The famous Tompion, who contributed greatly to bring the art of Horology in England to that reputation m hich it had acquired for a long period of sixty years or more, during which he practised it, made a 'scapement about 1695, and flattered himself with being very successful. The verge or axis of the balance was a small solid steel cylinder, cut across at the middle, and nearly half way down ; along the length way of this part cut across, a deep angular notch was made, forming a sort of pallet on the right hand side ; the balance wheel was flat, and much like Dr Hooke's; and the spaces between the teeth suffi ciently wide to allow the cylinder to turn freely between them. When a tooth of the wheel impelled the pallet, and when on escaping from it, the tooth following drop ped on the outside of the cylinder, near the left edge, resting on the cylinder during this vibration of the ba lance, after passing the left edge, and meeting a little recoil, it got on the pallet, and gave a new impulse, which was given only at every second vibration. An excellent property was observed in this 'scapement, that any inequa lities in the motive force made no deviation in its time keeping ; but the friction of the balance wheel teeth on the cylinder and its edges was so great and destructive, that it was given up in consequence of it.
Knowing what Tompion had been doing, being bred under him, Graham, a good many years after, set to work with the cylinder 'scapement, and ultimately succeeded. Although this 'scapement is now pretty generally known, yet we may be allowed to give an account of what he did. Plate CCCII. Fig. S. In place of Tompion's solid cylin der he made a hollow one ; on the points of the wheel teeth of Tompion were raised something like small pins or stems, on the tops of which a sort of inclined or curved wedge-like teeth were formed, of such a length as to have very little freedom when in the inside of the cylinder, and the outside of the cylinder to have the same freedom be tween the point of one tooth and the heel of the other. A notch or opening was made across the cylinder, not quite half way down the diameter ; the edges of the cylinder made by this opening were dressed so that the curved edge of the tooth might operate easily on them ; the right hand edge was flanched outward, the left one rounded ; when the balance was at rest, and the wheel in its place to 'scape, the point of the tooth got then just in on the cylinder edges, and no more ; a second notch was made below the other, to allow the hottom of the wheel to pass, leaving hardly a fourth of the circum ference of the cylinder, the other leaving more than a semicircle. The highest part of the wedge or curved tooth being on a circle, greater or beyond that on which the point was, it is evident that, if the wheel is urged for ward, it will make the cylinder to turn, and the angle of 'scapement will be according to the height of the wedge.
When a tooth of the c) finder wheel escapes from the left edge of the cylinder, the point of it falls into the inside of the cylinder, after reposing there, and then passing and impelling the right hand edge ; on escaping it, the point of the succeeding tooth drops on the outside of the cylin der, where it reposes ; on the return of the balance, it gets on the left hand edge, giving a new impulse, and so on. The teeth impel at both edges of the cylinder, giving by each a vibration to the balance.
This 'scapement being the best of any that had preced ed it, (llebaufre's perhaps excepted) procured for Gra ham's watches a very considerable reputation, as their performance was much superior to that of those of the old construction. However, on comparing the going of sonic of Graham's with those of a later date, we confess that none of his, though excellent, were ever equal to them in this. The cylinders were rather large in diame ter, the balance too light, the motive force too weak, and he had great difficulty in obtaining good pendulum spring wire, meeting sometimes with iron, where he expected steel wire. Watches having the cylinder 'scapement were not known in France till 1728, when Julien Le Roy commissioned one of them from Graham. They were losing their character here, some time before the intro duction of the duplex, which contributed afterwards still more to bring them down. The duplex will in its turn be supplanted, for reasons which will be afterwards noticed. Flat movements, shallow balance wheels, steel and brass of bad materials, from the difficulty of getting them good, injudicious execution, and low prices, must have tended to make the cylinder 'scapements so bad as they were of late ; many of the cylinders were destroyed and cut to pieces in a very few years, and some of them could not last so long. Let these he compared with the cylinder 'scapetnents of old Hull, many of which that we have seen, have little or no impression even on their edges, after having been in use thirty years and upwards. Of what did Hull's art consist ? There must have been some causes for it ; but what these are, we shall not at tempt to conjecture. As Graham, with whom he was instructed, did, Hull soldered in the plugs of his cylin ders, with silver solder, which caused a very tedious pro cess afterwards in making the cylinder ; but this is not offered as any reason for his excelling in the art of cylin der 'scapement making. The acting edges of the teeth have hitherto been made too thin, particularly for steel cylinders, with the view of lessening the friction ; but, from cutting soon, this friction increased, and was worse than a greater friction which was constant.