well Zemzem, in another part of the same square, of which they take large draughts, and undergo A final cerea thorough ablution in its waters. mony is the pilgrimage to Mount Arafat, about thirty miles to the south of the city. The climate

MECHANIC, adj. & n. s.

MECHANICS, n. s. MECHANICAL, adj. MECHANICALLY, adv. MECHANICALNESS, n. 8. MECHANICIAN, MECHANISM.

here is very sultry, and there are even instances recorded of persons being suffocated in the streets. The balm of Mecca is only to be found in small quantities, but is not manufactured here. Long. 40° 15′ E., lat. 21° 18′ 9′′ N.

Fr. mechanique; Lat. mechanicus; Gr. μηχανη. Re-lating to machinery: a constructor of machines: mechanics is the science or doctrine of their construction. See below. Mechanician, an adept in, or student of,

mechanics or mechanical constructions: mechanism, action, or construction, according to the laws of mechanics.

Know you not, being mechanical, you ought not to walk upon a labouring day, without the sign of your profession? Shakspeare.

Hang him, mechanical salt-butter rogue! I will stare him out of his wits; I will hew him with my cudgel.

Mechanick slaves,

Id.

With greasy aprons, rules, and hammers, shall
Uplift us to the view. Id. Antony and Cleopatra.
Do not bid me

Dismiss my soldiers, or capitulate
Again with Rome's mechanicks.
Id. Coriolanus.

In the youth of a state, arms do flourish; in the middle age of a state, learning; and then both of them together for a time; in the declining age of a Bacon. state, mechanical arts and merchandise.

To make a god, a hero, or a king, Descend to a mechanick dialect. Roscommon. Some were figured like male, others like female Screws, as mechanicians speak.

Boyle. Many a fair precept in poetry is like a seeming demonstration in mathematicks, very specious in the diagram, but failing in the mechanick operation.

the rest.

Dryden.

They suppose even the common animals that are in being, to have been formed mechanically among Ray. A third proves a very heavy philosopher, who sibly would have made a good mechanick, and have done well enough at the useful philosophy of the South. spade or the anvil.

pos

motion.

Harris.

Newton.

one,

but is the result from the mechanical composition of the whole animal.

Pope. The rudiments of geography, with something of mechanicks, may be easily conveyed into the minds of acute young persons. Watts's Improvement of the Mind. MECHANICS. In furnishing our readers with a condensed treatise on this important branch of physics, it may be advisable, in the first instance, to examine the simple mechanical powers; by means of which we are enabled to adapt animal labor to the useful arts of life: their combination in the construction of machines will then be better understood.

The tyro is frequently disposed to imagine that, by means of the mechanical powers, a real increase of power is obtained; this, however, is not the fact. For instance, if a man be just able to convey 1 cwt. from the bottom to the top of his house in one minute's time, no mechanical engine would enable him to convey 3 cwt. to the same height in the same time; but the engine will enable him to convey 3 cwt. in three minutes; which amounts to the same thing as to say that the man could, without the engine, carry the 3 cwt. by going three times to the top of the house, and carrying 1 cwt. at a time, provided the load admitted of its being so divided. Therefore the engine increases the effect of a given force by lengthening the time of the operation; or (since uniform velocity is proportional to the time), by increasing the velocity of that force or power.

The power, or acting force, is so far from being increased by any machine, that a certain part of it is always lost in overcoming the resistance of mediums, the friction, or other unavoidable imperfections of machines; and this loss in some compound engines is very great.

Every machine, however complex it may be, must consist of some combination of the follow

ing simple machines, which are commonly called the mechanical powers.

1. The lever.

2. The wheel and axle. 3. The pulley.

4. The inclined plane

5. The wedge.

6. The screw.

This classification of the elements of machinery, although very simple when considered with respect to the extent and power of the results which spring from it, may be still farther simplified; not because any of the six machines, which we have just enumerated, admits of being resolved into more simple parts, but because some of them are identical in principle, and different only in appearance. We shall show, hereafter, that the wheel and axle is, in fact, a lever, and that the wedge and screw are only modifications of the inclined plane; so that it follows, that all the varieties of simple machines may be reduced to three :

1. The lever. 2. The pulley.

3. The inclined plane.

The lever is the simplest of all machines, and is only a straight bar of iron, wood, or other material, supported on, and nioveable round, a prop, called the fulcrum.

In the lever there are three circumstances to be principally attended to: 1. The fulcrum, or prop, by which it is supported, or on which it turns as an axis, or centre of motion. 2. The power to raise and support the weight. 3. The resistance, or weight to be raised or sustained. The points of suspension are those points where the weights really are, or from which they hang freely.

