1 part, a fourth a fractional part, friction another part, and so on, till the whole 1000 parts are expended. In some large cotton, flax, or silk spinning establishments, a single water-wheel or steam-engine turns several thousands of spindles; each spindle, consequently, consumes a minute fraction of the originally impressed power. Whatever be the nature of the moving forces, it is generally sufficient for all purposes that they produce in the first instance rotary or circular motion, and either in a horizontal or vertical direction. It is, however, indispensable that the power be of that magnitude which will cause each part of the machine to fulfil its assigned office. If the power be too small or weak, the machine will move languidly and ineffectually; and if too great, it will either cause the machine to move too rapidly, or at least power will be expended uselessly. In the application of moving forces, it is always a matter of importance to regulate the power to the precise wants of the machinery.

The circular motion communicated in the first instance to a machine, is, by means of certain contriv ances, diffused through the whole organisation, and changed into every conceivable direction; some parts being caused to revolve, others to rise and fall, a third kind to move horizontally to and fro, and so forth, in all possible ways. The various parts may also be made to move with any degree of velocity; there being methods of transforming quick into slow motion, or slow motion into quick. Most minute and complex operations are thus performed by machines with a precision which often exceeds the skill of the most expert artisan; but these operations are all necessarily marked by the quality of uniformity of action. As machines cannot reason, or act arbitrarily in stopping, moving, or altering their process, according to circumstances, they proceed in a blind routine, whether right or wrong, mechanically as it is called, and in every case less or more require the superintendence of reasoning beings. This apparent defect, however, is really advantageous. A machine, by being composed of inanimate matter, destitute of feeling and unsusceptible of fatigue, proceeds unswervingly in its assigned duty, and may be forced to accomplish tasks which it would be both inhumane and impolitic to demand from living

creatures.

The purpose of machinery, therefore, is to lessen and aid human labour. At an inconsiderable expense, and with a small degree of trouble in supervision, a machine may be made to do the work of ten, fifty, or perhaps as many as five hundred men; and the work so simply effected by inanimate mechanism, serves to cheapen and extend the comforts and luxuries of life to the great body of the people.

The following are the chief clementary parts of machinery :

WHEELS.

A wheel moving on a central axis is a lever with equal arms radiating from the fulcrum at the centre, and is thus called a perpetual lever.

Wheels may be used in machines simply to transmit power from one point to another. This is done by means of toothed wheels. Projecting teeth or cogs are placed all round the circumference of a wheel, and, when the wheel is turned, these teeth work upon or press against the teeth of another wheel, and so cause it to turn also, but in an opposite direction. Fig. 42 represents two wheels

so working upon each other. As both of these wheels are of the same size, and consequently are levers with equal arms, they do not alter the effect of the power communicated to them.

Fig 42.

The motion of the axle in the wheel B is the same as
the motion of the first axle in the wheel A. Thus,
Jower may be transmitted from one point to another.
A long and large axle, in wheel-work, is called a shaft,

and shafts of small dimensions are termed spindles. The terminating points of axles, shafts, and spindles, where they rest and turn upon supports, are called their pivots or gudgeons. The sockets upon which the gudgeons bear in turning, are sometimes termed bushes.

WHEELS AND PINIONS.

W

When power has to be accumulated or increased in its effect in the course of its transmission, a large wheel is made to play upon a small wheel, by which means there is a diver sity in the lengths of the levers. Fig. 43 is a representation of a large wheel W, working on a small wheel or pinion P. The wheel is turned by the handle C. In all arrangements in which large wheels Fig. 43. are moved by small wheels, or small wheels by large, the small wheels are called pinions; and when these pinions are broad in their dimensions, they are termed trundles.

In this combination of a wheel and pinion, a long perpetual lever works against a short perpetual lever, by which a considerable mechanical advantage is gained. The wheel may be supposed to possess 48 teeth and the pinion 6 teeth; hence, by one revolution of the wheel, the pinion turns 8 times, which gives the axle of the pinion eight times the velocity of the axle of the wheel; and if we suppose that the diameter of the wheel is ten times the diameter of the pinion, the power is increased in effect ten times.

