140

The Lazy-tongs.-Cumulative Machines.

256. The arrangement of cross-jointed rods or wires, represented

a

in fig. 53, and called the Lazy-tongs, connects different velocities, and therefore different intensities of force. It has been applied to some curious purposes, but to none of much utility. By pressing the ends a and b towards each other, the rods, from being in the condensed position represented in the upper figure, immediately assume the outstretched position represented in the lower ; so that the end, c, darts forward much farther and faster than the ends a and b

b

Fig. 53.

approximate.

257. (ii.) The cumulation of a series of separate motions, impulses, or donations of Energy is often an important means of procuring either increased intensity or volume of mechanical Energy. This is effected at the expense of time, and there is, of course, no creation of mechanical force; yet such cumulative contrivances have, according to the old notion, some claim to be considered mechanic powers.

Of this class of machines are hammers, clubs, pile-engines, battering-rams, slings, brakes, fly-wheels, &c., all of which admit of the storage of a continued moderate effort being applied in a condensed form to overcome a resistance with which the unaccumulated effort would be totally unable to contend.

A man may have a purpose to effect which a very forcible downward push would accomplish; but, being too weak to give that push directly, he may employ a certain time in carrying a weight to such an elevation above his work that, when let fall. its momentum may do what is required. Thus, the continued effort of a man may be employed to lift a weight to a height of perhaps thirty feet, which may suffice to drive a pile or stake into the earth one inch.

By swinging a sling round and round the head, such a velocity of the stone is accumulated that, when the central constraint is released, it may be projected a long distance.

Brakes, though they are opposed to what are usually regarded as powers, are really the same in principle. When a brake is applied to the wheels of a railway carriage moving down an incline or approaching a station, the moving force of the whole train is gradually transferred and accumulated as a minute quiver of the particles of the brake.

The fly-wheel, which has often been ignorantly accounted a positive power, in common cases merely equalizes the effect of an irregular force. In using a winch to turn a mill, for instance, a man does not act with equal force all round the circle ; but a heavy wheel fixed on the axle moderates the irregularity of speed, by receiving or absorbing momentum while his action is above par, and returning that momentum while his action is below par-thus equalizing the movement. In the common instances of circular motion produced by a crank, as when by the pressure of the foot on the treadle, a lathe, or grindstone, or spinning-wheel, is turned. the force is applied during a small part only of the revolution, in the form of interrupted pushes; yet the motion goes on steadily, because the turning grindstone, or wheel, or lathe, becomes a fly and reservoir, equalizing the effect of the force. The alternate upward and downward pushes of the piston of a steam-engine are converted, by means of a heavy fly-wheel, into a steady rotatory motion.

A heavy wheel is, moreover, often used as a concentrator of force or a mechanic power, in the sense that motion or momentum being gradually accumulated in the wheel, may be made to expend itself in producing some sudden and proportionally intense effect. Thus a man may lift a very heavy weight by first in any way imparting motion to a fly-wheel, and then suddenly hooking a rope from the weight to the axle of the wheel, which rope being wound round the axle, lifts the weight.

A fly-wheel, containing the result of a man's action during perhaps one hundred seconds, when made to impel a screw-press, will, with one blow or punch, convert a piece of smooth metal into a perfect medal or coin; or will, by repeated blows, change a flat piece of silver into a graceful spoon or other utensil.

In the same way a spring may become a mechanical power. A person may expend some minutes in bending it, and may then let fly its accumulated Energy in an instantaneous blow. A gun-lock shows this on a small scale. The slow bending of a bow, which afterwards shoots its arrow with such striking velocity, is another instance.

258. (iii.) A modification of motive Energy, as to direction simply, is often an important means to mechanical ends.

It is this power of changing the direction of motion, added to the power of connecting and adjusting different intensities of force and resistance by the simple machines just described, that has

142

Conversion of Motion as to Direction.

enabled man to make complex machines rivalling in their performances the nicest work of human hands. It would be endless to enumerate the various modes in which the direction of motions may thus be changed, for it would be to enumerate and describe the whole apparatus of the arts and sciences.

We shall merely advert to a few as specimens :—

"Straight motion changed into Rotatory."

259. In utilising the Energies of nature-such as a waterfall, or a flowing river, or any falling heavy mass, or the force of the wind, or the Energy of heat expanding steam or air-the storage of the rectilinear force is effected by converting it into a rotatory motion of a heavy wheel, from which, by a series of connected parts, the motion is ultimately obtained in the desired form.

