common iron crow, for moving a block of stone or so, which rests on the one end, and the man applies his strength as a power at the other, while a small stone near the lower end serves as a fulcrum or prop on which the lever turns in its motion. Suppose a man press on the upper end of the crow with a power equal to 50 pounds, and at a distance of 48 inches from the prop, it is evident that he will be able to balance a weight of 400 pounds, placed within 6 inches of the prop, at the other end of the crow, (for 50 times 48 are equal to 6 times 400), and that by a little increase of his power, he will be able to raise that weight. Of the same sort is the poker in stirring the fire; the hand is the power, the coal in the grate is the weight to be moved, and the bar on which the poker rests is the fulcrum or prop. Of this kind of lever is also the common balance, although it be supported by suspension from above, and not upon a prop fixed below; but as the prop is placed in the middle of the lever, it is of no service in raising weights, because the power must be greater than the weight before the latter be moved. From this property of the first sort of lever is derived the Roman statera, or steel-yard, before mentioned. The point of suspension is placed very near one end of the beam; but that is made so massy as exactly to equal the weight of the long slender arm. At the end of the short arm is hung the scale containing the goods to be weighed, and the long arm is divided into a number of parts, each equal to the shorter arm. If any weight as I pound be placed at the 1st division, it will just balance 1 pound in the scale at the short end: but if it be placed at the 2d division, it will balance or weigh 2 pounds in the scale; if at the 3d division, 3 pounds, at the 4th division 4 pounds, &c. The 2d sort of lever has the prop at one end of the lever, and the power at the other, and the weight to be raised between them. The advantage gained by this sort, as in the former, is as great as the distance between the power and the prop exceeds that between the weight and the prop. When a man employs a crow or handspike, to move a block of stone in a forward direction, the point of the crow rests on the ground, under the block, which presses upon the crow a little above the end, and a moderate power raising up the other end of the crow, forces the block to move forward. Two men carrying a load upon a pole resting upon their shoulders, have the weight divided equally between them: but if the load be placed nearer to the one than the other, the nearest man will have a greater share to bear, in proportion as the distance between the load and the other man is increased; and if the nearest man come directly under the load, the whole weight will fall upon him. Suppose two men carry a load of 180 pounds on the middle of a pole resting on their shoulders, 6 feet asunder: in this case each will carry one half or 90 pounds: but let the load be placed within 2 feet of the hindmost man, and 4 feet from the foremost, the distances being as 1 to 2, the hindmost man will have to carry 120 pounds while the foremost carries only 60. By attention to this circumstance two porters of very unequal strength, may be made to bear just that share of a common burthen which is suited to the respective powers of each; daily experience, however, shows how little it is attended to, or rather how little it is understood. In the same way two horses, oxen, &c. of unequal strength, might be yoked so as to draw in proportion to their power, by placing the weaker animal as much farther from the drawing point of the beam as his strength is inferior to

that of the stronger. The oar of a boat is a lever of the 2d blade rests in the water as a prop, the power, of the rower i the other end, and the weight to be moved, that is the boat, the two. Every door of a house or room is a lever of the s hinges are the props or centres of motion, the weight is the of the door, and the power is the hand applied to the outer door, by which it may be easily moved. But if, by fixing a a nail in the door, near to the hinges, we try to open and force required will be very considerable, because so near t motion. Cutting knives fastened at one end, used by coopers and chaff-cutters, are levers of the 2d order. In levers of th the weight is at one end and the prop at the other, and th applied between them. To this sort of lever are referred t a man's arm; for when he lifts a weight by the hand, the exerts its power is fixed to the bone, about one tenth part of t of the hand below the elbow, which is the prop or centre the muscle must then exert a power ten times as great as to be raised by the hand. This, however, has no relation weight from the ground by the arm stretched down perpend which a much greater weight may be raised. This kind of the least advantageous of all, is therefore seldom employed occasions, however, it is indispensable, as in raising a long against a high wall, when one end is kept fast on the groun ladder is raised by strength of men's arms. When we draw of a piece of wood by means of a clawed hammer, we emp of the 1st order. If the shaft of the hammer be 5 times as iron claws, which draw the nail, the lower part of the head the board as a prop, then by pulling the end of the shaft ba man will draw the nail with one fifth part of the force that requisite to draw it out with a pair of pincers.