Any degree of velocity greater than that of the first rotary motion, may be imparted to the parts of a machine, by making these parts so much smaller than the primary moving parts. Thus, if a large wheel, having a thousand teeth in its circumference, work upon and turn a small wheel having only ten teeth in its circumference, the small wheel will go round one time for every ten teeth of the large wheel which it touches; or in other words, it will go round one hundred times for one time of the large wheel. The respective velo cities of wheels in a machine are, in this manner, always proportionate to their diameters, or size, unless when specially arranged to be otherwise.

A combination of wheels acting as perpetual levers, is represented in fig. 44. Three wheels are placed in a row close to each other, and it is supposed they are fixed by three axles to some upright object. On the side of the first wheel A, there is attached a small toothed pinion or wheel F, which, by the pressure of its teeth on the

W

P

To calculate the power or mechanical advantage to be gained by such a machine, suppose the number of teeth on the first wheel to be six times less than the number of those on the circumference of the second wheel, then the second wheel would turn round only once, while the first wheel turned six times. And, in like manner, if the number of teeth on the circumference of the third wheel be six times greater than those on the axle of the second wheel, then the third wheel would turn once, while the second wheel turned six times. Thus, the first wheel will make 36 revolutions, while the third wheel makes only one. The diameter of the first wheel being three times the diameter of the axle of the third wheel, and its velocity of motion being 36 to 1, three times 36 will give the weight which a power of 1 pound at P will raise at W. Three times 36 being 108, one pound at P will balance 108 pounds at W.

WORKING OF TOOTHED WHEELS.

In the working of toothed wheels one upon another, or of wheels working on pinions, it is essential to set them in opposition with such exact adjustment, that the teeth of one will fall into the hollows betwixt the teeth of the other. When the teeth of each do not work with this nicety, they are apt to jar upon and break each other, and so damage the machine. In some cases teeth are made of a round or pointed form at their extremities, by which a very small degree of grinding or pressing on each other takes place. Fig. 45 is an example of a wheel and pinion with rounded and pointed teeth. From the centre of the axis of the pinion L to the centre of the wheel C, a dotted line is marked, called by mechanics the line of centres. The dotted circle 00 round the pinion, and the dotted circle PP round the wheel, indicate the true point of working or contact of the teeth upon each other. are seen to join with exactness at A.

Fig. 45.

These two circles

ALTERING THE DIRECTION OF MOTION.

Motion often requires to be altered in its direction in the course of its transmission. For example, rotary horizontal motion requires to impart rotary vertical motion, or rotary vertical motion to impart horizontal motion. By means of a peculiar mode of setting the wheels, and a corresponding peculiarity in the shape of their teeth, any alteration may be effected in the direction of the motion.

Fig. 46 represents a plan of changing the direction of motion. A is a pinion or trundle working with its shaft horizontally on a wheel B, whose shaft is turning vertically. As the case may happen to be, the horizontal movement is causing the vertical movement, or the vertical movement is causing the horizontal movement.

BEVEL WHEELS.

Fig. 46.

TRANSMISSION OF POWER BY BELTS.

A common plan of transmitting power from one point to another, when the interval is considerable, is by a flat leather band, strap, or belt, communicating from a wheel at the source of power to a wheel connected with the machine."

A

C

The wheels upon which straps work are usually called pulleys. They have flat and broad rims, and these rims have sometimes narrow ledges, to prevent the belt from slipping off. The rims must also be rather rough on their surface, so as to give the belt a sufficient friction or power of pulling in performing its revolutions. Fig. 48 represents the transmission of power by a belt. A is the first pulley, which has received the power from its source, and C is the second pulley, moved by a belt, which passes over both pulleys. In this case, the motion of A is transmitted by the belt to C, which it causes to turn Fig. 48. in the same direction as A. If these two pulleys were of precisely the same diameter, and the belt did not relax or slip, the second pulley would unavoidably go at the same velocity as the first, because the belt has exactly the property of a toothed wheel, and simply transmits the power it has acquired. As C appears to be somewhat smaller than A, it would consequently turn more frequently than A; therefore, we have here an example of the mode of increasing the velocity while transmitting power.

