The detached escapement used by Mr. Earnshaw, is represented in fig. 6, which is taken from his statement presented to the board of longitude. This escapement is similar to that of Arnold's, already de scribed, except in small variations, which will be easily perceived on a comparison of the two figures. It is besides asserted, and it appears probable from every cir cumstance relative to these constructions, that the late Mr. Arnold had made use of this form of escapement long before Mr. Earnshaw, but that he had laid itaside, in order to adopt the escapement with cy cloidal teeth, which he esteemed far pre ferable. In the escapement we are now considering, the escape wheel is shaped as appears in the figure ; and, on the in spection of this representation, it will be easily observed, that the teeth presenting a right line, and escaping by a sharp point, their action cannot be so smooth, and the wear of the wholemust be greater, than in the construction with protuberant cycloid+. al teeth. The detent is of the same kind as the other, and only differs from it, in be ing stopped by the inside of the head of the adjusting screw, instead of the extre mity of the screw itself, and unlocking outwards and not towards the centre.
The two constructions, which may be considered as the same, differ from the French detached escapements, such as those of F. Berthoud, which we have al ready explained in the detent. In the new detent, the pivots are abolished, and the centre of motion is established in the locking piece itself; which, for that pur pose, is made flexible near the extremity by which it is fixed. The elasticity of the detent, or locking piece, supplies also the office of a strait auxiliary spring, placed behind the lever of the detent, or a spiral spring, which has been sometimes applied to the axis of the pivots, to keep the de tentin the proper situation.
The pivots of the old detent are so slender, that its performance cannot be supposed subject to any considerable de gree of friction ; and watches, with that kind of detent, have been known to go ve ry well. Some able artists, upon that ac count, think that the new detent is only preferable to the other, because it saves work and is less expensive ; but while the spring detent is allowed to perform as well, if not better, than the detent with pivots, which its universal use in this country seems to prove, that property, combined with the economy in the manu factory, must secure to the mechanism in question the character of an improve ment in the construction of time-keepers.
To whom are we indebted for the in vention of the spring'detent ? The general opinion attributes it to the late Mr. Ar nold; and we do not see any reason of sufficient weight to refuse him that merit. Mr. Earnshaw has claimed it in his own favour ; but Mr Arnold's labours have, at least, the advantage of priority and the strength of this advantage not having been done away by any proofs, which in our opinion can be esteemed satisfactory, must decide our judgment in the present case, as in the like controversies upon other points, which have been considered in the course of this inquiry. The con trivance of the locking spring, or spring detent, therefore, appears to us to be due to the late Mr. Arnold. With regard to this mechanism, it is also worthy of re mark, that the invention is entirely Eng lish, not a single passage existing in the writings of the French authors, by which any one of them might claim it with rea son, or even plausibility. The first men
tion of any thing like the locking spring, to be found in foreign publications, is the detent without pivots, given by F. Ber thoud in his " Supplement au Traite des Horloges Marines ;" but that book was published in 1787, that is, five years after Mr. Arnold had taken out his patent, and when many watches upon that construc tion had been in circulation. We eannot, therefore, allow him the credit of this thought; nor do we find that other French artists have availed themselves of that hint, to carry the spring detent to the great degree of simplicity which it has attained in this country.
A little afterthe invention of the detach ed escapement, the isochronism of the vi brations of the balance, by means of the spiral spring, was, if not newly discover ed, at least perfected and brought into ge neral notice, and that principle added a great value to the detached escapement, while this mechanism secured the utility of the principle, by offering the species of insulated balance which it required. From some theories and experiments long known to the world, it would appear that the vibration of a spring might be always supposed of equal duration ; and that ad vantage Dr. Ilooke asserted himself to have attained with his invention in watch es, which had been shown to several per sons. The principle, however, could not be generally trusted, according to Dr. Hooke himself, who, in the postscript to his descriptionof Helioscopes,(p.29,) de clares, that he had explained how the vi brations might be so regulated, as to make their durations, either all equal, or the greater, slower, or quicker, than the less, and that in any proportion assigned. We must suspect that these ideas were not properly digested, or regret that their communication by the author, in his lec tures in Gresham College, was not suffici ently explicit to give precise rules for prac tice, and fix the attention of watch-makers upon the subject. After those hints, the principle seemstohave been very little at tended to for many ears, and the isochro nism was frequently attempted to be ef fected by means of mechanical contriv ances in the escapement. Harrison endea voured to accomplish that important ob ject, by the form of the back of the pallets and on the return of the voyage to Jamaie a. added for the same purpose the cycloid:, pin, to regulate the balance spring; but this method of adjustment never ap peared satisfactory or certain. P. le Hoy, in his " Mernoire sur la meillure Maniere de mesurer le Temps en Ater," rep: added in 1766, first announced distinctly the discovery of a general principle, proper to produce the isochronism, by means of the balance spring, which is expressed as follows : that in every spring sufficiently long, a certain portion of it will be iso chronal, whether long or short ; that the length of this portion being found, if it be lessened, th elong vibrations will be quick er than the short ones ; and that, on the contrary, lithe length be increased, the small arcs will be performed in less time than the great arcs." This important property of the spring enabled P. le Roy to bring to a happy issue his labours for the improvement of chronometry : and the art is indebted to him for the practi cal utility of that discovery, as much as for the invention of the detached escape ment.