experienced. If the zinc be placed beneath the tongue, and the silver between the lips and gums, near the eyetooth, and the two metals be brought into contact, a distinct flash of light will be visible.*

Various facts in common life are explained by Galvanism. The superior flavour of porter when drunk from a pewter pot has been attributed to this action, the pewter being a mixture of two different metals; as also snuff having a different flavour when taken from a tin box in which part of the iron has become exposed, from that which it has when in contact with tin alone.

Electricity and Voltaism are probably the most interesting of all the sciences, from the numerous and splendid experiments which may be performed by their means, as also from their intimate connexion with the various operations of nature; yet, notwithstanding the most unwearied investigations of men of the highest science and talent, much yet remains undiscovered and unknown.

MECHANICAL POWERS.

Origin and advantage of Mechanical Powers-The different kinds of LeversWheel and Axle-Pulley-Inclined Plane-Wedge and Screw, each explained, with the principle of its action-Examples, &c. &c.

THE Mechanical Powers are engines used for raising great weights, moving heavy bodies, &c., without the aid of which many stupendous erections, as massive statues, lofty buildings, and various similar works of art, could not have been effected. Man, in the early stage of society, may be supposed to have soon discovered his

*This is caused by the irritation of the optic nerve, in a manner similar to that produced by a blow on the eye.

At the meeting of the British Association, lately held at Bristol, Mr. Crosse, of Broomfield, Somerset, stated that he had kept the Voltaic power in force for twelve months by water alone. He had obtained water from a crystallized cave, and by the action of the Voltaic Battery he had succeeded in producing from that water numerous rhomboidal crystals resembling those of the cave. He had also obtained blue and gray carbonate of copper, phosphate of soda, and 20 or 30 other specimens; and had not the least doubt but that diamond itself might be produced by Voltaic action.

weakness, and to have directed his attention to the attainment of artificial aid. It is natural to suppose that the Lever would be the first power of which he availed himself, as it is the most simple, and is also that on which all the other mechanical powers depend.

The Mechanical Powers are six in number; viz., the Lever, the Wheel and Axle, the Pulley, the Inclined Plane, the Wedge, and the Screw: in the various combinations of these all machines exist.

The Lever is chiefly used to raise heavy weights to small heights, as a handspike or crow, and is divided into three kinds. A lever of the first kind, is when the weight to be raised is at one end and the power at the other, and the fulcrum, or prop, is between the two. Thus, if a man be required to raise or remove a heavy log of wood, he will thrust a handspike, or any similar instrument, under the wood, and by placing another piece of wood or stone as a fulcrum under the handspike, and pressing down at the further end, he will probably effect his purpose. If the log to be raised be five hundred weight, and he can press with the force of one hundred weight, he will be able to raise it by placing a lever six feet long over a fulcrum at one foot from the weight to be raised, and pressing with the above-named weight at the other end. For, as the distance of the weight from the fulcrum is to the distance from the fulcrum to the power, so is the force applied to the effect produced.

It is a general rule in mechanics that what is gained in power is lost in time; so in the above there will be a gain of power in proportion of five to one, but then the hand of the operator will go over a space five times that of the log. The steelyard used in weighing hay, meat, and other things, is a lever of this kind. Pincers, snuffers, scissors, &c., are also compound levers of this kind, the joint round which they move being the fulcrum.

A Lever of the second kind is when the fulcrum is at one end, the power at the other end, and the weight to be raised between the two. This is exemplified by a man drawing a plug; he thrusts the end of the handspike through the ring, and resting it on the ground, he elevates the end he holds, and thus draws the plug. The advan

tage gained is in the same proportion as in the former lever. Cutting-knives used by patten-makers, doors, oars of a boat, the rudder of a vessel, &c., are levers of this kind. A pair of bellows is a compound lever of the second kind. This lever shows how two men carrying a weight on a pole, as brewers' men do a cask, or chairmen a sedan-chair, may bear an unequal proportion of the weight. If the weight be exactly in the middle of the two, each will support an equal share of the burden; but if it be nearer the one than the other, he to whom it is nearest will have the heaviest weight, and that in inverse proportion to his distance from it, as compared with that of the other man.