SHAFTS AND PULLEYS.

When power requires to be carried to a distance beyond that which belts can conveniently manage, the transmission is effected by a long shaft; and, if it be necessary to change and re-change the direction of the motion, bevel wheels are added. Or the transmission may take place by a long flat chain acting like a belt, but caused to travel over small wheels or pulleys, to prevent the chain hanging down in any part of its course. A chain of this nature is called an endless chain.

Motion is often required to be communicated to many different machines, at different points, from one source of power. This is effected by means of a shaft and pulleys. From the pulley which receives the first motion, a belt is sent to a pulley fixed upon a shaft, which shaft is generally hung horizontally from the roof over the machines. As the shaft turns through its whole extent, it is able to turn pulleys fixed at any point upon it, and from these pulleys, belts are sent down to pulleys at the respective machines.

D

B

L

Fig. 49. represents an apparatus of a shaft and pulleys. A is the pulley receiving motion from the source of power, and, by means of the belt L, turns the pulley B on the end of the shaft S. At the same time, the pulley D at the oppo

site end of the

Fig. 49.

A

the direction of motion. The wheels

in this case are bevelled. A bevel wheel is a wheel with teeth placed in a sloping or oblique direction on its circumference. When two bevel wheels are placed at right angles to each other, their respective teeth work against each other, and so a harmonious joint motion ensues. This is exemplified in the figure, in which a horizontal shaft with a bevel wheel, is seen turning a smaller bevel wheel above it, Flaced on a vertical shaft.

Fig. 17.

other belt descends to turn E. Thus, an extended axle or shaft from C will turn a machine, and an extended axle or shaft from E will turn another machine. The apparatus can turn two machines.

Shafts with pulleys, working on the plan now stated, are to be seen at almost every considerable manufactory in which machinery is employed; and the power, by means of bevel wheels and upright connecting shafts, is carried upwards from storey to storey in a building, giving motion to hundreds of wheels, spindles, and other parts of the mechanisın.

CHANGING VELOCITY.

It is sometimes necessary that a machine, or part of a machine, should be propelled with a velocity which is

not equable, and is continually changing from fast to slow and slow to fast. This happens in cotton-mills, where it is necessary that the speed of certain parts of the machinery should continually decrease from the beginning to the end of an operation. To effect this, an apparatus is used, as represented in fig. 50. Two cones, or conically shaped drums, are used, having their larger diameters in contrary directions. They are connected by a belt, which is so governed by proper mechanism, that it is gradually shifted along from one extremity of the cones to the other, thus acting upon circles of different diameter, causing a continual change of velocity in the driven cone with relation to that which drives it. The shifting of bands from large to small wheels, and from small to large, has similar effects.

Fig. 50.

PRESERVING REGULARITY OF MOTION BY A VARIABLE FORCE. In some mechanical contrivances, the force which is applied varies in its intensity, while the wheels of the machinery require to be kept at an uniform speed. This is generally the case when the force is communicated from a steel spring, which, after being wound up, is suffered to relax. Fig. 51 is a spring suited for operations of this kind. It is represented in a state of relaxation, and is wound up into a compact form

Fig. 51.

by means of a spindle fixed to its inner extremity. The coiling of a strip of paper round the finger, and allowing it to unwind itself, is a familiar illustration of the action of a spring of this description.