The power and the weights are always supposed to act at right angles to the lever, except it be otherwise expressed.

The lever is distinguished into three sorts, according to the different situations of the fulcrum, or prop, and the power, with respect to

each other.

In the lever of the first kind the fulcrum is placed between the power and the resistance; in the second kind the resistance is between the fulcrum and the power; and in the third kind the power is applied between the fulcrum and the resistance; and the power gained will in all cases be in proportion to the relative lengths of the two arms in the saine lever. Plate I. MECHANICS, fig. 1, represents a lever of the first kind in one of its most common applications; viz. raising a large stone or weight d, and when used in this way it is, in practice, most commonly called a crow-bar, or hand-spike. The entire bar ef is, however, a lever, and the stone, or block g, is the fulcrum upon which it turns ; consequently, from g to f (where the power is to be applied) will be the acting part of this lever, and from g to e its resisting part. The power gained by this form of the lever is as the resist ing is contained in the acting part, or as fg is to ge; so that if the lengths fg and ge be taken, and fg be divided by ge, the quotient will be the power gained. In the plate eg is contained twice in gf, or will gain a power of two, so that, whatever the weight of d may be, it will only appear to weigh half as much at f; and, if the fulcrum g were removed so much nearer to e that the new distance between e and g should be contained twenty times in the distance gf, then a power of twenty would be gained, or d would be lifted at f with a power equal to one-twentieth of its real weight. In this case, however, f would

have to descend twenty times as far as e woL rise, and therefore a very small motion word b communicated to d.

By the same rule, if moving the fulcra towards the resistance d increases the power: the lever at f, so will removing it in a conte direction, or towards f, decrease it. Thus #ga supposed to be placed exactly half way bever e and f, so as to make the acting and reas. armis equal, then no power at all will be g atƒ; because, whatever may be the we placed on e, an equal power will be necessary: move it at f; and as both e and ƒ will is a ing perform equal arcs of circles, or move th equal distances in equal times, so no power be gained, this being in direct opposition to te laws of motion. Again, if the fulcrum g best posed to be placed nearer to f than it is to g,the when moved will pass through a larger space tha f, therefore e will generate more momentunt can be generated at f; and consequently apparent weight of d, as felt at f, will be te j creased, or a greater power must be exerted a than is equivalent to the weight of d. Ca such an arrangement it will be evident t although the whole is a lever, it cannot be az chanic power, unless it is admitted that from fr g is the resisting end, and from g to e the a one; in this case, the power must be applied o c instead of ƒ, and now it will be seen that it a same lever as in the first case, but with its reversed.

In speaking of the power of levers, under their several modifications, it must be obsere. that they must be considered as without we and, in making accurate experiments upon th the weights of their several parts must be balanced, otherwise the result will be affec Thus, in the preceding figure, if the bar ej of equal size throughout, it is evident te would be twice as much matter in the arm gra in the shorter one eg, and this additional qu tity of matter would preponderate and down the end f; thus appearing to increase assist the power. This difficulty will be me: theory by considering the whole bar ef as w out weight, and in practice or experiment it mus be obviated by hanging on a weight, or make the shorter arm eg so much thicker, as to cas it to be an exact balance for the longer arg when the lever is not in action.

A sufficient illustration of the first form of th lever may be found by reference to an ordiary poker. This, when employed to stir the te will have for its axis the bar upon which it rest the coals are the weight to be overcome, wh's the hand may be considered as the power ployed; and it is also with reference to this r chanical power that we construct the steel-y

BA, fig. 2, is a lever of the first kind. spported by an inflexible point O; and, as the to tance from O to B is twice as great as it sc O to A, the hand C, by exerting a force of o pound, may be placed in equilibrio with a wer of two pounds, or twice the amount. From this it will be seen that the length of the at OB, multiplied into the power C, equals (/i multiplied into the weight D, and as such the power and the weight must be to each co

inversely as their distances from the fulcrum. To further illustrate the matter let us suppose OB, the distance of the power from the prop, to be twenty inches, and ÓA, the distance of the weight from the prop, to be eight inches, it will then be found that a weight of five pounds may be supported by a power of only two pounds, because the distance of the weight from the f.crum eight, multiplied into the weight five, makes forty, and as such the longer arm will only require a power of two, which, multiplied by twenty, will produce the same amount.

