This 'scapement being a detached or free 'scapement, can at pleasure be converted either into a recoiling or a dead beat one, without so much as once disturbing or stop ping the pendulum a single vibration. To make a dead beat of it, put in a peg of wood, or a small wire to each, so as to raise the detents free of the pallets ; and these being left so as to keep them in this position, the pin-teeth will now fall on the circular parts of the pallets, and so on to the flanch, and the 'scapement is then, to all intents and purposes, a dead beat one. To make a recoiling one of it, let there be fixed to each arbor of the detents a wire, to project horizontally from them about 3i or 4 inches long ; the outer ends of the wires must be tapped about half an inch in length ; provide two small brass balls, half an ounce weight each, having a hole through them, and tapped so as to screw on the wires ; the balls can be put more or less home, and be adjusted proportionably to the force of the clock on the pendulum. No recoil will be seen by the seconds' hand ; yet these balls will alternately oppose and assist the motion of the pendulum, as much as any recoiling pallets can possibly do ; and as their effects on the pendu lum will be exactly the same, it may be considered as a good recoiling 'scapement. This sort of detached 'scape ment, by becoming a dead beat, or a recoiling one, at any time when required, makes it convenient for making vari ous experiments with the different 'scapements.
We shall now proceed to describe a clock whose pallets require no oil, invented by the la r John Harrison, who received the parliamentary reward of 20,0001. for a marine time-keeper.
In Plate CCC1. Fig. 6. S \V is the swing wheel, whose teeth are shorter than usual. On the verge is a brass arm, of a sort of cross and flat pronged form, as may be seen at e, c, e, Fig. 6. and at e, e, Figs. 7. and 8. Upon this arm are screwed two brass-cocks, marked d, d, in Fig. 6. and d in Figs. 7 and 8 ; the upper pivots of the pallet arbors, as seen at a, Figs. 7. and 8. run in these cocks, and the lower pivots in the end of the prongs. On the lower end of the pallet arbors is a brass socket to each, having freedom to move easily on them, and also a proper end-shake between the prongs and the pallet arms. On the end of the sock ets, next the pallet arms, is rivetted a thin piece of brass to each, the piece on the socket of the driving pallet being shaped as seen at h, h, Figs. 6. and 7. and having two holes in it : one of these holes has a range, limited by a pin fixed to the brass arm from the verge ; the other hole, which is at the outer end, allows range to a pin, which is fixed to an arm on the pallet arbor, as may be seen at Figs. 6. and 7. The piece of brass on the socket of the leading pallet arbor is shaped as seen at k, k, Figs. 6. and 8. having a tail which comes to rest on the outer edge of the cock d, after being carried a little way by the motion of the pallet ; at the outer end, at k, is a small brass screw, serving as a counterbalance to the opposite arm or pallet hook. In this pallet arm is an opening, through which the swing wheel comes, as may be seen at 1, Fig. 8. the arm at the other end being filed thin down, leaving a sort of shoulder on it. AB, Fig. 6. is a stout piece of brass, rivetted or screwed to the verge collet ; CC is the steel crutch, having another arm, which comes up on the inside of the piece of brass ; the ball or paurue of the crutch is kept to the verge collet by a sort of spring collet, which has two screws outside, and through to the verge collet, the crutch having liberty to turn on the verge. The piece of brass AB has two short knees turned up, having a hole tapped in each to re ceive the two screws s, s, Fig. 6. whose ends bear on the upper arm of the crutch, and serve to move the arm to one side or the other, so as to put the pendulum or clock on beat ; p, ft, is a piece of hard wood put on the lower end of the crutch, having an opening in it, to clip or take in with the middle rod of a gridiron pendulum.
The parts of this 'scapement being described, it now re mains to explain their action. The tooth of the swing
lvheel, which has hold of the hook of the leading or right band pallet, carries it on, until another tooth meets with the hook or notch at the end of the driving pallet arm. When this takes place, the wheel is made to recoil a little back ; at this instant, the hook of the leading pallet gets free of the tooth, and is made to rise clear of the top of it, by means of the counterbalancing of the brass arm, and the screw k at the end of it. The tooth of the swing wheel, which has now got into the notch at the end of the driving pallet arm, carries it forward, until another tooth, meeting with the hook of the leading pallet, causes the wheel again to recoil. This allows the notch of the driving pallet to get free of the tooth; and the brass piece, which is on the pallet arbor, falls down, till it comes to rest on the pin in the brass cross piece, making the pallet notch get quite clear of the top of the tooth, and so on. There is a great deal of ingenuity dis played in the contrivance of this 'scapement, yet the nice and ticklish balancing of the pallets occasion some degree of uncertainty in their operations ; and whether the great recoil which it has may not be against the time-keeping of the clock, remains, perhaps, yet to be proved. Was it this 'scapement which was in a clock of Mr Harrison's, at his Ilse in Orange Street, of whose going Mr Short said, " 'Mt he could depend on it to one second in a -month," and " that it had been going for fourteen years at this rate ?" The properties of a good 'scapement arc, that the im pelling force should be applied in the most uniform and eirect way, and with the least friction and loss of motive force : that it require little oil, or none ; and that the oscil lations of the regulator, whether it is a pendulum or a ba lance, be made in as free and undisturbed a manner as pos sible. The nice execution required in a 'scapement, whe ther for a clock or a watch, for merly engrossed so much of the attention of workmen, that they, in some measure, lost sight of the properties of the pendulum, as well as that of the spiral or balance pendulum spring, and thought that the time-keeping of their machines depended more on the 'scapement than on any other thing, without consider ing, that this, from principle, lies wholly, or almost wholly, in the pendulum and in the spiral spring. Berthoud im putes a notion like this to Harrison, for attempting to make the 'scapement in his time-keeper so that the long and short vibrations should be made in equal times. Whereas he says, " he ought to have looked for this in the isochro nous property of the spiral or balance spring. But this property (he adds) was unknown at that time to the Eng lish artists ; and was a discovery of those in France, from whom the English artists afterwards obtained it." If this be the case, how did it happen that Mr Mudge, long be fore the period when Le Roy and Berthoud disputed about the property of this spiral spring, each claiming the merit of having first made the discovery, mentioned in his tract, published in 1763, " That the pendulum or balance spring, from physical principles, made the balance perform the long and the short vibrations to equal times ?" He learned this from Dr Hooke's works, with which he was well acquainted; and this property of springs was known to Dr Hooke, and pointed out by him nearly a hundred years before Mudge published his pamphlet. It is but too true, that few or none of the English artists seem to have been acquainted with these properties till very lately, though Mr Mudge had pointed them out so long before, and though they were contained in the works of Dr Hooke. Lepaute's book was published at Paris in 1767, and does not contain the most distant hint of these properties of the balance spring; hence they were not known there at the time when Lepaute wrote, otherwise he would have mentioned them. It was soon after this, that the disputes commenced be tween Le Roy and Berthoud regarding this subject.