The force communicated by the relaxing of the spring varies in its intensity. The force is greatest when it begins to relax, and it gradually weakens till its expansive energy is exhausted. To compensate this defect, a very ingenious plan is adopted, and which is put in operation in the apparatus of the common watch. Fig. 52 represents the apparatus of motion of a watch, somewhat magnified. The spring is confined

in a brass cylinder or barrel B. To this barrel the spring is attached by a slit at its outer extremity. The inner extremity of the spring is fixed by a similar slit to the central axis or spindle. F is a brass cone, broad at bottom and narrow at top, with a path winding spirally round it as an inclined plane. This cone is called the fusee, and has also a central axis or spindle K, to which it is fixed. To a point on the lower inclined path of the fusee, a small steel chain C is attached, and the other extremity of this chain is attached to the top part of the barrel. When the spring is relaxed, the chain is almost altogether round the barrel. To set the apparatus in motion, the watch-key is made to turn the spindle K, by which the chain is drawn from the barrel to the fusee, filling up the inclined path to the summit. The chain in leaving the barrel causes it to turn, and consequently to wind up the spring inside. The process of unwinding or relaxing ensues, and now the ingenious plan for regulating the motion is to be remarked. At first, when the force of the spring is greatest, the chain acts upon a small round of the fusee; in other words, it pulls with a small lever-for, as already explained under the head Wheel and Axle, a wheel or round object on an axis is simply a perpetual ver. In proportion as the intensity of the force

weakens, and the barrel takes off the chain from the fusee, and winds it about itself, so does the chain act upon a longer lever, or so does it gain a greater lever advantage, by drawing at a wider part of a cone. Thus, the gradual loss of force is counterbalanced by a gra dual increase of lever advantage. (The case resembles that of a strong man working with a short lever, and a weak man working with a long lever; both are equal in effect in balancing any resistance.) The wheelwork of the watch is moved by teeth on the lower circumfe

rence of the cone.

ALTERNATE OR RECIPROCATING MOTION-ECCENTRIC
WHEELS.

Alternate or reciprocating motion is applied to movements which take place continually backwards and forwards in the same path. In most complex machines, both rotary and reciprocating motion occur, and these motions may be converted into each other by various contrivances.

A common contrivance for gradually raising and depressing an object by machinery, is that of an eccentric wheel.

OA

Fig. 53.

די

W

An eccentric wheel is a wheel with an axis not in its centre, but at a point nearer one side than the other. Fig. 53 represents the action of a wheel of this kind. W is the wheel, and A the axis upon which it is fixed. When the axis turns, the wheel turns with it. As the axis never moves out of its place, the wheel necessarily describes a path of gradual rising and falling in its revolutions. Suppose an object, as T, pressing upon the upper edge of the wheel, so as to accommodate itself to the motion, it is obvious that, by the action of the wheel, this object will be alternately raised and allowed to fall. Or suppose that a rod is hung from a point of the wheel near where T rests, it is similarly obvious that the rod would be raised or depressed, according as the wheel turned. Thus a rising and falling motion may be effected by an eccentric wheel.

Eccentric wheels are made of different forms. Ac cording as they may be required to act, they are circular, oval, heart-shaped, or pointed at one end, and so forth-the object in each case being to produce alternate motion, by continually altering the distance of some moveable part of the machine, from the axis about which they revolve. Technically, the projecting parts of eccentric wheels are called cambs.

In some cases, eccentric wheels are not required to perform entire revolutions on their axes. It is perhaps sufficient for the purpose of the mechanism, if they gradually rise to the height of their power, and then, without turning round, gradually descend by retracing their course.

H

When alternate rising and falling is required thrice, by only one revolution of an axle, an eccentric wheel is used having three projecting cambs on its circumference, and as each camb comes round, it lifts and lets fall any object presented to it. An example of this apparatus is given in fig 54. The object required is to work a heavy hammer upon an anvil for beating iron. W is the wheel with the three cambs, and it turns by an axle in upright supports. In turning, each camb, with its rounded or convex side, presses down the end of the handle of the hammer, so as to raise the heavy head H at the opposite end. After pressing down the handle and escaping, the head of the hammer falls with a heavy blow on the anvil A. There it remains till raised up and let fall by the next camb, and so on.