The third kind of lever is when the power is between the fulcrum and the weight. A ladder raised by a man against a wall is a lever of this kind, and so is the human arm, and the limbs of animals generally. In this kind of lever there is a loss of power in proportion to the distance between the power and the fulcrum, compared with the distance between the power and the weight. The muscles of the human arm for this reason exert a force of one hundred pounds to raise a weight of ten pounds, the distance from the acting muscles to the elbow being about one-tenth of the distance from the muscles to the hand. The disadvantage is here made up by the compactness and convenience of the motion, the muscles being sufficiently powerful to produce the effect required.

The Wheel and Axle consist of a wheel fixed to the end of an axle, so that they both turn round together. This power is sometimes named the perpetual lever, it being in reality a lever on whose arms the power and weight may always act perpendicularly, although the lever turns round its fulcrum. Cranes are of this nature; church-bells are moved by the wheel and axle, as also are the helms of ships. The advantage gained is in proportion as the diameter or circumference of the wheel is to the diameter or circumference of the axis: thus, if the diameter of the axis be one foot, and that of the wheel be ten feet, a man with a force of one hundred weight will be able to raise ten hundred weight.

The Pulley is a small wheel turning on an axis, with

a rope passing over it. In a single fixed pulley there is no advantage except convenience; for, as the weight is at one end of the rope and the power at the other, a person will be able to use only his weight in raising a body, and not his strength. As no advantage is obtained, this cannot be correctly said to be a mechanical power. In a single moveable pulley the advantage is as two to one. In a system of pulleys the advantage is found by multiplying the number of moveable pulleys by two.

The Inclined Plane is usually made by a plank laid aslope, by means of which a heavy body may be easily raised or lowered. The advantage obtained in the inclined plane is in proportion as the length of the plane is to its height: thus, if a person who can use a force of one hundred weight has to raise a weight of ten hundred weight one foot, he can effect it by means of a plank ten feet long, but the body will have to travel over ten times the space. A body moving down an inclined plane acquires the same velocity as in falling perpendicularly through the height of the plane, and its rate of moving is in proportion to the square of the time: thus, it moves four times as far in two seconds as in one. The Chisel, the Adze, and similar instruments, are referred to this power.

The Wedge may be considered a double inclined plane, and is useful in cleaving blocks of wood and stone. The advantage gained by the wedge is as the thickness of the back is to the length of the two sides. The wedge possesses a great superiority over the other mechanical powers, as it is capable of being impelled by percussion, so that by the simple blow of a heavy mallet an immense resistance may be overcome. To the wedge may be referred the Axe, the Spade, Knives, Needles, and all sorts of instruments which begin from edges or points, and become gradually thicker as they lengthen. Birds flying are of a wedge-like shape, and so are Fish swimming, for the purpose of cleaving their respective elements. The ancients occasionally drew up their armies in the shape of a wedge, in order to break the line of their opponents.

The Screw is a kind of inclined plane wound round a cylinder, the power of which is increased by a handle or

lever. The uses to which the screw is applied are various; bodies are pressed together by it, as with the common tablecloth and napkin press, the bookbinder's press, the packer's press, &c. The screw is also sometimes used to raise the parts of a building that may have settled or sunk into the ground, through the foundation giving way; and if the material be sufficiently strong, one man with a good leverage may raise an immense weight. The power gained by the screw is in proportion as the circumference made by the moving power is to the distance between the threads of the screw thus, if the distance between the threads of the screw be half an inch and the lever used be three feet, the circumference made by the moving power will be eighteen feet, or 216 inches. The advantage obtained will then be as 216 are to one-half, or as 432 to one, so that a man who can push with the force of a hundred weight may produce a power (setting friction aside) of nearly equal to

22 tons.

In the screw and the wedge there is a very great friction to overcome, independent of the weight, sometimes as much as nearly amounts to the weight raised; but then the friction is of use to retain the weight and the machine in their places after the power is removed. If machines could be made without friction, the least degree of power beyond that which balances the weight would be sufficient to raise it.

In the lever the friction is very little, as also in the wheel and axis; in pulleys it is very considerable, and in the inclined plane, wedge, and screw, it is very great. In machines in general one-fourth and sometimes onethird is allowed for friction. Mr. Wyatt considers onefourth lost in a steam-engine by friction. In pulleys friction may be reduced one-half by grease. The least friction is produced when polished iron moves on brass. The mechanical arts are of the greatest importance to mankind, and the extent to which machinery has been carried is truly wonderful. It is by a skilful combination of the mechanical powers that such astonishing effects are produced, as are witnessed in the manufactory of different articles, and which have raised England so much above other nations. At Stroud, in Gloucestershire, there is a