Let AG, fig. 3, represent an horizontal lever at rest. At the point B, equi-distant from A and G, is placed the fulcrum D, and at the extremities A and G are hung the equal weights E and F. Then the lever will continue at rest, the weights E and F being in equilibrio. For it is evident that, if the line AG be moved on the fulcrum B, its extremities A and G will each be carried with equal velocities in the periphery of the same circle. And, because AG is horizontal, the actions of the weights E F will be in direction at right angles to its length; that is to say, they will act in the direction of tangents to the said circle at the points A and G; or they will act in the direction of those particles of the periphery which may be imagined to coincide with the tangents. Each pressure, therefore, tends to move the correspondent extremity of the line AG in that very line of direction in which only it can move. Suppose the pressure at G to be removed, and the whole pressure at A will be employed in depressing the point A, or, which is necessarily in this case the same, in raising the point G: on account therefore of the equal velocities of the points A and G, the action of E at the point A will be the same as if it were exerted at G in the direction of the tangent CG. But again, suppose the equal weight F to be restored, and the point G will then be acted on by two equal and opposite forces, which, destroying each other's effect, will not produce motion, consequently the lever will continue at rest.

It is likewise evident that, if the radii A Band BG be not in a right line, the equal forces will nevertheless be in equilibrio, if they be applied in the directions of the tangents: thus, if BG be bent to the position BK, and the force F be there applied in the direction KH, the equilibrium will remain as before.

What is called the hammer lever differs in nothing but its form from a lever of the first kind. Its name is derived from its use, that of drawing a nail out of wood by a hammer.

Suppose the shaft of a hammer to be five times as long as the iron part which draws the nail, the lower part resting on the board, as a fulcrum; then, by pulling backwards the end of the shaft, a man will draw a nail with one-fifth part of the power that he must use to pull it out with a pair of pincers; in which case the nail would move as fast as his hand; but with the hammer the hand moves five times as much as the nail, by the time that the nail is drawn out.

Let AC B, fig. 4, represent a lever of this sort, bended at C, which is its prop or centre of motion. P is a power acting on the longer arm A C, at A by means of the cord DA going over

the pulley D; and W is a weight or resistance acting upon the end B of the shorter arm C B. If the power be to the weight, as CB is to CA, they are in equilibrio: thus, suppose W to be five pounds, acting at the distance of one foot from the centre of motion C, and P to be one pound, acting at A, five feet from the centre C, the power and weight will just balance each other. Of levers of the first kind there are numerous instances. Scissars, pincers, snuffers, and all similar instruments, consist of two levers, of which the rivet by which they are united is the common fulcrum.

The balance, an instrument of very extensive use in comparing the weight of bodies, is a lever of the first kind, whose arms are of equal length. The points from which the weights are suspended, being equally distant from the centre of motion, will move with equal velocity; consequently, if equal weights be applied, their momenta will be equal, and the balance will remain in equilibrio.

In order to have a balance as perfect as possible, it is necessary to attend to the following circumstances.

1. The arms of the beam ought to be exactly equal, both as to weight and length.

2. The points from which the scales are suspended should be in a right line, passing through the centre of gravity of the beam; for by this the weights will act directly against each other, and no part of either will be lost, on account of an oblique direction.

3. If the fulcrum, or point upon which the beam turns, be placed in the centre of gravity of the beams, and if the fulcrum and the points of suspension be in the same right line, the balance will have no tendency to one position more than another, but will rest in any position it may be placed in, whether the scales be on or off, empty or loaded.

If the centre of gravity of the beam, when level, be immediately above the fulcrum, it will overset by the smallest action; that is, the end which is lowest will descend; and it will do this with more swiftness, the higher the centre of gravity be, and the less the points of suspension be loaded.

But, if the centre of gravity of the beam be immediately below the fulcrum, the beam will not rest in any position but when level; and if disturbed from that position, and then left at liberty, it will vibrate, and at last come to rest on the level. In a balance, therefore, the fulcrum ought always to be placed a little above the centre of gravity. Its vibrations will be quicker, and its horizontal tendency stronger, the lower the centre of gravity, and the less the weight upon the points of suspension.

4. The friction of the beam upon the axis ought to be as little as possible; because, should the friction be great, it would require a considerable force to overcome it; upon which account, though one weight should a little exceed the other, it will not preponderate, the excess not being sufficient to overcome the friction, and bear down the beam. The axis of motion should be formed with an edge like a knife, and made very hard: these edges are at first made sharp, and then rounded with a fine hone, or piece of

buff leather, which causes a sufficient bluntness, or rolling edge. On the regular form and excellence of this axis depends chiefly the perfection of this instrument."

5. The pivots, which form the axis or fulcrum, should be in a straight line, and at right angles to the beam.

6. The arms should be as long as possible relatively to their thickness, and the purposes for which they are intended, as the longer they are the more sensible is the balance.

