Imágenes de páginas
PDF

and the friction proceeding from this circumstance may he considered 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 of friction 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 which may be necessary for the action of the escape wheel, to give it a new impulse. This ingeniousidea 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 gone 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 escapement.

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, ttieneh it has been contended that Madge had the mere notion much earlier. But Mr. Camming, to whom utir art, and the other branches of mechanics are highly indebted for his labours, and his Treatise on Clock and Watch Making, made bis detents separate from his pallets, and by that means avoided the chief defect of the construction afterwards adopted by Mudjee. 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 constructions used at present, by the English watchmakers ; and shall begin with that of the late Mr. Arnold, 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. B 15 d represents the detent, which is formed of a flexible piece or spring, bending between C and N; 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 «; and, passing under the adjusting screw, F, extends a little beyond the extremity, d, of the detent itself. HHH 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 tod, presses against the end of the discharging spring, n e; and, carrying it together with the locking spring, B B d, disengages the locking piece, a, out of the internal angle 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 aper- ture of the circular piece. As soon as this is done, the spring detent, or locking spring,

Mia back to it* position against the adjusting screw, F; and the pallet, by receiving or interrupting the next tooth, stops the motion of the escape wheel. When the balance returns from d to c, 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. Eamshaw 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 nse of this form of escapement long before Mr. Eamshaw, but that he 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 it 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 cycloidal teeth. The detent is 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 purpose, 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 detent in the proper situation.

The pivots of the old detent are so slender, that its performance cannot be supposed subject to any considerable degree of friction ; and watches, with that kind of detent, have been known to go very 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 improvement 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 deride 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 mention of any thing like the locking spring, to be found in foreign publications, is the detent without pivots, given by F. Berthoud in his "Supplement au Traite des Horlnges 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 vibrations 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. 39,) declares that he had explained how 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 rules for practice, and fix the attention of watchmakers upon the subject. After those hints, the principle seems to have been very little attended to for many years, and the isochronism was frequently attempted to be effected 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 cycloiilal pin, to regulate the balance spring; but this method of adjustment never appeared satisfactory or certain. P. le Roy, in his "Memoire sur la meillenre Maniere de mesurer le Temps en 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 incr, ased, 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 chrono

metry; and the art is indebted to him for the practical utility of that discovery, a> much as for the invention of the detached escapement.

Berthoud found that the spiral spring, In order to be isochronal, must have an ascend ing force in arithmetical progression, and that this property may be effected, 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 timekeepers.

The late Mr. Arnold applied to the balauce the cylindrical or helical sprint, 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 docs not po»sess 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, Eamshaw, in the explanation of his timekeepers, presented to the board of longitude, after noticing the insufficiency of the cylindrical spring, states, that he had, by long perseverance, 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 timepieces 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 hit general remedy.

Fig. 7, represent! the balance of a chronometer, or time-piece, as usually made by our artists. A circular groove is turned in the tlat face of a piece of steel, and into this groove a piece of good brass is driven, and ■ little of the solution of borax is applied to prevent oxydation. This compound piece being then put into a crucible; is made sufficiently hot to melt the brass; which in these circumstances adheres firmly to the steel without requiring any solder. Tin- race of the steel is then cleaned, and by proper application 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 cut through and diminished in thenlength as in the figure; and near that extremity of each arc which is farthest from its radius, a piece or weight is put on, which can be slidc.il 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 bychange 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.

Fig. 8. shews a balance according to a construction used by Arnold, and specified by him to the commissioners of longitude. The expansion weights are cylindrical, and are adjusted upon the arm by screwing: and there is an inner rim upon which three weights are adjusted by sliding. These serve to regulate the going of the timepiece in different positions.

If an uniform ring, with two or more radii placed at equal regular distances, and in all other respects alike, were to be poised on its axis, as a balance, no part Mould preponderate, but it would remain

