the top of the barrel, all the acceleration is undone, and is to begin again. The motion of such a machine is very hobbling but the superplus of accelerating force at the beginning of a returning stroke will not make such a change in the motion of the machine if we connect the fly with it. For the accelerating momentum is a determinate quantity. Therefore, if the radius of the fly be great, this momentum will be attained by communicating a small angular motion to the machine.

The momentum of the fly is as the square of its radius; therefore it resists acceleration in this proportion; and although the overplus of power generates the same momentum of rotation in the whole machine as before, it makes but a small addition to its velocity. If the diameter of the fly be doubled, the augmentation of rotation will be reduced to one-fourth. Thus, by giving rapid motion to a small quantity of matter, the great acceleration during the returning stroke of the piston is prevented. This acceleration continues, however, during the whole of the returning stroke, and at the end of it the machine has acquired its greatest velocity. Now the working stroke begins, and the overplus of power is at an end. The machine accelerates no more; but if the power is just in equilibrio with the resistance, it keeps the velocity which it has acquired, and is still more accelerated during the next returning stroke. But now, at the beginning of the subsequent working stroke, there is an over plus of resistance, and a retardation begins, and continues during the whole rise of the piston; but it is inconsiderable in comparison of what it would have been without the fly; for the fly, retaining its acquired momentum, drags forward the rest of the machine, aiding the impelling power of the wheel. It does this by all the communications taking into each other in the opposite direction. The teeth of the intervening wheels are heard to drop from their former contact on one side, to a contact on the other. By considering this process with attention, we easily perceive that, in a few strokes, the overplus of power during the returning stroke comes to be so adjusted to the deficiency during the working stroke, that the accelerations and retardations exactly destroy each other, and every succeeding stroke is made with the same velocity, and an equal number of strokes is made in every succeeding minute. Thus the machine acquires a general uniformity with periodical inequalities. It is plain, that by sufficiently enlarging either the diameter or the weight of the fly, the irregularity of the motion may be rendered as small as we

please. It is much better to enlarge the diameter. This preserves the friction more moderate, and the pivot wears less. For these reasons a fly is in general a considerable improvement in machinery, by equalizing many exertions that are naturally very irregular. Thus, a man working at a common windlass exerts a very irregular pressure on the winch. In one of his positions, in each turn he can exert a force of near 70 lbs. without fatigue, but in another he cannot exert above 25 lbs; nor must he be loaded with much above this in general. But if a large fly be connected properly with the windlass, he will act with equal ease and speed against 30 lbs.

This regulating power of the fly is without bounds, and may be used to render uniform a motion produced by the most desultory and irregular power. It is thus that the most regular motion is given to mills that are driven by a singlestroke steam-engine, where, for two, or even three seconds, there is no force pressing round the mill. The communication is made through a massive fly of very great diameter, whirling with great rapidity. As soon as the impulse ceases, the fly, continuing its motion, urges round the whole machinery with almost unabated speed. At this instant all the teeth, and all the joints, between the fly and the first mover, are heard to catch in the opposite direction.

If any permanent change should happen in the impelling power, or in the resistance, the fly makes no obstacle to its producing its full effect on the machine; and it will be observed to accelerate or retard uniformly, till a new general speed is acquired exactly corresponding with this new power and resistance.

Many machines include, in their construction, movements which are equivalent with this intentional regulator. A flour mill, for example, cannot be better regulated than by its mill-stone; but in the Albion Mills, a heavy fly was added with great propriety; for if the mills had been regulated by their mill-stones only, then, at every change of stroke in the steam-engine, the whole train of communications between the beam, which is the first mover, and the regulating millstone, which is the very last mover, would take in the opposite direction. Although each drop in the teeth and joints be but a trifle, the whole, added together, would make a considerable jolt. This is avoided by a regulator immediately adjoining the beam. This continually presses the working machinery in one direction. So judiciously were the movements of that noble machine contrived, and so nicely were

E

they executed, that not the least noise was heard, nor the slightest tremor felt in the building.

Mr. Valoué's beautiful pile engine, employed at Westminster bridge, is another remarkable instance of the regulating power of a fly. When the ram is dropped, and its follower disengaged immediately after it, the horses would instantly. tumble down, because the load, against which they had been straining hard, is at once taken off; but the gin is connected with a very large fly, which checks any remarkable acceleration, allowing the horses to lean on it during the descent of the load; after which their draught recommences immediately. The spindles, cards, and bobbins, of a cotton mill,. are also a sort of flies. Indeed, all bulky machines of the rotative kind tend to preserve their motion with some degree of steadiness, and their great momentum of inertia is as use ful in this respect as it is prejudicial to the acceleration or any reciprocation when wanted. There is another kind of regulating fly, consisting of wings whirled briskly round, till the resistance of the air prevents any great acceleration. This is a very bad one for a working machine, for it produces its effect by really wasting a part of the moving powers. Frequently it employs a very great and unknown part of it, and robs the proprietor of much work. It should never be introduced into any machine employed in manufactures.