The force of the wind or water may be made to act directly at the circumference of a suitable wheel, and thus pass at once into an Energy of rotation.

The alternate rising and falling of the piston of a steam-engine is made by means of a crank to turn the great fly-wheel, from which the motions of all the other parts are derived.

The crank is the regular contrivance by means of which a series of interrupted straight pushes or motions is converted into rotation, as is seen when the human foot acts on a treadle turning a grindstone, a lathe, or a sewing-machine.

"Conversion of rotatory motion."

260. As the steam-engine is the principal and the type of the socalled mechanical Prime Movers, the general problem which the science of mechanism has to solve is

Given a uniformly rotating heavy mass, such as a fly-wheel, to find the construction and connection of solid parts that shali reproduce this motive Energy in any required form.

If the required form be another circular motion-as for the grinding of corn or the sawing of timber—the motion of the fly-wheel is conveyed by rolling contact—that is, by trains of wheels serrated or toothed on the edge, or else by endless bands or cords. This method is of constant application in all kinds of mill-work, where by means of toothed wheels the direction and rate of motion may be changed to any extent; a horizontal rotation being converted into a vertical, and the slow, steady motion of the fly-wheel being converted into the deafening rattle of hundreds of smaller wheels.

261. If the rotatory Energy of the prime mover has to be reproauced as a rectilinear movement, this may be effected by the crankcontrivance, or by the modification of it called an eccentric, or by a toothed wheel gearing into a toothed bar or rack, or, most simply, by merely winding a flexible chain or rope on an axle, as in raising water or minerals from a depth.

The conversion of a rotatory into a perfectly straight or rectilinear motion by rigid connections is, however, not so easy as may at first sight appear; and, as it is a frequent and an important requirement in machines, a great amount of ingenuity has been expended in the attempt to solve the problem of a perfectly straight or parallel conversion of circular motion. The difficulties of the case may appear from the following example :

D

B

262. Suppose that we have to work a pump, P (fig. 54), with a steam-engine. The rotatory power is applied by means of the crank, C, to the rod, A, of the piston of the pump. Clearly, as the crank turns, the top, A, of the piston-rod will be moved from side to side, producing a useless straining and irregularity of action. The difficulty may be met, so far, by confining the piston-rod within guides on each side, the straïning being then expended in friction against these guides, which will consequently get worn in course of time and cease to be true guides to straight motion.

It is found, however, that if the end, A, be jointed to the centre of

Fig. 54.

a bar, or link, E F, connected with two other links, ED and FB, moving round D and B as centres, these links will guide the motion of A, so that it deviates extremely little from the straight line so long as A has but a moderate range. This does away with the disadvantages of a sliding guide; still the parallelism of A's motion is by no means perfect.

263. We are indebted to the illustrious Watt for a much more exact parallel motion, the arrangement having been devised by him for the simultaneous working of two piston-rods, by connecting them with the beam of his steam-engine. Fig. 55 will give a general idea of Watt's parallel motion. ABF is the half of the

144

Peaucellier's Perfect Parallel Motion.

beam of the engine, which moves up and down, describing a portion of a circle round A as a centre. B D E F is a parallelogram of bars or links jointed at the four corners, B, F, E, D. At the joint, D,

of a similar circle round C, that is, in a contrary direction, there will be some intermediate point, P, of the link, which will incline to neither centre of motion, but will describe a rectilinear path up and down. To this point, therefore, the head of the piston-rod is attached. Geometrical considerations show that there will be another point in the line, A P, produced, which will move exactly parallel to P; and the dimensions of the jointed parallelogram, B D E F, with respect to the beam A B F, are usually calculated so that this second point of parallelism falls at E.

Even this motion is but an approximation to a perfectly rectilinear one. It is only quite lately that a really perfect parallel motion, independent of any gliding guides has been devised.

This is so beautiful in its simplicity that we shall give a general idea of its principle.

P

"Peaucellier's exact Parallel Motion."

264. Peaucellier, an officer of engineers in the French army, first

A

B

Fig. 56.

published, in 1864, his discovery in the form of a question in the Annales de Mathématique. This exact parallel motion consists of seven bars or links connected together and moving, like Watt's parallel motion, round two fixed centres (see fig. 56). Four equal links. P C, C Q, P D, D Q, are jointed together in the form of a diamond; and two other equal arms, C E, D E, are hinged to the

diamond corners ard move round a fixed centre, E.