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friction; and, not being liable to the wear which takes place in metal rubbing upon metal, the , machine, with that addition, not only becomes more durable, but acuires a degree of uniformity in the motion of the pieces, which is very favourable to the regularity of its going. That extraordinary man John Harrison having produced the first portable machines, which, upon repeated trials, met with success, to the extent required for the great reward offered by the British parliament, must be reckoned the father of modern chronometry; and his long and active career has proved extremely useful, by stimulating with so bright an cxample other artists to similar endeavours. The principles of Mr. Harrison's watches are well known ; and, as most parts of his construction have been superseded by more simple contrivances, we shall only mention the principalinventions of which he appears to be the author, and which are still used by the watchmakers of the present day. The going fusee is one among those inventions which have proved the most ge. nerally useful in practice. By this simple mechanism, the main spring, while the watch is going, acts on an intermediate. short spring, which Harrison calls the secondary spring, and is constantly kept bent to a certain tension by the former ; and when the watch is winding up, and the principal spring ceases to act, the secondary spring being placed in a ratchet wheel, which is hindered from retrograding by a click, continues the motion without alteration. Other contrivances have been proposed, and executed, to make time-pieces go while winding up ; but none which, like this, combines the advantage of simplicity, and the property of providing a supplementary power, which is equal to that of the main spring when its action ceases. And it is to be observed, that the utility of the going fusee, which has induced manufacturers to introduce it into all good watches, is peculiarly important in those time-pieces which have not the power of setting themselves in motion, as is the case with the best modern escapements. Harrison invented also a compensation for the effects of heat and cold; which at the time was perfectly new, and has led to the improvements made afterwards in that essential requisite of time-keepers. The alterations to which the length, of the pendulum is liable, by the different degrees of heat and cold, affect the going of clocks with that sort of regulator, (see PENDULUM); and watches, with a balance,

are still more subject to irregularity from

that source; because not only the ba

lance expands or contracts, according to

the rise or fall of the thermometer, but

the regulating spring itself, while it suf.

fers similar changes, becomes weaker or

stronger; so that, from these causes, a

time-piece must go slower or faster in too

great a proportion to be overlooked or neglected. Graham (Philosophical Transactions, 1726) is the first who thought of applying two metals, of different expansibility, to correct the errors proceeding from the variation of temperature in a pendulum; but as he seemed to have had in view to effect it immediately, without the aid of mechanism, he was obliged to fix on steel and mercury, these being the metals which offered to him the greatest difference of dilatation and contraction, Harrison, by multiplying the bars, increased the total length of the two metals acting on one another, without exceeding the limits of the pendulum; and thereby produced a sufficient compensation with brass and steel, in the compound or gridiron pendulum, which has been almost universally adopted ever since. This contrivance could not be easily applied to balances; but Harrison, following still the

