever is that in which the fulcrum is at one end, the weight at the other, and the power placed between them. At one time this was called the losing lever, because the power had to be greater than the weight. The advantages of it are now discovered and appreciated, consisting, as it does, in a small power causing the extreme point of a long arm to move over a great space; and it is one of those wonderful adaptations of the Divine Being in the construction of the appropriate mechanism of animals and man.

The mechanical power of the muscles of man, acting on the bones as levers, is one of a surprising nature in the combination of power, velocity, and beauty of construction. In the arm the elbow is the fulcrum, the muscles the moving power, and the weight raised the resistance. Thus, if the weight raised be 50 lbs., and the elbow passes through a space of 20 inches, the muscles springing from the shoulder will contract 1 inch, and the force be equal to 1000 lbs. The muscles being

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near the joints or fulcra, give great velocity to the other end of the lever, generating great momentum. In the human body, sometimes the fulcrum is between the power and resistance, as the elbow between the muscles of the shoulder and humerus, and the hand with the weight. In other places the resistance is intermediate, and the fulcrum at the end, as the toes on the floor, and the hinge of the lower jaw; and in parts the fulcrum is at the end and the power intermediate, as the weight of the arm has its fulcrum in the shoulder-bone, and the power is in the muscle covering and proceeding from the shoulder.

"Mechanics and Mechanism," by R. S. BURN.

1. LEVER, that which lifts or raises. (Lat. levo, to raise.) 2. FULCRUM, a prop or support, the fixed point on which a lever moves. Plural, fulcra. (Lat. fulcio, to prop.)

3. ARCHIMEDES, a celebrated geometrician, born at Syracuse 287 B.C. He attended the lessons of Euclid, and gained great distinction by his discoveries in mechanics, engineering, &c.

4. MANUAL, pertaining to the hand; a thing done, made, or used by the hand. (Lat. manus, the hand.)

5. MOMENTUM, the quantity of motion in a body.

to move.)

6. HUMERUS (Lat. the shoulder).

(Lat. moveo,

THE WHEEL AND AXLE.

THIS simple machine consists of a wheel w fixed upon an axle a. Suppose it took 3 inches of rope to go round the axle, and 1 foot of rope to go round the wheel, then the proportion would be as 1 to 4, and a weight of 1 lb. at the wheel, would support a weight of 4 lbs. at the axle. If the rope be wound round the axle in a different direction from that on the wheel, and an increase of weight be attached to the rope at w, then it would unwind and the

weight descend, while the rope on the axle would wind up and lift the weight fastened to it. Thus one power is made to act against another, as pointed out in the lever.

The wheel and axle are called a perpetual lever. In the diagram it will be seen how the term may properly be applied. The power p is the weight hanging from the wheel, the fulcrum is the centre of the axle, and the weight to be raised is that hanging from the axle. Now, if the distance from the edge of the wheel to the centre of the axle be 8 inches, and from the centre to the edge of the axle be 1 inch, and 1 lb. be the power hanging from the wheel, it will balance 8 lbs. hanging from the axle. A slight addition of power, then, would raise up the 8-lb. weight; but for every inch the weight rises, the power would descend through 8 inches of space. A handle inserted would act the same as a wheel. A lever would only raise the weight through a small space, while the wheel and axle will act as long as the length of rope will allow.

The larger the wheel and the smaller the axle the more powerful is the machine, but thegreater time is taken in raising the weight.