While Dutertre was engaged with Hooke's 'scapement, an artist in England, whose name is unknown, produced a 'scapernent with the dead beat, which seems at that time to have been the great object of pursuit. Julien Le Roy having got one of these watches, showed it to Sully in No vember 1727, and told him that it was a 'scapement very deserving of notice. Thiout mentions it as a 'scapcment of Flamenville, having two pallets of repose; and says that it had much attracted the attention of the English watchmakers, who had made it for three or four years. Plate CCCII. Fig. 9. (See page 108, plate xliii. fig. 26. of his first volume.) With our workmen it went by the name of the 'scapement with the tumbling pallets. The axis of the balance had two semicylindrical pallets, whose faces stood in the same plane or centre of the axis ; the balance wheel was the common crown wheel one, the teeth of which got a very small hold of the pallets. When escaping from the face of one pallet, a tooth on the opposite side dropped on the semicylindrical part of the other pallet, where it rested during the going and coming of the vibra tion; getting then on the face, it gave new impulse, escap ing in its turn ; the pallet on the opposite end of the verge received a tooth on the semicylindrical part, and so on. After having been laid aside for some time, it was of late years taken up by several, who no doubt must have thought well of it. Among these was Kendal, a man possessed of no common talents. He transformed it into one having two crown wheels on the same pinion arbor, the tops of the teeth in the one pointing to the middle of the spaces in the other, and with only one pallet, the diameter of the semicylinder being of any size. (See Plate CCCIII. Fig. 1.) About thirty years ago we had some watches made with this 'seapement, and after a few years trial gave them up. The principle of the 'scapement is good, as long as the parts of it remain unimpaired, and the oil continues fresh ; but the acting parts having such a small hold of one another, get soon altered, which causes a great devia tion from the rate of time with which it first sets out. They cannot be expected to last long, unless with a diamond pallet, and a steel wheel of the hardest temper.
The free or detached 'scaltement, is that in which the greater part of the vibrations of the balance is free and in dependent of the wheels, the balance wheel being then locked; when unlocked, it gives impulse, which only takes place at every second vibration. In Mudge's detached 'seapement, the impulse is given at every vibration. The progress which has of late years been made in improving the detached 'scapement has been very wonderful, when we consider that half a century ago the name of this 'scapement was unknown. The first rude draught of any thing like it, appears to be that of Thiout's, described at p. 110 of the first volume of his work, and shown in Plate xliii. fig. 30, which he calls " A 'scapement of a watch, the half of whose vibrations appear independent of the wheel work, during the time they are made. A hook retains the ratchet or balance wheel ; the return of the vibration brings the pallet to its place of being impelled by the wheel ; in the returning, the hook is carried outwards, and leaves the wheel at liberty to strike the pallet, and so on. This sort of .'scapement cannot act without the aid of a spiral or pendulum spring " Peter Le Roy's 'scapement is the next step that was made towards this invention. He contrived it in 1748, and, like Thiout's, it has hardly ever been made use of. Both of them have a great recoil to give the wheel before it could be disengaged, and their arcs of free vibration are not much extended. Berthoud informs us, that in 1754 he made a model of one, which he gave to the Royal Aca demy of Sciences. Camus, on its being shown to him at that time, told him that the late Dutertre had made and used such a 'scapement, having a long detent and free vi brations. Nothing appears now to be known of the con struction of Dote' tre's, and Le Roy seems to have acknow ledged the priority of it to the one he contrived in 1748. " My thought, or invention," he says, " was not so new as I had imagined. Dutertre's sons, artists of considerable repute, showed me very soon alter, a model of a watch in this way by their late father, which the oldest Dutertre must still have. This model, very different from my con struction, is, however, the same with respect to the end proposed." The detached 'scapement in Le Roy's time-keeper, which was tried at sea in 1768, is very different from that of 1748.