principle of the different expansibility of

metals, applied it in a manner which had not been thought of before, and made it act on the spiral spring, in order to produce the desired compensation in the regulator. This method is described as follows: (Principles of Mr. Harrison’s Timekeeper, p. xii. notes.) “The thermometer kirb is composed of two thin plates of brass and steel riveted together in several places, which, by the greater expansion of brass than steel by heat, and contraction by cold, becomes convex on the brass side in hot weather, and convex on the steel side in cold weather; whence, one end being fixed, the other end obtains a motion corresponding with the changes of heat and cold, and the two pins at the end, between which the balance spring passes, and which it touches alternately as the spring bends and unbends itself, will shorten or lengthen the spring, as the change of heat and cold would otherwise require to be done by the hand, in the manner used for regulating a common watch.” This kind of compensation has been since applied in other ways; but the method, in general, is liable to some material objections, on account of its altering the length of the balance spring, and the difficulty, perhaps impossiblity, of ef. fecting with it an accurate adjustment. Pierre le Roy, eldest son and successor to Julien le Roy, the companion and friend of H. Sully, had the merit of accomplishing the great desideratum of making the compensation on the balance itself. In the chronometer, which was presented to the king of France the 5th August, 1766, and obtained the prize of the Academy of Sciences of Paris the 31st of the same month, that author executed a compensation in the balance, which he has fully explained in his description of that machine. (“Mémoires sur la meilleure Manière de mesurer le Temps en Mer, qui a remporté le Prix double au judgement de l'Academie Royale des Sciences. Contenant la Description de la Montre a Longitudes, presentée à sa Majesté le 5 Aout, 1766.” Par M. le Roy, Horloger du Roi, pp. 41 to 44.) This memoir accompanies the account of Cassini’s voyage in 1668, published in 1770. This compensation is composed (fig. 4.) of two thermometers, t K t A K, of mercury and spirits of wine, made eachin the form of a parallelgogram, except in the upper branch, which bears the ball containing the spirits of wine, and is a little bent downwards; the mercury is in the lower part, and the vertical branch of the tube, A K, is open at the upper end. These two thermometers are placed opposite one another, the axis of the balance being in the same plane with the centrical lines of the tubes; and the thermometers and balance are solidly attached together, and form a well poised and steady regulator. At the middle temperature of the atmosphere, the quicksilver stands at K At Ki, but, when an increase of heat occurs, the alcohol, by its expansion, forces the mercury from the exterior branch, t K t, towards A K, and a portion of the mass of the regulator contracts by approaching the centre. On the contrary, if the variation consist of an additional degree of cold, the mercury moves to: the exterior branch, and the weight towards the circumference of the balance becomes greater. Thus, if the thermometers are well adjusted, the effects of all the changes of temperature in the balance will be compensated, and the regulator will act with the same uniformity as if its figure were not liable to such alterations. Peter le Roy mentions his knowledge of Harrison's expansion curb, acquired soon after he made his thermometrical balance; and he constructed a balance accordingly on the principle of Harrison's curb, the arms or arcs of which act by flexure, and are adjusted by moveable weights. He proved the effect of such

arms by experiment; but gave the preference to his own mercurial compensation. Peter le Roy’s second invention is the same as is now used: but though so publicly declared in the face of the French government and academy in 1766, Arnold took an English patent for it in 1782. We have not yet taken any notice of improvements made in the escapement, because after all the plans proposed for this most essential part of chronometers, the principle of what is called the detached escapement is the only one now used; and, being established upon long experience, seems to merit the prefer. ence given to it over all the constructions proposed till now. We shall content ourselves with stating in a general manner the beginning and progress of that escapement. In all the escapements known till the middle of the last century, the escape wheel was in continual contact with the F. belonging to the axis of the baance wheel; and the friction proceeding from this circumstance may be consider. ed as a principal source of irregularity in the going of the watches. Suppose that a regulator should be made so perfect as to be exactly isochronal, while vibrating in a free situation, that advantage would be diminished or lost as soon as it was placed in connection with a train of wheels; and the errors would be more or less,'according to the nature and quantity offriction in the escapement. It would be, therefore, extremely useful to secure to the regulator a perfect liberty of vibration, except during the short intervals of time .. may be necessary for the action of the escape wheel, to give it a new impulse. This ingenious idea was also started by P. le Roy, and carried into execution by the same artist, in a model which he presented in 1748 to the Academy of Sciences of Paris, and is described in the collection of machines approved by that society for the same year, vol. vii. p. 385. The effect or action of le Roy's escapement in few words is this: An escape wheel is kept in repose by a lever detent. . The balance unlocks the detent, and receives an impulse or stroke on a pallet through a part of every second vibration; and during great part of its course it is free and detached. About the year, 1755, according to Count de Bruhl, the late Mr. Thomas Mudge invented a detached escapement, and applied it to a watch which he made for the king of Spain, Ferdinand VI. This is the same escapement that was

