It is more than fifty years since we saw a small spring clock having this 'scapement, made by a very ingenious clockmaker of this place, whose name was Robert Brack enrigg. It may be supposed to have been made a very few years after Thiout's work was published.
In 1727, Peter leRoy gave an account of a 'scapement which he had made, having one pallet on the axis of the balance, and a notch below it, a wheel of arrete, and one of impulse, as described in the preceding 'sea pement ; so that one half of the vibrations were independent of the wheel work. Dutertre claimed the pretended invention of Le Roy, who, finding it not to answer his expectations, gave it up. That Dutertre made the one which is represented in Plate xli. of Thiout, we hav'e no doubt ; and there is un questionable authority, that he brought Dr Hooke's to the improved state which has just been mentioned. It is said, that he had made a free or detached 'scapement ; but no account whatever has been given of it.
The duplex 'seapement, as it is now called, was intro duced into its native country about thirty years ago or more, under the name of Tyrer's 'scapement, the name, it is supposed, of him who put the last hand to improve that which came in a lineal descent from Dr Hooke. In place of the notch being made right across the arbor, as has been mentioned before, Tyrcr's had a very small cylin der or roller, whose diameter was .03 of an inch, into which was made, in a longitudinal direction, a deep angu lar notch of 30 or 40 degrees. Plate CCCII. Fig. 8. The cylinder was sometimes of steel, but most frequently of ruby. When the points of the teeth of the wheel of re pose fall into the notch, they meet with a very small recoil by the balance, in what may be called the returning vibra• tion. This goes so far as to make the point of the teeth for a little to leave the notch, at the side opposite to that by which it came in. The balance on returning, is now in the course of that vibration, when it is to receive impulse from the wheel, which takes place immediately on the tooth of the wheel of repose leaving the notch and the small cylinder, and as soon as the tooth of impulse escapes from the pallet,—the next tooth of repose falls to rest on the small cylinder, and so on.
This 'scapement of Tyrer's is much superior to that of the cylinder, or horizontal one ; it is almost independent of oil, requiring only a very little to the points of the wheel teeth of repose. It can carry a balance of much greater momentum, and, when well executed, performs most admirably. But there are so many circumstances or minuti? to be attended to in it, that some of them may at times escape the eye of the most judicious and careful ; the watch may stop, and yet the 'scapement be in every other respect as complete as possible. This has often given the wearer cause to complain, and to suspect the qualities of his watch, and hence watchmakers have been induced to abandon this 'scapement, and adopt inferior ones. The pallets of Tyrer's were at first very thin. We frequently urged the necessity of having them made much thicker, and were pleased to see that this was. gradually adopted. 'Why should they not be made as thick as the pallet of a detached 'scapement ? There is no 'scapement which re quires to have the balance wheel teeth more correctly cut, or the steady pins of the cock and potence more nicely fitted to their places in the potence plate. The minuti? alluded to were, too much or too little drop of the impulse teeth on the pallet, the 'scapement not set quite so near to beat as might be, the balance rather heavy, or the points of the teeth of repose too much or too little in on the small cylinder. In a good sizeable pocket watch, the wheels having fifteen teeth, the ratio of the diameter of the wheel of repose to that of impulse may be as .520 of an inch to .400, the cylinder .030. The angle of 'scapement will be 60 degrees, taking from the escape of the impulse tooth, to that of the tooth of repose falling on the cylinder ; the balance passes 20 degrees of these, before the impulse tooth gets again on the pallet, consequently it has only 40 degrees for the acting angle of the 'scaiLment. There is a variety of 'scapements in Berthoud's liristoire, which ap peared in 1802, many of which are of very inferior note to that of Tyrer's, and yet he takes no notice of the latter. This is remarkable, as he surely must have seen it, con sidering the great number of them which were made.