OBLIQUE ACTION.

A

Fig. 54.

A mechanical advantage, which is frequently of a very serviceable nature, is obtained by causing the points of two straight bars to meet each other, but fixed

loosely, so as to be free to move from an oblique to a straight direction, and the reverse. The power consists in bringing the bars to the straight, by which they force asunder or press hard upon any object presented to their outer extremities. In the adjoining figure, the bars are seen first in their oblique position, and next when brought towards a straight. Betwixt the two points a small hollowed piece of metal is inserted, in which the points work, and against which the power is exerted to produce the action. The straightening and bending of the apparatus resembles the action of the knee-joint in animals. The pressure produced by the forcing downwards of the outer extremity of the lower

Fig. 55. bar (the upper working against a fixed beam), is very easily and rapidly accomplished, and is almost unlimited; and these advantages, as well as the extreme simplicity of the mechanism, have led to the application of the power to the printing-press wrought by the hand, instead of screw pressure.

CRANKS.

The crank affords one of the simplest and most useful methods of changing an alternate rising and falling motion into rotary motion.

A crank resembles a common handle or winch for turning a machine by the hand; the chief difference being, that a rod or shaft jointed to the handle, and going up and down, works the machine. If the crank be made double, it will turn two wheels or machines. Fig. 56 represents a double crank in action. S is the rod or shaft ascending and descending, and attached by a joint to the lower part of the crank C, which it alternately pulls up and pushes down, so as to cause Wthe axles W W to turn a wheel at each side. Take away one of the sides of the crank and its support, and the apparatus becomes a single crank.

S

Fig. 56.

principle, the battering rams, or engines for beating down the fortifications of towns in ancient times, were constructed, and the force of their blows was as great as that of a cannon ball; nevertheless, the power of their blows never could exceed the accumulated power of the impulses given to them in order to produce these blows.

The forcible expenditure of accumulated power in the swing apparatus, resembles that which is observable in the case of a person occupying several minutes in bending a spring-that is, accumulating power-and then allowing the spring to unbend itself by one violent effort, which effort is nothing more than the giving out of the accumulated power.

A boy taking a race to gain force before making a leap, is another familiar example of accumulating power and expending it instantaneously. The boy is gathering up power at every step he runs, and the force of his leap corresponds exactly with the quantity of the power he has acquired.

In the same manner, the lifting of a hammer, axe, or other instrument, to an elevation as far as our arm can reach, in order to give a blow with good effect, is a method we naturally pursue to gain an accumulation of power.

In contrivances in the arts, power is sometimes accumulated in order to be given out in the form of a rapid and effective blow. This may be done by means of a horizontal bar or lever, poised on a central axis, and loaded at each end with a heavy ball of lead or iron. After communicating to the machine a sufficient power of rotation, it will proceed with an enormous accumulated energy and momentum, till it expend its force either by friction in turning, or upon some fixed obstacle presented to it.

EQUALISATION-FLY-WHEELS.

Turning-lathes, knife-grinders' machines, and similar apparatus, are usually turned by cranks wrought by an alternate pressing and raising of the foot of the operator; a rod going upwards from the foot-board to the crank, causing the wheel or spindle to go round. The crank has been hitherto indispensable in the action of the steam-engine.

ACCUMULATION.

Power is susceptible of accumulation-that is, of increasing little by little-and of being expended either gradually or in one or more violent efforts; the efforts being entirely the concentrated amount of the previous accumulation. The apparently wonderful powers displayed through the agency of levers and other simple machines, are all a natural consequence of an accumulation of any degree of force into a small space; by which effects take place that could never have been accomplished by the original force.

In consequence of this convenient accumulation of power in machines, plans have been devised for establishing reservoirs of power, as they may be called, in connexion with moving machinery.

