These modest means then were the first steps toward the exact subdivisions of time which we now enjoy. Unrest, progress, discontent with things that be, we must acknowledge, have, from the appearance of the first clock to the present hour, been the powers which have driven on the inventive genius of watch and clockmakers to designate some new and more acceptable system for regulating the course of the movement. In consequence of this restless search after the best, a very considerable number of escapements have been invented and made up, both for clocks and watches; only a few, however, of the almost numberless systems have survived the test of time and been adopted in the manufacture of the timepiece as we know it now. Indeed, many such inventions never pa.s.sed the experimental stage, and yet it would be very interesting to the professional horologist, the apprentice and even the layman to become more intimately acquainted with the vast variety of inventions made upon this domain since the inception of horological science. Undoubtedly, a complete collection of all the escapements invented would const.i.tute a most instructive work for the progressive watchmaker, and while we are waiting for a competent author to take such an exhaustive work upon his hands, we shall endeavor to open the way and trust that a number of voluntary collaborators will come forward and a.s.sist us to the extent of their ability in filling up the c.h.i.n.ks.
PROBLEMS TO BE SOLVED.
The problem to be solved by means of the escapement has always been to govern, within limits precise and perfectly regular, if it be possible, the flow of the motive force; that means the procession of the wheel-work and, as a consequence, of the hands thereto attached. At first blush it seems as if a continually-moving governor, such as is in use on steam engines, for example, ought to fulfil the conditions, and attempts have accordingly been made upon this line with results which have proven entirely unsatisfactory.
Having thoroughly sifted the many varieties at hand, it has been finally determined that the only means known to provide the most regular flow of power consists in intermittently interrupting the procession of the wheel-work, and thereby gaining a periodically uniform movement.
Whatever may be the system or kind of escapement employed, the functioning of the mechanism is characterized by the suspension, at regular intervals, of the rotation of the last wheel of the train and in transmitting to a regulator, be it a balance or a pendulum, the power sent into that wheel.
ESCAPEMENT THE MOST ESSENTIAL PART.
Of all the parts of the timepiece the escapement is then the most essential; it is the part which a.s.sures regularity in the running of the watch or clock, and that part of parts that endows the piece with real value. The most perfect escapement would be that one which should perform its duty with the least influence upon the time of oscillation or vibration of the regulating organ. The stoppage of the train by the escapement is brought about in different ways, which may be gathered under three heads or categories. In the two which we shall mention first, the stop is effected directly upon the axis of the regulator, or against a piece which forms a part of that axis; the tooth of the escape wheel at the moment of its disengagement remains supported upon or against that stop.
In the first escapement invented and, indeed, in some actually employed to-day for certain kinds of timekeepers, we notice during the locking a retrograde movement of the escape wheel; to this kind of movement has been given the name of _recoil escapement_. It was recognized by the fraternity that this recoil was prejudicial to the regularity of the running of the mechanism and, after the invention of the pendulum and the spiral, inventive makers succeeded in replacing this sort of escapement with one which we now call the _dead-beat escapement_. In this latter the wheel, stopped by the axis of the regulator, remains immovable up to the instant of its disengagement or unlocking.
In the third category have been collected all those forms of escapement wherein the escape wheel is locked by an intermediate piece, independent of the regulating organ. This latter performs its vibrations of oscillation quite without interference, and it is only in contact with the train during the very brief moment of impulse which is needful to keep the regulating organ in motion. This category const.i.tutes what is known as the _detached escapement_ cla.s.s.
Of the _recoil escapement_ the princ.i.p.al types are: the _verge escapement_ or _crown-wheel escapement_ for both watches and clocks, and the _recoil anchor escapement_ for clocks. The _cylinder_ and _duplex escapements_ for watches and the _Graham anchor escapement_ for clocks are styles of the _dead-beat escapement_ most often employed. Among the _detached escapements_ we have the _lever_ and _detent_ or _chronometer escapements_ for watches; for clocks there is no fixed type of detached lever and it finds no application to-day.
THE VERGE ESCAPEMENT.
The _verge escapement_, called also the _crown-wheel escapement_, is by far the simplest and presents the least difficulty in construction. We regret that the world does not know either the name of its originator nor the date at which the invention made its first appearance, but it seems to have followed very closely upon the birth of mechanical horology.
Up to 1750 it was employed to the exclusion of almost all the others. In 1850 a very large part of the ordinary commercial watches were still fitted with the verge escapement, and it is still used under the form of _recoil anchor_ in clocks, eighty years after the invention of the cylinder escapement, or in 1802. Ferdinand Berthoud, in his "History of the Measurement of Time," says of the balance-wheel escapement: "Since the epoch of its invention an infinite variety of escapements have been constructed, but the one which is employed in ordinary watches for every-day use is still the best." In referring to our ill.u.s.trations, we beg first to call attention to the plates marked Figs. 145 and 146.
This plate gives us two views of a verge escapement; that is, a balance wheel and a verge formed by its two opposite pallets. The views are intentionally presented in this manner to show that the verge _V_ may be disposed either horizontally, as in Fig. 146, or vertically, as in Fig.
145.
