SCAPEMENT

, in Clock-work, a general term for the manner of communicating the impulse of the wheels to the pendulum. The ordinary Scapements consist of the swing-wheel and pallets only; but modern improvements have added other levers or detents, chiefly for the purposes of diminishing friction, or for detaching the pendulum from the pressure of the wheels during part of the time of its vibration. Notwithstanding the very great importance of the Scapement to the performance of clocks, no material improvement was made in it from the first application of the pendulum to clocks to the days of Mr. George Graham; nothing more was attempted before his time, than to apply the impulse of the swing-wheel. in such manner as was attended with the least friction, and would give the greatest motion to the pendulum. Dr. Halley discovered, by some experiments made at the Royal Observatory at Greenwich, that by adding more weight to the pendulum, it was made to vibrate larger arcs, and the clock went faster; by diminishing the weight of the pendulum, the vibrations became shorter, and the clock went slower; the result of these experiments being diametrically opposite to what ought to be expected from the theory of the pendulum, probably first roused the attention of Mr. Graham, and led him to such farther trials as convinced him, that this seeming paradox was occasioned by the retrograde motion, which was given to the swing-wheel by every construction of Scapement that was at that time in use; and his great sagacity soon produced a remedy for this defect, by constructing a Scapement which prevented all recoil of the wheels, and restored to the clock pendulum, wholly in theory, and nearly in practice, all its natural properties in its detached simple state; this Scapement was named by its celebrated inventor the dead beat, and its great superiority was so universally acknowledged, that it was soon introduced into general use, and still continues in universal esteem. The importance of the Scapement to the accurate going of clocks, was by this improvement rendered so unquestionable, that artists of the first rate all over Europe, were forward in producing each his particular construction, as may be seen in the works of Thiout l'ainé, M. J. A. Lepante, M. le Roy, M. Ferdinand Bertoud, and Mr. Cummings' Elements of Clock and Watchwork, in which we have a minute description of several new and ingenious constructions of Scapements, with an investigation of the principles on which their claim to merit is founded; and a comparative view of the advantages or defects of the several constructions. Besides the Scapements described in the above works, many curious constructions have been produced by eminent artists, who have not published any account of them, nor of the motives which have induced each to prefer his favourite construction: Mr. Harrison, Mr. Hindley of York, Mr. Ellicot, Mr. Mudge, Mr. Arnold, Mr. Whitehurst, and many other ingenious artists of this country, have made Scapements of new and pecnliar constructions, of which we are unable, for the above reason, to give any farther account than that those of Mr. Harrison and Mr. Hindley had scarce any friction, with a certain mode and quantity of recoil; those of all the other gentlemen, we believe, have been on the principle of the dead beat, with such other improvements as they severally judged most conducive to a good performance.

Count Bruhl has just published (in 1794) a small pamphlet, “On the Investigation of Astronomical Circles,” to which he has annexed, “a Description of the Scapement in Mr. Mudge's first Timekeeper, drawn up in August 1771.” Before entering upon the Description, the Count premises a few observations, in one of which he recognizes a hint concerning the nature of Mr. Mudge's Scapement, thrown out by this artist in a small tract printed by him in the year 1763, which is this: “The force derived from the mainspring should be made as equal as possible, by making the mainspring wind up another smaller spring at a less distance from the balance, at short intervals of time. I think it would not be impracticable to make it wind up at every vibration, a small spring similar to the pendulum spring, that should immediately act on the balance, by which the whole force acting on the balance would be reduced to the greatest simplicity, with this advantage, that the force would increase in proportion to the arch.” From this hint, Count Bruhl is surprised that no other artists have taken up Mr. Mudge's invention. He then gives the Description of that invention as follows: “Mr. Mudge's Timekeeper has five wheels, with numbers high enough to admit pinions of twelve, and yet to go eight days. The Scapement consists of a wheel almost like that of a common crown wheel, and acts on pallets, each of which has a separate axis lying in the same line. To each pallet a spring is fixed in the shape of a pendulum spring; these springs are would up alternately by the action of the last wheel upon the pallets, which is performed in the following manner: —Whenever one of the pallets (for instance the upper one) is set in motion by a tooth of the wheel sliding upon it, and then resting against a hook, or, rather a bearing at its end, the balance is entirely detached from it, being then employed in carrying the other pallet the contrary way. When the balance returns from that vibration (partly by the force of the pendulum spring, and partly by that of one of the two small springs which it had bent by the motion of that pallet which it had carried along with itself) it lays hold of the upper pallet and carries it round in the same manner as it did before the lower one, and, of course, in the same direction which the upper pallet had received from the power of the mainspring at the time that it was quite unconnected with the balance. The communication of motion from the balance to the pallets, and vice versa, is effected by two pins fixed to a crank, which in following the balance, hit each its proper pallet alternately. By what has been said, it is evident that whatever inequality there may be in the power derived from the mainspring (provided the latter be sufficient to wind up those little pallet springs) it can never interfere with the regularity of the balance's motion, but at the instant of unlocking the pallets, which is so instantaneous an operation, and the resistance so exceedingly small, that it cannot possibly amount to any sensible error. The removal of this great obstacle was certainly never so effectually done by any other contri- | vance, and deserves the highest commendation, as a probable means to perfect a portable machine that will measure time correctly. But this is not the only, nor indeed the principal advantage which this timekeeper will possess over any other; for, as it is impossible to reduce friction to so small a quantity as not to affect the motion of a balance, the consequence of which is, that it describes sometimes greater and sometimes smaller arcs, it became necessary to think of some method by which the balance might be brought to describe those different arcs in the same time. If a balance could be made to vibrate without friction or resistance from the medium in which it moves, the mere expanding and contracting of the pendulum spring, would probably produce the so much wished-for effect, as its force is supposed to be proportional to the arcs described; but as there is no machine void of friction, and as from that cause, the velocity of every balance decreases more rapidly than the spaces gone through decrease, this inequality could only be removed by a force acting on the balance, which assuming different ratios in its different stages, could counterbalance that inequality. This very material and important remedy, Mr. Mudge has effected by the construction of his Scapement; for his pallet springs having a force capable of being increased almost at pleasure, at the commencement of every vibration, the proportion in their different degrees of tension may be altered till it answers the intended purpose. This shews how effectually Mr. Mudge's Scapement removes the two greatest difficulties that have hitherto baffled the attempts of every other artist, namely, the inequalities of the power derived from the main spring, and the irregularities arising from friction, and the variable resistance of the medium in which the balance moves. Although at the time I am writing this account of his invention, the machine is not yet finished; I am not the less confident that whenever it is, it will be found to be one of the most useful of any which has as yet appeared.”

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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SAUVEUR (Joseph)
SCALE
SCALENE
SCALIGER (Joseph Justus)
SCANTLING
* SCAPEMENT
SCARP
SCENOGRAPHY
SCHEINER (Christopher)
SCHEME
SCHOLIUM