The good qualities of a balance depend much upon the circumstances. The arms of the beam ought to be exactly as to weight and length: the points from which the scales h be in a right line passing through the centre of gravity of for by this the weights will act directly against one anoth fulcrum or point of suspension of the beam be placed in th gravity, the beam will have no tendency to take one position another, but will remain steady in any, in which it may if the centre of gravity of the beam be immediately above th it will overset by the smallest action, the lowest end descer the upper part of the beam, if at liberty to turn over, bed lower-but if the centre of gravity of the beam be immedia the point of suspension, the beam will never rest in any p when perfectly level or horizontal; and if put out of that po then left at liberty, the beam will vibrate up and down un at last to the level. From these particulars it follows, that, balance, the point of suspension or support should be pla above the centre of gravity of the beam; its vibrations or up and down will be quicker, and its tendency to the level p be stronger, the lower the centre of gravity, and the less upon the point of support. The friction of the beam upo ought to be reduced as much as possible; the axis should with an edge like a knife and extremely hard.

The wheel and axle are employed in a great variety of ways; the power being applied to the wheel, and the weight to the axle. Suppose a wheel of 6 feet diameter, and consequently of 18 feet 10 inches circumference, be fixed on an axle of 1 foot diameter, or 3 feet 1 inch circumference. Let a rope be placed round the rim of the wheel by which the moving power is to be applied, and another round the axle, but in a contrary direction, by which the weight is to be raised. If the rope round the wheel be pulled until it go just once round, just as much rope will be drawn off as is equal to the circumference of the wheel. But while the wheel turns once round the axle does the same, and consequently the rope by which the weight is suspended will wind once round the axle, and the weight will be raised through a space equal to the circumference of the axle. The velocity of the power will therefore be to that of the weight in the proportion of the circumference of the wheel, to that of the axle; or the power will be to the weight raised as the diameter of the axle is to that of the wheel. But the diameter of the axle being 1 foot, and that of the wheel 6 feet, it follows that a power of 1 pound applied to the circumference of the wheel will balance a weight of 6 pounds suspended from the axle, and consequently that 6 pounds will be raised by a little more than 1 pound at the wheel. In this, and similar machines, the wheel and axle act as perpetual levers, of which the support is the centre of the axle of both, and the arms are the diameters of both. Hence it is evident, that the greater the diameter of the wheel in proportion to that of the axle, the greater will be the power of the machine; but the weight will rise proportionally dower. The capstan and the windlass of a ship act in the same way as the wheel and axle, in raising the anchor, or other weight; the bars or handspikes being like the spokes of a wheel without a rim. An axle moved by a crooked handle or winch acts precisely on the same principle.

If teeth be cut in the circunference of two wheels of the same size, A and B, and in the same number, if these wheels are made to work in one another, it is evident that they will both turn round in the same time, although in opposite directions; and a weight attached to the axle of the second wheel B, will be raised in the same time as if it had been attached to the first wheel A. But if the teeth of B be made to work not in the teeth of the circumference of A,but in others cut in its axle, then A must go round as much faster than B, as the number of teeth in B exteeds that of the teeth in the axle of A. If the teeth in the circumference of B be 72, and those in the axle of A be 12, it is evident that A must make 6 revolutions in the time of one revolution of B. In the same manner, three, four, or more wheels may be made to work in one another, and by duly proportioning the several wheels and axles to each other, any requisite degree of power may be produced. To this sort of engines belong cranes for raising great weights: but in these the wheel often has cogs instead of teeth, and a small trundle is made to work in the cogs, and turned by a winch. In all machines of this sort, it is necessary to apply a ratchet wheel, on the end of the axle, to which the weight is attached, with a catch to fall into its teeth. This will at any time support the weight, and keep it from descending, if the person who turns the handle should, through accident or carelessness, quit his hold of the winch, while the weight is raising. By this means the

danger would be prevented which has but too often happened, by the running down of the weight when unsupported.