They should also be made as stiff and inflexible as possible; for, if the beam be too weak, it will bend, and become untrue.

7. The rings, or the piece on which the axis bears, should be hard and well polished, parallel to each other, and of an oval form, that the axis may always keep its proper bearing, or remain always at the lowest point.

8. If the arms of a balance be unequal, the weights in equipoise will be unequal in the same proportion. The equality of the arms is of use, in scientific pursuits, chiefly in the making of weights by bisection. A balance with unequal arms will weigh as accurately as another of the same workmanship with equal arms, provided the standard weight itself be first counterpoised, then taken out of the scale, and the thing to be weighed be put into the scale, and adjusted against the counterpoise. Or, when proportional quantities only are considered, the bodies under examination may be weighed against the weights, taking care always to put the weights in the same scale, for then, though the bodies may not be really equal to the weights, yet their proportions amongst each other will be the same as if they had been accurately so.

9. Very delicate balances are not only useful in ice experiments, but are likewise much more expeditious than others in common weighing. If a pair of scales, with a certain load, be barely sensible to one-tenth of a grain, it will require a considerable time to ascertain the weight to that degree of accuracy, because the turn must be observed several times over, and is very small. But if no greater accuracy were required, and scales were used which would turn with 100th of a grain, a tenth of a grain, more or less, would make so great a difference in the turn that it would be seen immediately.

10. If a balance be found to turn with a certain addition, and is not moved by any smaller weight, a greater sensibility may be given to the balance, by producing a tremulous motion in its parts. Thus, if the edge of a blunt saw, a file, or other similar instrument, be drawn along any part of the case or support of the balance, it will produce a jarring, which will diminish the friction in the moving parts so much, that the turn will be evident with one-third, or one-fourth of the addition that would else have been required. In this way, a beam which would barely turn by the addition of the tenth of a grain, will turn with the thirtieth or fortieth of a grain.

The usefulness of having good balances for the weighing of substances, is not limited to the due performance of nice experiments, but they

also save much time in weighing when a less degree of accuracy is required. If a pair of scales, loaded with a certain weight, be barely sensible to one-tenth of a grain, it will require a considerable time to ascertain the weight to that degree of accuracy, because the turn must be observed several times over, and is very small; but if scales were used which would turn with the part of a grain or less, supposing that the weight was not required to any greater accuracy than the tenths of grains, a single tenth of a grain, more or less, would make so great a difference in the turn of the scale that it would be seen imme diately.

For the determination of the specific gravity of various substances, Muschenbroek says, he used a balance which turned within a fortieth of a grain when loaded with 200 or 300 grains; hence his balance determined the weight to the 12,000th of the weight.

Mr. Bolton's larger balance (mentioned in the Philosophical Transactions, vol. 66) loaded with a pound, probably of the common avoirdupo 5, would turn with one-tenth of a grain; so that the weight is determined to the 70,000th part of the weight.

Mr. Bolton's small balance, capable of weighing half an ounce, turned with the 100th part of a grain, which is the 24,000th part of the weight.

Mr. Reid's balance, mentioned in the same volume, when loaded with 55 lbs. avoirdupois, turned readily with less than a pennyweight, and very distinctly with four grains; so that it determined the weight to the 96,000th part of the weight in the scale; and, although so strong, was decidedly the best common balance whose performance has been recorded.

Mr. Whitehurst's balance, mentioned in the same volume, which weighed one penny-weight, was sensibly affected with the 2000th part of a grain, or the 48,000th part of the weight.

Mr. Nicholson's balance, noticed by him in his Dictionary of Chemistry, when loaded with 12,000 grains in each scale, turned with the th part of a grain, or the 84,000th part of the weight; so that this was a very good balance.

Mr. Alchome's balance (mentioned in the Philosophical Transactions vol. 77), with 15 lbs. at each end, turned with two grains; these were probably troy pounds, and so the accuracy of this balance went only to the 43,200th of the weight.

Dr. G. Fordyce (in the Philosophical Transactions, vol. 75) mentions a balance, made by Ramsden, turning on points instead of edges, which, when loaded with four or five ounces Troy, could ascertain the weight to the 160th part of a grain; that is to say, the 30,400th part of the weight in the scale.

Mr. Magellan's balance, mentioned by Nicholson in his Dictionary of Chemistry, with a pound in each scale, showed distinctlyth of a grain, or the 70,000th part of the weight.

The Royal Society's balance, made by Ramsden, and turning on steel edges upon planes of polished crystal, is said to be sensible to the 7,000,000th part of the weight.

The comparison of the weights of different countries, as made by Mr. Tillet, with the