A well-known method of accumulating power consists in suspending a heavy body by a chain or strong rope of considerable length-forming what is called by young persons a swing. This body may be put in motion by a very small degree of power, and will acquire a vibrating motion like a pendulum. By continuing the impulse as the body returns, it will continually acquire greater and greater force, the arcs through which it moves becoming continually larger, until at last it might be made to overcome almost any obstacle. Upon this

The irregularities in the motion of machinery, from whatever cause they arise, are remedied by giving to each machine a reservoir of power, from which force may be given at all times to equalise the motion according as it may be required. These reservoirs of power are usually in the form of fly-wheels.

A fly-wheel is generally made of iron, and consists of a heavy rim or circumference, joined to a central axis by cross bars or spokes. In most cases it is placed in close connexion with the first moving force, the effect of which it equalises in its passage to the machine.

FRICTION.

Moving bodies, as machines and wheel carriages, are less or more retarded in their velocity by friction, and the resistance of the atmosphere, while vessels moving on water are retarded by the resistance both of the atmosphere and of the liquid in which they are buoyant. Friction is an effect of the action of rubbing of bodies one upon another.

This effect is produced by inequalities of surface. No such thing is found as perfect smoothness of surface in bodies. In every case there is, to a lesser or greater extent, a roughness or unevenness of the parts of the surface, arising from peculiar texture, porosity, and other causes; and, therefore, when two surfaces come together, the prominent parts of the one fall into the hollow parts of the other. This tends to prevent or retard motion. In dragging the one body over the other, an exertion must be used to lift the prominences over the parts which oppose them, and this exertion is similar to that of lifting or drawing of bodies up inclined planes or over upright protuberances.

Friction acts as a retarding influence in the action of all mechanical contrivances, and a due allowance must in every case be made for it. In many instances it destroys more than a half of the power employed, and seldom destroys less than a third. However small it may be, it sooner or later causes the wearing down and destruction of mechanism, and therefore forms an insurmountable obstacle to the lasting duration of bodies and the perpetuity of motion.

Friction is found to depend on the following circumstances:-1st, The degree of roughness of the surfaces. 2d, The weight of the body to be moved. 3d, The extent of surfaces in certain bodies presented to the action of rubbing. 4th, The nature of the bodies. 5th, The degree of velocity of the motion. 6th, The manner of the motion.

Roughness. It is of the utmost importance to smooth the surfaces. An apparently insignificant piece of matter, or even particles of dust, will greatly retard the motion of a body. But there is a limit beyond which it would be imprudent to smooth the surfaces of bodies having a close texture. If the surfaces be highly polished and levelled, the bodies will adhere by the effect of attraction of cohesion, even when the atmospheric air is not entirely expelled from between them, and more forcibly when the air is completely expelled. Practically, roads, railways, and similar bodies, cannot be made too smooth.

Weight.-Friction from weight differs in different bodies, and depends on concurring circumstances, as nature of surface, and so forth. Friction always increases in exact proportion as the weight increases, when all other circumstances remain the same. The parts of machinery, therefore, should be made as light as possible, consistent with strength and durability.

Extent of surfaces.-Rough bodies are more easily drawn along when their surface of contact is narrow than when they are broad. For example, it is easier to draw two narrow brushes across each other, than two broad ones of the same weight. Friction may, therefore, be diminished in rough bodies by lessening the extent of surfaces in contact. But there is a limit to this diminution. If the moving surface be very thin, and the other soft, the thin surface will plough a groove in the soft one, and thus the friction will be increased, and the machine injured.

Nature of Bodies.-It is a remarkable truth that two bodies which are of the same nature, or homogeneous, produce greater friction in movement than bodies which are different in their nature, or heterogeneous. Thus, iron working against iron, steel against steel, or brass against brass, causes in each case greater friction, and wearing of parts, than when iron or steel is made to work against brass. This circumstance is always attended to in the construction of machinery. Frequently, a small piece of leather is adjusted round an axle, to prevent the metals from coming in contact.

Degree of Velocity.-Friction is a uniformly retarding force, except in the case of small velocities, when it is greater in proportion. The reason for it being greater in small velocities is, that in these cases time is allowed for the prominences of the moving body to sink deeply into the hollows of the surface on which it is moving, which has a retarding effect.