The Pulley is a small wheel turning on an axis, with a drawing rope passing over the wheel: the wheel is usually called the sheave, and it is so fixed in the case or block as to turn on a pin, passing through its centre. Pullies are of two sorts, fixed and moveable. When a pulley is fixed to some elevated place, two equal weights attached to the ends of a rope passing over it, will balance each other; and if one of them be pulled down a certain distance, the other will rise an equal distance. This kind of pulley, therefore, gives no mechanical advantage; so that a man can raise no greater weight by it than he could by his natural strength. It is nevertheless of great use by changing the direction of the power and applying it with convenience. By it a man can raise a weight from the ground to any height, without stirring from his place, instead of carrying it up a long stair; by it he can employ not only his strength, but his bodily weight, in raising a load from the hold of a ship or a cellar: by it also, a number of men may be employed in raising a heavy body, which only two or three could conveniently handle without it; for any number of hands may be applied to the rope. The moveable pulley is fixed to the weight, and rises or falls with it. One - end of the rope passing round this pulley is made fast to the frame above, by which the whole is supported, and the other is carried over another pulley made fast to the same frame above: the weight is suspended by a hook immediately under the centre of the moveable pulley. This pulley acts as a lever of the 2d order; the side of the wheel next to the fast end of the rope, is the fulcrum or prop, the power is the moveable rope at the other side of the wheel, and the weight is applied midway between the two. Consequently, any given weight at the centre may be raised by one half of that weight at the moveable rope, a man may therefore, by this moveable pulley, which rises with the weight, raise double the weight that he could raise by the fixed pulley. Or the effect of the moveable pulley may be considered in this way: the pulley is supported by two parts of a rope, the one fixed, the other moveable, which are equally stretched; each part must therefore bear one half of the weight; consequently a power equal to one half of the weight, exerted on the moveable part of the rope will be sufficient to balance the weight. In this way a number of pullies may be combined, by which very considerable weights may be raised by comparatively small powers. From the nature of blocks and ropes, however, and from the great friction occasioned by heavy loads, the power of a combination of pullies is much more limited in practice than might be expected. Considerable improvements were lately made on blocks, by employing, instead of a number moving separately, two of solid brass cut into grooves of different diameters: by this contrivance, the power was increased to the proportion of ten to one. If instead of one continued rope going round all the pullies, the rope belonging to each pulley be made fast at top, a different proportion between the power and the weight will be produced. In this case, each succeeding pulley will double the power of the one before it; if two pullies be employed, the power will sustain four times the weight, if three pullies, eight times the weight, if four pullies, sixteen times the weight, and so on. A pair of blocks with the rope fastened round them is termed a tackle. The inclined plane is of great use in rolling heavy bodies, as caska,

wheel-barrows, &c. up to a height to which it would be very difficult to lift them directly. The force with which any body descends upon a slope, or inclined plane is to the force with which it would fall perpendicularly to the same level, as the height of the slope is to its Leugth. Let the length of the slope be twelve feet, and the difference of level between its upper and lower ends be four feet, the force by which a body would roll down the slope, will be to that by which it would fall that height, is as four to twelve. Consequently reversing the case, a power equal to four, would be sufficient to support the body on the slope, while it would take a power three times as great, or equal to twelve, to support the body in the open air. Hence a cask, a cylinder, or any other rolling body may be moved up such an inclined plane, by a power a little more than one third part of that which would be necessary to lift the body directly to the same height.

From hence it is evident, that the longer the slope, in proportion to its height, the less power will be necessary to roll any body up it: and to this inclined plane may be reduced hatchets, chisels, and other edged tools, sloped only on one side.

The Wedge may be considered as formed of two planes equally inclined and joined together, and is used for cleaving and separating wood, stone, &c. This effect it would be impossible to produce by the lever, the wheel and axle, or the pulley: for the force of the blow applied to the back of the wedge, not only separates the parts which it touches, but shakes the adjoining parts, and renders them more disposed to separate when another blow is given to the wedge. It was befure said that the momentum of any body in motion, is always equal to its weight, or quantity of matter multiplied by its velocity. The force given to a wedge is generally applied by a smart stroke, and not by dead pressure: for a sharp blow with even a small hammer will overcome more resistance than the constant pressure of a heavy weight; and a blow from a blacksmith's sledge-hammer will, in cleaving wood and stone, overcome a resistance of several tuns weight: nor have we any way of comparing the force of a blow with that of mere pressure or dead weight. In certain parts wedges of dry wood are used to separate blocks and layers of stone, by driving a number into the line where the separation is intended, and then moistening their outward ends with water, which penetrates into the wood, swells it, and thus by a gradual action, without violence, raises the bed of stone above the wedges from the lower mass. Axes, and such chisels as are sloped off to an edge from each side, act on the principle of the wedge.

The Screw can scarcely be considered as a simple mechanical power: for it is never used without the assistance of some of the others, as a lever handle or winch to turn it. The nature of the screw may be understood in this way. Out of a piece of strong paper cut a right-angled triangle, having the perpendicular much longer than the base. Fasten this base to a cylinder, and wind the paper round it, making the perpendicular side always to fall in the same place, by which means the inclined side will, at every turn of the cylinder, draw nearer and nearer to the perpendicular side; and at last coincide with it, at the angular point of the paper. In this way the inclined side of the paper will represent the winding or spiral groove or thread of the screwWhen this spiral thread is formed on the outside of a cylinder, it becomes a male screw; and when the is formed in the inside of a groove