Some rare cases occur where a very different regulator is required: where a certain determined velocity is found necessary. In this case the machine is furnished, at its extreme mover, with a conical pendulum, consisting of two heavy balls hanging by rods, which move in very nice and steady joints at the top of a vertical axis. It is well known, that when this axis turns round, with an angular velocity suited to the length of those pendulums, the time of a revolu tion is determined. Thus, if the length of each pendulum be 39 inches, the axis will make a revolution in two seconds very nearly. If we attempt to force it more swiftly round, the balls will recede a little from the axis, but it employs as long time for a revolution as before; and we cannot make it turn swifter, unless the impelling power be increased beyond all probability; in which case the pendulum will fly out from the centre till the rods are horizontal, after which every increase of power will accelerate the machine very sensibly. Watt and Boulton have applied this contrivance with great ingenuity to their steam-engines, when they are employed for driving machinery for manufactures which have a very changeable resistance, and where a certain speed cannot be much

departed from without great inconvenience. They have connected this recess of the balls from the axis (which gives immediate indication of an increase of power or a diminution of resistance) with the cock which admits the steam to the working cylinder. The balls, flying out, cause the cock to close a little, and diminish the supply of steam. The impelling power diminishes the next moment, and the balls again approach the axis, and the rotation goes on as before, although there may have occurred a very great excess or deficiency of power.

A fly is sometimes employed for a very different purpose from that of a regulator of motion-it is employed as a collector of power. Suppose all resistance removed from the working point of a machine furnished with a very large or heavy fly immediately connected with the working point. When a small force is applied to the impelled point of this machine, motion will begin in the machine and the fly begin to turn. Continue to press, uniformly, and the machine will accelerate. This may be continued till the fly has acquired a very rapid motion. If at this moment a resisting body be applied to the working point, it will be acted on with very great force; for the fly has now accumulated in its circumference a very great momentum. If a body were exposed immediately to the action of this circumference, it would be violently struck. Much more will it be so, if the body be exposed to the action of the working point, which, perhaps, makes one turn while the fly makes a hundred. It will exert a hundred times more force there (very nearly) than at its own circumference. All the motion which has been accu mulated on the fly during the whole progress of its accumu lation, is exerted in an instant at the working point, multiplied by the momentum depending on the proportion of the parts of the machine. It is thus that the coining press performs its office; nay, it is thus that the blacksmith forges a bar of iron. Swinging the great sledge hammer round his head, and urging it with force the whole way, this accumulated motion is at once extinguished by impact on the iron. It is thus also we drive a nail, &c. This accumulating power of a fly has occasioned many to imagine that a fly really adds power or mechanical force to an engine; and, not understanding on what its efficacy depends, they often place the fly in a situation where it only adds a useless burden to the machine. It should always be made to move with rapidity. If intended for a mere regulator, it should be near the first mover; and if it be intended to accumulate force in the

working point, it should not be far separated from it. In a certain sense, a fly may be said to add power to a machine, because by accumulating into the exertion of one moment the exertions of many, we can sometimes overcome an obstacle that we never could have balanced by the same machine, unaided by the fly. And it is this accumulation of force which gives such an appearance of power to some of our first movers.

ANIMAL STRENGTH.

ANIMAL STRENGTH has been very differently estimated by different authors; but this is not to be wondered at when we consider the many difficulties that ever must attend any attempt to subject it to an estimate. Physical causes must sensibly affect the extent and duration of animal exertion, either in man or beast; and the only way of coming to any thing like an accurate result, is to compare the experiments of the different philosophers who have attended to the subject. This has been already done by Dr. Young, in the second volume of his Philosophy, whose valuable tables we here present to our readers.

Comparative table of mechanical forces.

In order to compare the different estimates of the force of moving powers, it will be convenient to take a unit which may be considered as the mean effect of the labour of an active man, working to the greatest possible advantage, and without impediment. This will be found, on a moderate estimation, sufficient to raise 10 pounds, 10 feet in a second, for ten hours in a day; or to raise a 100 pounds, which is the weight of twelve wine gallons of water, one foot in a second, or 36,000 feet in a day; or 3,600,000 pounds, or 432,000 gallons, one foot in a day. This we may call a force of one continued 36,000"

Immediate force of men, without deduction for friction.

A man, weighing 133 pounds, Fr. ascended 62 feet, Fr. by steps, in 34", but was completely exhausted.Amontons

A sawyer made 200 strokes of 18 inches, Fr. each, in 145", with a force of 25 pounds, Fr. He could not have gone on above three minutes.--Amontons....

A man can raise 60 pounds, Fr. one foot, Fr. in 1", for

eight hours a day.--Bernouilli

A man of ordinary strength can turn a winch with a force of 30 pounds, and with a velocity of 34 feet in 1", for 10 hours a day.-Desaguliers