at rest in any position; and if we suppose the axis itself to be a spring, such as a piece of stretched wire, and we overlook the difference of tension in the wire, which might arise from the weight of the balance itself', in different positions, it is obvious that all the vibrations of that balance, through equal arcs, would be performed in equal times, whether the balance were made to vibrate parallel to the horizon, or in any other position. But in the balance of a time piece, the pivots of its axis bear very differently, according to the position of the chronometer; and it requires some management to make the frictions the same, whether the axis be turning on ore of its ends, or upon the two cylindrical faces of the pivots. And still more than this, since the balance itself has a permanent figure, compared with the spring, which in every part of the vibration alters its distance from the axis, and in every part of its length has a different degree oi'rotatary motion, it cannot be expected, nor does it happen, that a balance, which is found to be in poize, along with its spring, when out of the chronometer, will make equal vibration, as to time, iu all positions when in its place. And in addition to these difficulties, there is one part of the arc of vibration where the force of the spring, and the inertia of the balance, are not simply in opposition to each other, but are combined with the maintaining power; namely, during the action of escape. The remedy fur all these difficulties, which is happily adopted in thermometers for use at sea, is to place the axis in a vertical position; by which means the balance itself is not affected by gravity; but for pocket time-pieces, the ingenuity of the artist is called upon for expedients, of which it would be not easy to exhibit a complete theory. The general principle commonly used, and also adopted by Arnold and Karnshaw, as tar as can be leathered fi om the little they say in their specifications to the commissioners of longitude, is to consider the balance, when out of adjustment tor position, as a pendulum weight above and below the centre of suspension, acted upon by gravity,— and at the same time urged to a quiescent point by the force of elasticity. In these circumstances the vibrations will be quickest when, the point ot stable equilibrium is downwards, and they will be slowest in the opposite position of the machine. This leads to the remedy of diminishing either the weight or the radius, on that side which is lowest when the rate is most quick. Thus if one of the two adjusting screws in fig. 7, were downwards, in the position of quickest rate, that screw would require to be screwed a very little quantity inwards, and the opposite screw to be screwed a like quantity outwards, in order to remedy this imperfection without much change in the other adjustments. And if a like imperfection were found in the vibrations of the balance, when tried in a vertical position, having the lowest point at rest, in a line at right angles to the line passing through the radii, a similar alteration must be made in the expansion weights, either by a careful flexure of the circular arcs, or by altering the quantities of those weights; or else by means of small screws tapped into the weights themselves, and directed towards the centre, like the weights at the extremities of the radii.

By there, and other correspondent means, the balance may be made to keep time, in all those positions wherein its plane shall be perpendicular to that of the horizon ; but even in these trials, very great pains and labour may be required to produce a high degree of accuracy; and in this course of operation, the skilful workman may be under the necessity of preparing a great number of expansion weights, and regulating screws, to he applied in trial, whenever the course of adjustment in one part shall carry him beyond the general conditions of the whole machine. And after all, as the quantity of action, in the spring, must alter the quantity of pendulous effect, in this curious and delicate time-measurer, it may be doubted, whether the adjustments for position in the vertical balance can be effectual any longer than while the arcs of vibration continue permanent. '1 his consideration leads us to the necessity of an adjustment in the maintaining power, in order that the vibrations shall not fall off; at the same time that it affords one of the strongest arguments in favour of a remontoire, notwithstanding the experiment of Arnold, which showed that a sea-chronometer (face up) kept the same rate, when those arcs were greatly varied.

When the balance has been adjusted for position in the vertical situation, it does not follow, as a matter of course, that it will keep the same time with its plane horizontal, or face up and face down. In the former cases, the effect of gravity still appears to be combined with that of elasticity, though under circumstances of adjustment;

in the latter, gravity seems to be out of the question. If the rate should demand adjustment with the axis vertical, in order to make it agree with that which obtains when the balance is vertical, it appears necessary, that either the inertia or the elastic force should be altered. The former seems to demand such changes of the screw* or weights, as may alter the effective radius of the balance; the latter requires a change of the spring itself. The artists with whom the writer of this article has conferred, did not seem to have clear notions of any direct method for effecting the purpose here pointed out. It appeared, that they have recourse to several expedients; but that, in general, the rate, face up, of a time-piece, which has been well adjusted in other respects, does not require much additional adjustment.

From all these considerations, with others into which brevity forbids us to enter, together with those which relate to the choice and preparation of materials, and the delicacy and truth of workmanship, the reader will be able to form some judgment of the intelligence and skill with which, chiefly under the sanction of the British government, this important manufacture has bern pursued, and likewise of the ample field for improvement, which remains for the exertions of future artists. See Pendulum, Clock, Train, and Watch-work.

HOROPTER, in optics, a right line drawn through the point where the two optic axis meet parallel to that which joins the centres of the two eyes or the two pupils.

HOROSCOPE, in astrology, is the degree of the ascendant, or the star that rises above the horizon at a certain moment, which is observed in order to predict some future event, as the success of a design, the fortune of a person who was at that instant born, &c. The same name is also given to a scheme or figure containing the twelve houses, in which are marked the situation of the heavens and stars, in order to form predictions. See House.

Horoscope Unar, the point from whence the moon proceeds when the sun is in the ascending point of the cast.

HORS de son fee, an exception to avoid an action brought for rent issuing out of certain lands, by him that ,pretends to be the lord, or for some customs and services; for if the defendant can prove the land to be without the compass of his fee, tie action fails.

« AnteriorContinuar »