used by the late Josiah Emery, in his chronometers, some of which have sone very well. It differs from the constructions which we have already explained, both in the detent and in the communication of the impulse, which in this mechanism takes place at every vibration: but the date will not suffer us to consider it as the first invention of the detached escapeinent. This justly celebrated artist afterwards made a chronometer, in which the vibrations of balance were kept up by secondary springs attached to two pallets, each of which was wound up by the last wheel of the train during the time employed by the balance in its vibration, unconnected with that pallet. Though this invention is highly ingenious, and was rewarded by parliament, it is now generally considered as unsafe in the locking of the hooks, or detent parts, which terminate the pallets. Mr. Alexander Cumming executed a similar escapement for clocks long before Mudge actually carried his idea into effect, though it has been contended that Mudge had the mere notion much earlier. But Mr. Cumming, to whom our art and the other branches of mechanics are highly indebted for his labours, and his Treatise on Clock and Watch 'Making, made his detents separate from his pallets, and by that means avoided the chief defect of the construction afterwards adopted by Mudge. Our limits will not, however, allow us to pursue these and other improvements and variations, adopted by our own and by foreign artists.

We must confine ourselves to the construction used at present by the English watchmakers; and shall begin with that of the late Mr. Anold, as described in his statement, presented by his son to the board of longitude.

The teeth of the escape wheel (fig. 5.) are of a cycloidal shape, in the face part, which is intended for action, the section of which, with those of the two other sides, form a sort of mixed triangle. BB d represents the detent, which is formed of . a flexible piece or spring, bending between C and N i and in the part N B d, which is stronger than the other, is fixed the locking pallet, a, opposite an adjusting screw F. The pallet, projecting below the spring detent, locks upon the interior angle of the tooth; suspending the motion of the escape wheel, and leaving the balance to vibrate free, as pointed out in the preceding escapements. The action of the spring detent (for the joint of the detent is itself a spring) presses the

locking pallet against the screw, F, except at the time of unlocking the wheel. A very delicate spring N e, called the discharging or unlocking spring (and also the tender spring,) is attached by one end, N, to the spring detent, C B N B ai and, passing under the adjusting screw, F, extends a little beyond the extremity, d, of the detent itself. : H H H is a circular piece attached to the axis of the balance, and, o, the discharging pallet. This pallet, when the balance is in motion from e to d, presses against the end of the discharging spring, n e i and carrying it, together with the locking spring, B B d, disengages the locking piece, a, out of the internalangle of the tooth, with which it was in contact; and the escape wheel

then communicates a new power to the

balance, by its impulse on a pallet, m, which is fixed or set in the aperture of the circular piece. As soon as this is done, the spring detent, or lockingspring, falls back to its position against the adjusting screw, F , and the pallet, by receiving or intercepting the next tooth, stops the motion of the escape wheel. When the balance returns from d to e, the unlocking pallet acts again on the extremity of the discharging spring ; but this, being very delicate, gives way without disturbing the detent or locking spring; and the balance, after suffering a trifling degree of resistance by that contact, continues its free vibrations. At the next vibration the unlocking takes place ; and the action of the escapement proceeds successively, as explained before. 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 described, 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 circumstance 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 it aside, in order to adopt the escapement with cycloidal teeth, which he esteemed far preferable. In the escapement we are now considering, the escape wheel is shaped as appears in the figure; and, on the inspection 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 whole must be greater, than in the construction with protuberant cycloidalteeth. The detentis of the same kind as