Berthoud, in his Traite des Horologes Marines, published in 1773, has given, in No. 281, an account of the principle on which 'the model was made in 1754 ; and, in No. 971, a particular description of the parts composing it, which are represented in plate xix. fig. 4. of that work. It may be somewhat interesting to lay before our readers what is contained in No. 281. " I composed," says he, " in 1754, an 'scapement upon a principle, of which I made a model, in which the balance makes two vibrations in the time that one tooth only of the wheel escapes, that is to say, the time in which the balance goes and comes back on itself; and, at the return, the wheel escapes and restores, in one vibra tion, the motion that the regulator or balance had lost in two. The 'scapement wheel is of the ratchet sort, whose action remains suspended (while the balance vibrates free ly) by an anchor, or click, fixed to an axis carrying a lever with a (leer's-foot joint, the lever corresponding to a pin placed near the centre of the axis of the balance. When the balance retrogrades, the first vibration being made, the pin which it carries turns a little back the deer's-foot joint, and the balance continuing freely its course, its liberty not being disturbed during the whole of this vibration, but by a very small and short resistance of the deer's foot joint spring. When the balance comes back on itself and makes
the second vibration, the same pin which it carries raises the deer's foot lever in such a way, that the anchor which it carries unlocks the wheel, in order that it may restore to the balance the force which it had lost in the first vibra tion. This effect is produced in the following manner : In the instant that the deer's-foot jointed lever is raised, the wheel turns and acts upon the lever of impulsion, formed with a pallet of steel which acts upon the wheel, and with another arTi which acts on a steel-roller placed near the axis of the balance ; and, in the same instant that the wheel acts upon the lever of impulsion, the second arm, which its axis carries, and which is the greatest, stays on the roller, and the motion of the wheel is communicated to the balance almost without loss and without friction, and by the least decomposition of force. As soon as the wheel ceases to act on the lever of impulsion, it falls again, and presents itself to another tooth." " To render the vibra tions of the balance more free and independent of the wheel work," continues Berthoud in No. 282, 44 and diminish as much as possible the resistance it meets with at every vi bration, the pin must be placed very near the centre of the balance, so that the lever may not be made to describe a greater course than that required to render the effect of the click perfectly sure, and while the balance turns, and makes its two vibrations, prevent only one tooth of the wheel from escaping ; an effect which would be danger ous, by the seconds' hand, which is carried by the wheel, announcing more seconds, or time, than the balance by its motion would have measured. It was the dread of such a defect that made me then give this 'scapement up, which, I confess, seemed to be rather flattering; but it did not give to the mind that security in its effects which is so necessary, pAt ticularly in marine thne-keepers, the use of which is of too great consequence, to allow any thing suspicious in them to be hazarded." The principle given here by Berthoud is the same as that of the detached 'scapement now made, although the parts of the model are more complex. This 'scapement had received a variety of modifications under his hand. In 1768, he had five marine clocks planned to have spring detents to their 'scapements, the lifting spring being placed on the roller, or pallet, which received the impulse. These were not finished till 1782. Subsequent improve ments, made by the late Mr Arnold and others, can hardly be considered as differing very materially from those of Berthoud. This 'scapement in pocket watches may some times come under such circumstances as have been noticed with Tyrer's ; but no other can well be admitted into box chronometers, whether it is made in the manner of Ar nold, or in that of Earnshaw. In the 'scapcment of Ar nold, (see Plate CC CIII. Fig. 2.) that part of the face of the pallet at the point, or nearly so, on meeting the cycloi dal curved tooth to give impulse, rolls, as it were, down on this curve, for one half of the angle, and in the other goes up ; or it may be thus expressed—the curve goes in on the pallet for the first part of the impulse, and comes out during the last. In making this curve too circular near the point of the tooth, as has been done by some, when the drop is on the nice side, the pallet has to turn a little way before the wheel can move forward, which has sometimes caused stopping ; but, where attention is given to the pro per form, this is not likely to happen. In that of Earn shaw, (see Fig. 3 ) the face of the pallet is considerably undercut. Here, the point of the tooth will slide up for the first part of the impulse, and down in the last ; in the first it seems to have little to do, and may acquire some velocity in order to overcome the part it has to perform in the last. The face of the pallet being undercut, had been found requisite from experience, as is said, in order to prevent cutting or wearing. In Berthoud's box-chro nometers, or time-keepers, the face of the pallet is made straight, or in a line to the centre. One of these, after twenty-eight years going, the greater part of which was from Europe to India, and in the Chinese seas, was put into our hands, and neither the face of the steel pallet, or that of the detent, had the least appearance of being any way marked. This was the more remarkable, as the wheel was uncommonly thin. It must have been made of very fine brass. The wheel had ten teeth ; the. ratio of the diameter to that of the roller, or pallet, was .530 to .340; the balance weighed 174 grains, and made two vibrations in a second. The balance was suspended by a short and weak spring, which had been broken by some accident be fore we got it. The length of this suspension spring re quired to be .9 of an inch, and of so delicate a nature, that many were made for it before the chronometer could be brought any way near to time. It seemed, indeed, to have more influence on the timing than any spiral spring could possibly have. Each of the balance pivots turned between the rollers, which were more than one inch in diameter ; and from them and the suspension sluing, perhaps, arose that ease and freedom in the motion of the balance, in con sequence of which the balance wheel teeth had little to do when impelling the roller or pallet ; and this may have been one cause why the pallet face was not cut or marked. It may be observed, that it had the common spiral balance spring, and a compensation consisting of two laminx, or blades of brass and steel, pinned together and rivetted ; and in the moveable end was a screw, which, by its connection with an arm in which the curb pins were placed, served to regulate for mean time. Three screws in the balance were also used for t his purpose.