Puslapio vaizdai

longer arm. It may, perhaps, be objected to pulleys, that a longer time is required to raise a weight with their aid than without it. That is true, for it is a fundamental law in mechanics, that what is gained in power is lost in time; this applies not only to the pulley, but to the lever and all the other mechanical powers. It would be wrong, however, to suppose that the loss was equivalent to the gain, and that we derived no advantage from the mechanical powers; for since we are incapable of augmenting our strength, that science is of wonderful utility which enables us to reduce the resistance or weight of any body to the level of our strength. This we accomplish, by dividing the resistance of a body into parts, which we can successively overcome; and if it require a sacrifice of time to attain this end, you must be sensible how very advantageously it is exchanged for power. The greater the number of pulleys connected by a string, the more easily the weight is raised; as the difficulty is divided amongst the number of strings, or rather of parts, into which the string is divided by the pulleys. Several pulleys thus connected form what is called a system, or tackle of pulleys. You may have seen them suspended from cranes to raise goods into warehouses, and in ships to draw up sails. Here both the advantages of an increase of power and change of direction are united; for the sails are raised up the masts by the sailors on deck, from the change of direction which the pulleys effect; and the labour is facilitated by the mechanical power of a combination of pulleys. Pulleys are frequently connected, as described, both for nautical and a variety of other purposes; but, in whatever manner pulleys are connected by a single string, the mechanical power is the same in its principle.

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The third mechanical power is the wheel and axle. Let us suppose the weight w to be a bucket of water in a well, which is to be raised by winding the rope, to which it is attached round, the axle; if this be done without a wheel to turn the axle, no mechanical assistance is received. The axle without a wheel is as impotent as a single fixed pulley, or lever, whose fulcrum is in the centre; but add the wheel to the axle, and you will immediately find the bucket is raised with much less difficulty. The axle acts the part of the shorter arm of the lever, the wheel that of the longer arm. The velocity of the circumference of the wheel is as much greater than that of the axle, as it is further from the centre of motion; for the wheel describes a large circle in the same space of time that the axle describes a small one, therefore the power is increased in the same proportion as the circumference of the wheel is greater than that of the axle. If the velocity of the wheel were twelve times greater than that of the axle, a power nearly twelve times less than the weight of the bucket would be able to raise it.

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The fourth mechanical power is the inclined plane. This is nothing more than a slope, or declivity, frequently used to facilitate the drawing up of weights. It is not difficult to understand, that a weight may with much greater ease be drawn up a slope than it can be

raised the same height perpendicularly. But in this, as well as the other mechanical powers, the facility is purchased by a loss of time; for the weight, instead of moving directly from a to c, must move from в to c, and as the height of the plane is to its length, so much







is the resistance of the weight diminished. Thus, if a pulley be fixed at F, so that the string from F to w may be parallel to в c, and a string fixed to the weight w were connected with another weight P; then if P bear the same proportion to w that the line a c does to the line B C, the two weights will balance each other, a considerable portion of the weight w being supported by the plane B C, and only the residue by the power P.

The wedge, which is the next mechanical power, is composed of two inclined planes. Woodcutters some


times use it to cleave wood. The resistance consists in the cohesive attraction of the wood, or any other body which the wedge is employed to separate; and the

advantage gained by this power is in the proportion of half its width to its length. The wedge, however, acts principally by being struck, and not by mere pressure; the proportion stated, is that which expresses its power when acting by pressure only.

All cutting instruments are constructed upon the principle of the inclined plane, or the wedge. Those that have one edge sloped, like the chisel, may be referred to the inclined plane; whilst the axe, the hatchet, and the knife, (when used to chop or split asunder,) act on the principle of the wedge. But a knife cuts best when drawn across the substance it is to divide, as it is used in cutting meat; for the edge of a knife is really a very fine saw, and therefore acts best when used like that instrument.


The screw, which is the last mechanical power, is more complicated than the others. It is composed of two parts, the screw and the nut. The screw s is a cylinder, with a spiral protuberance coiled round it, called the thread; the nut N is perforated to contain the screw; and the inside of the nut has a spiral groove, made to fit the spiral thread of the screw; just like the lid of a box which screws The handle which projects from the nut is a lever, without which the screw is never used as a mechanical power. The nut,




with a lever L attached to it, is commonly called a winch. The power of the screw, complicated as it appears, is referable to one of the most simple of the mechanical powers, the inclined plane. If a slip of paper be cut in the form of an

inclined plane, and wound round a
pencil, which will represent the cylin-
der, it will describe a spiral line corres-
ponding to the spiral protuberance of
the screw.
The nut then ascends an
inclined plane, but ascends it in a spiral
instead of a straight line. The closer
the thread of the screw, the more easy

is the ascent; but the greater are the number of revolutions the winch must make; so that we return to the old principle, what is saved in power is lost in time. The power of the screw may be increased, also, by lengthening the lever attached to the nut; it is employed either for compression or to raise heavy weights. It is used in cider and wine presses, in coining, in bookbinding, and for a variety of other purposes.

All machines are composed of one or more of the six mechanical powers we have examined. One more remark must be made relative to them, which is, that friction in a considerable degree diminishes their force. Friction is the resistance which bodies meet with in rubbing against each other. There is no such thing as perfect smoothness or evenness in nature. Polished metals, though they wear that appearance, more than any other bodies, are far from really possessing it; and their inequalities may frequently be perceived through a good magnifying glass. When, therefore, the surfaces of two bodies come into contact, the prominent parts of the one will often fall into the hollow parts of the other, and occasion more or less resistance to motion. In proportion as the surfaces of bodies are well polished, the friction is diminished; but it is always considerable, and it is usually computed to destroy one-third of the power of a machine. Oil or grease is used to lessen friction; it acts as a polish by filling up the cavities of the rubbing surfaces, and thus making them slide more easily over each other. It is for this reason that wheels are greased, and the locks and hinges of doors oiled. In these instances, the contact of the rubbing surfaces is so close, and the rubbing

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