the other, and only differs from it, in being stopped by the inside of the head of the adjusting screw, instead of the extremity 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 already 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 purose, is made flexible near the extremity y 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 detent in the proper situation. The pivots of the old detent are so slender, that its performance cannot be supposed subject to any considerable de#. of friction; and watches, with that ind of detent, have been known to govery well. Some able artists, upon that account, 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 manufactory, 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 invention of the spring detent The general opinion attributes it to the late Mr. Arnold; 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 contrivance 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 remark, that the invention is entirely English, not a single passage existing in the writings of the French authors, by which any one of them might claim it with reason, 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. Berthoud in his “Supplément au Traité 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 construction had been in circulation. We cannot, 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 after the invention of the detached escapement, the isochronism of the vi, brations of the balance, by means of the spiral spring, was, if not newly discovered, at least perfected and brought into general 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 advantage Dr. Hooke asserted himself to have attained with his invention in watches, which had been shown to several persons. The principle, however, could not be generally trusted, according to Dr. Hooke himself, who, in the postscript to his description of Helioscopes, (p. 29,) declares, that he had explained bow the vibrations 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 lectures in Gresham College, was not sufficiently explicit to give precise rulesfor practice, and fix the attention of watch-makers upon the subject. After those hints, the principle seems to have been very little attended to formany years, and the isochronism was frequently attempted to be ef. fected by means of mechanical contrivances in the escapement. Harrison endeavoured to accomplish that important object, by the form of the back of the pallets; and on the return of the voyage to Jamaica, added for the same purpose the cycloidal pin, to regulate the balance spring; but this method of adjustment never apeared satisfactory or certain. P. le Roy, in his “Mémoire sur la meillure Manière de mesurer le Tempsen Mer,” rewarded 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 isochronal, whether long or short ; that the length of this portion being found, if it be lessened, the long vibrations will be quicker than the short ones; and that, on the contrary, if the 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 escapement. Berthoud found that the spiral spring, in order to be isochronal, must have an ascending force in arithemetical progression, and that this property may be affected, not only by the length of the spring, but by the number of coils, and the tapering or decreasing thickness from the centre to the extremity, &c. He adds, besides, the proportions of the tapering in many springs, which he had actually tried, and gives minute accounts of the experiments made with them in several time-keepers. The late Mr. Arnold applied to the balance the cylindrical or helical spring, which had been employed long before to a variety of purposes instead of the spiral, which had been constantly used in watches since the time of Dr. Hooke and M. Huygens. This is one of the articles of his patent of 1782, whence it would appear, that provided the spring be made of that form, the vibrations cannot fail to be isochronal; but experience is contrary to that, notion, and artists are obliged to attend to a variety of circumstances in the application of the helical, as well as that of the spiral spring. At present, some watchmakers think that the helical spring does not possess any advantage with regard to that property; but as the opinion of other persons is in the affirmative, while all the manufacturers, as far as our knowledge goes, agree in considering the cylindrical form as more easily managed than the other, its application seems entitled to the merit of a practical improvement. Mr. Earnshaw, in the explanation of his time-keepers presented to the board of longitude, after noticing the insufficieny of the cylindrical spring, states, that he

had, by long preseverance, found how to make springs increasing in thickness to the outer end, in order to effect the isochronism of the vibrations. This method of obtaining isochronal vibrations had been long before explained by Berthoud, with regard to the spiral spring, in that part of his treatise on marine time-pieces which we have already quoted.

This artist states as a considerable discovery, that the balance spring falls off or tires in its strength, and he gives an allowance for it; but the fact is neither so constant nor so equable as to admit of his general remedy.

Fig. 7, represents the balance of a chronometer, or time-piece, as usually mad; by our artists. A circular groove is turned in the flat face of a piece of steel, and into this groove a piece of good brass is driven, and a little of the solution of boraxis applied, to prevent oxydation. This compound piece being then put into a crucible, is made sufficently hot to melt the brass; which in these circumstances adheres firmly to the steel without requiring any solder. The face of the steel is then cleaned, and by properapplication of the mechanical means of turning, boring, and filing, the superfluous steel is taken away, and the balance is left, consisting of two or sometimes three radii, and a rim, the external part of which is brass, and the internal part steel, the former metal being about twice the thickness of the latter. Some artists solder the metals together; and others plunge the steel balance into melted brass, and suffer them to cool together, but the method we have described appears to be the best. In this state the arcs of the rim are then cutthrough and diminished in their length as in the figure; and near that extremity of each arc which is farthest from its ra. dius, a piece or weight is put on, which can be slided along the arm so as to be adjusted, at that distance, which upon trial shall be found to produce a good performance, under the different changes of temperature. For it scarcely need be observed, that the flexure of these arms, by change of temperature, will carry the weights nearer to the centre in hot than in cold weather; and the more, the greater the distance of the weights from the radius. The small screws near the ends of the radii afford an adjustment for time, as the balance will vibrate more quickly, the further these are screwed in ; and the contrary will be the case, if they be unscrewed or drawn further out,

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