Manner of the Motion. The least advantageous manner in which one body can be moved upon another, is to cause it to slide or drag. The most advantageous manner is to cause it to roll or turn. The causing of a body to roll instead of to slide, is one of the chief means of diminishing friction. The opposition presented by inequalities of surface to a rolling wheel, is overcome with ease, in proportion to the extent of diameter of the wheel. On a perfectly horizontal plane, the friction of wheels on the plane is very inconsiderable; the chief seat of friction in such cases being in the axles working in their sockets.

Friction is greatly diminished by lubricating the ubbing surfaces with an oily or greasy substance, which ance forms a medium of small soft particles betwixt

the bodies, and so prevents the tendency to grind or wear down the surfaces. Water or any similar fluid will also act as a medium to prevent friction, but the effects are only temporary, and would frequently be injurious, as the substance speedily evaporates, and would corrode metals. Practically, fine pure oil is found to be the best unguent for machinery.

One of the first considerations on the part of contrivers of mechanism, should be how to provide for and diminish the effects of friction in their machines. For want of forethought on this important point, thousands of ingenious schemes, which seemed perfect in the form of models and drawings on paper, have been completely frustrated when attempted to be brought into use.

Whatever may be the retarding and frequently inconvenient effects of friction, in reference to the action of mechanism, it is certain that friction is indispensable in the economy of both nature and art, and serves as an essential auxiliary to gravitation. It is a property which is frequently necessary, in order to allow one kind of matter to possess a hold upon another, without actual cohesion. We walk and maintain our erect posture by means of gravitation and action and reactionin other words, we are held to the earth by gravitation, and our pressure with our feet exemplifies action and reaction; but if there was no such property as friction, we should either stick to the earth by attraction of cohesion, or slide along it as upon the smoothest ice. In order to keep our feet from sliding when on ice, if we received any impulse, we either tie rough substances on our shoes, or scatter ashes in our path; and thus we receive the benefit of friction. It is by friction that rains wear down hills, and that rivers wear away their banks, by which ceaseless process the external configuration of the globe is constantly undergoing a change. The operations in art, of washing, cleaning, scouring, sharpening, polishing, cutting, bruising, beating, and so forth, are all effected less or more by friction. The hold which one fibrous substance has on another, or mutual friction, permits the operations of weaving cloth, twisting ropes and threads, and the tying of one body to another. Thus, friction is of universal service; and the only known instances in nature in which it is not required, and therefore not present, are the movements of the heavenly bodies, which revolve in a vacuum, and are consequently not impeded in their motions.

RESISTANCE OF AIR AND WATER.

Atmospheric air and water are fluids of different densities, and both present an obstacle to the motion of solid bodies through them.

There is a rule in respect to the resistance presented in moderate velocities, which applies both to air and water. It is, that the resistance is proportional to the square of the velocity. For example, a velocity of twenty miles an hour causes a resistance four times greater than a velocity of ten miles an hour, for the square of twenty (which is 20 times 20, or 400) is four times the square of ten (which is 10 times 10, or 100). Thus, by increasing the velocity of bodies through the air or water, we must increase the power in a greater proportion, in order to compensate the loss caused by resistance.

Although the above rule is nearly correct for moderate velocities, it deviates considerably from what is observable in the case of great velocities, such as that of a cannon-ball. When the velocity is upwards of 1000 feet per second through the air, the quick passage of the body is believed to cause a partial vacuum behind it, which causes a retardation of its motion.

Resistance to motion in fluids is greatly modified, also, by the form of the moving body. The form that gives least resistance is nearly that of a parabola, or a form somewhat resembling the breast of a duck, the head of a fish, or the rounded bow of a vessel, sharpened to cleave the fluid through which the body passes.

Printed and published by W. and R. CHAMBERS, Edinburgh. Sold also by W. S. Orr & Co., London.