132 



HOROLOGY. 



Free or 

 detached 



then lockd ; when unlocked, it gives impulse which 

 only takes place at every second vibration. In Mudge's 

 detached 'scapement, the impulse is given at every vi- 

 bration. 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 page 110 of the first volume of 

 his work, and shewn 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 li- 

 berty 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 be- 

 fore it could be disengaged, and their arcs of free vibra- 

 tion are not much extended. Berthoud informs us, that 

 in 1751 he made a model of one, which he gave to the 

 lioy al Academy 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 vibrations. Nothing appears now to be known 

 of the construction of Dutertre's, and Le Roy seems to 

 have acknowledged the priority of it to the one he con- 

 trived in 1 748. " My thought, or invention," he says, 

 " was not so new as I had imagined. Dutertre's sons, 

 artists of considerable repute, shewed me very soon af- 

 ter, 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 construction, 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 Horloges Marines, pub- 

 lished in 1773, has given, in No. 281, an account of the 

 principle on which the model was made in 1754; and, 

 ent< in No. 971, a particular description of the parts com- 

 posing it, which are represented in plate xix. fig. 4. of 

 that work. It may be somewhat interesting to lay be- 

 fore our readers what is contained in No. 281. "I 

 composed," says he, " in 1751, an escapement upon a 

 principle, of which I made a model, in which the ba- 

 lance 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 vi- 

 bration, 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 freely) by an anchor, or click, fixed to an axis 

 carrying a lever with a deer's-foot joint, the lever cor- 

 responding 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 conti- 

 nuing 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 

 lecond vibration, the same pin which it carries raises 



Bcrthoud's 

 model of a 

 detached 



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 vibration. This effect is produced in the fol- 

 lowing manner : In the instant that the deer's-foot joint- 

 ed 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 arm 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 mo- 

 tion of the wheel is communicated to the balance al- 

 most 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 

 vibrations of the balance more free and independent of 

 the wheel-work,'' continues Berthoud in No. 282. " and 

 diminish as much as possible the resistance it meets with 

 at every vibration, the pin must be placed very r.ear 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 ba- 

 lance turns, and makes its two vibrations, prevent only 

 one tooth of the wheel from escaping ; an effect which 

 would be dangerous, 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, particularly in ma- 

 rine time-keepers, the use of which is of too great con- 

 sequence, to allow any thing suspicious in them to be 

 hazarded." 



The principle given here by Berthoud is the same as 

 that of the detached 'scapements now made, although 

 the parts of the model are more complex. This 'scape- 

 ment 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 recei- 

 ved the impulse. These were not finished till 1782. 

 Subsequent improvements, 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 sometimes come under such cir- 

 cumstances as have been noticed with Tyrcr's ; but no 

 other can well be admitted into box-chronometers, whe- 

 ther it is made in the manner of Arnold, or in that of 

 Earnshaw. In the 'scapement of Arnold, (see Fig. 2.) 

 that part of the face of the pallet, at the point or nearly 

 so, on meeting the cycloidal 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 ex- 

 pressed 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 caus- 

 ed stopping ; but, where attention is given to the proper 

 form, this is not likely to happen. In that of Earnshaw, 

 (see Fig. 3.) the face pf the pallet is considerably under- 

 cut. 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 



Rsetpv 

 ments. 



''"/ - ' 

 Free or de- 

 tached 



'icapcment 



Arnold's 

 'scapemefit 



PLATE 

 CCCIIK 

 Fig. 2. 



Earrishaw 

 'scapeinei 



Fig- 3. 