[Ill.u.s.tration: Figs. 145 and 146]
[Ill.u.s.tration: Fig. 147]
Let us imagine that our drawing is in motion, then will the tooth _d_, of the crown wheel _R_, be pushing against the pallet _P_, and just upon the point of slipping by or escaping, while the opposite tooth _e_ is just about to impinge upon the advancing pallet _P"_. This it does, and will at first, through the impulse received from the tooth _d_ be forced back by the momentum of the pallet, that is, suffer a recoil; but on the return journey of the pallet _P"_, the tooth _e_ will then add its impulse to the receding pallet. The tooth _e_ having thus accomplished its mission, will now slip by and the tooth _c_ will come in lock with the pallet _P_ and, after the manner just described for _e_, continue the escapement. Usually these escape wheels are provided with teeth to the number of 11, 13 or 15, and always uneven. A great advantage possessed by this form of escapement is that it does not require any oil, and it may be made to work even under very inferior construction.
OLDEST ARRANGEMENT OF A CROWN-WHEEL ESCAPEMENT.
[Ill.u.s.tration: Fig. 148]
Plate 147 shows us the oldest known arrangement of a crown-wheel escapement in a clock. _R_ is the crown wheel or balance wheel acting upon the pallets _P_ and _P"_, which form part of the verge _V_. This verge is suspended as lightly as possible upon a pliable cord _C_ and carries at its upper end two arms, _B_ and _B_, called adjusters, forming the balance. Two small weights _D D_, adapted to movement along the rules or adjusters serve to regulate the duration of a vibration. In Fig. 148 we have the arrangement adopted in small timepieces and watches: _B_ represents the regulator in the form of a circular balance, but not yet furnished with a spiral regulating spring; _c_ is the last wheel of the train and called the _fourth wheel_, it being that number distant from the great wheel. As will be seen, the verge provided with its pallets is vertically placed, as in the preceding plate.
[Ill.u.s.tration: Fig. 149]
Here it will quickly be seen that regarded from the standpoint of regularity of motion, this arrangement can be productive of but meager results. Subjected as it is to the influence of the slightest variation in the motive power and of the least jar or shaking, a balance wheel escapement improvided with a regulator containing within itself a regulating force, could not possibly give forth anything else than an unsteady movement. However, mechanical clocks fitted with this escapement offer indisputable advantages over the ancient clepsydra; in spite of their imperfections they rendered important services, especially after the striking movement had been added. For more than three centuries both this crude escapement and the cruder regulator were suffered to continue in this state without a thought of improvement; even in 1600, when Galileo discovered the law governing the oscillation of the pendulum, they did not suspect how important this discovery was for the science of time measurement.
GALILEO"S EXPERIMENTS.
[Ill.u.s.tration: Fig. 150]
Galileo, himself, in spite of his genius for investigation, was so engrossed in his researches that he could not seem to disengage the simple pendulum from the compound pendulums to which he devoted his attention; besides, he attributed to the oscillation an absolute generality of isochronism, which they did not possess; nor did he know how to apply his famous discovery to the measurement of time. In fact, it was not till after more than half a century had elapsed, in 1657, to be exact, that the celebrated Dutch mathematician and astronomer, Huygens, published his memoirs in which he made known to the world the degree of perfection which would accrue to clocks if the pendulum were adopted to regulate their movement.
[Ill.u.s.tration: Fig. 151]
An attempt was indeed made to s.n.a.t.c.h from Huygens and confer upon Galileo the glory of having first applied the pendulum to a clock, but this attempt not having been made until some time after the publication of "Huygens" Memoirs," it was impossible to place any faith in the contention. If Galileo had indeed solved the beautiful problem, both in the conception and the fact, the honor of the discovery was lost to him by the laziness and negligence of his pupil, Viviani, upon whom he had placed such high hopes. One thing is certain, that the right of priority of the discovery and the recognition of the entire world has been incontestably bestowed upon Huygens. The escapement which Galileo is supposed to have conceived and to which he applied the pendulum, is shown in Fig. 149. The wheel _R_ is supplied with teeth, which lock against the piece _D_ attached to a lever pivoted at _a_, and also with pins calculated to impart impulses to the pendulum through the pallet _P_. The arm _L_ serves to disengage or unlock the wheel by lifting the lever _D_ upon the return oscillation of the pendulum.
[Ill.u.s.tration: Fig. 152]
[Ill.u.s.tration: Fig. 153]
A careful study of Fig. 150 will discover a simple transposition which it became necessary to make in the clocks, for the effectual adaptation of the pendulum to their regulation. The verge _V_ was set up horizontally and the pendulum _B_, suspended freely from a flexible cord, received the impulses through the intermediation of the forked arm _F_, which formed a part of the verge. At first this forked arm was not thought of, for the pendulum itself formed a part of the verge. A far-reaching step had been taken, but it soon became apparent that perfection was still a long way off. The crown-wheel escapement forcibly incited the pendulum to wider oscillations; these oscillations not being as Galileo had believed, of unvaried durations, but they varied sensibly with the intensity of the motive power.
THE ATTAINMENT OF ISOCHRONISM BY HUYGENS.
Huygens rendered his pendulum _isochronous_; that is, compelled it to make its oscillations of equal duration, whatever might be the arc described, by suspending the pendulum between two metallic curves _c c"_, each one formed by an arc of a cycloid and against which the suspending cord must lie upon each forward or backward oscillation. We show this device in Fig. 151. In great oscillations, and by that we mean oscillations under a greater impulse, the pendulum would thus be shortened and the shortening would correct the time of the oscillation.
However, the application of an exact cycloidal arc was a matter of no little difficulty, if not an impossibility in practice, and practical men began to grope about in search of an escapement which would permit the use of shorter arcs of oscillation. At London the horologist, G.
Clement, solved the problem in 1675 with his rack escapement and recoil anchor. In the interval other means were invented, especially the addition of a second pendulum to correct the irregularities of the first. Such an escapement is pictured in Fig. 152. The verge is again vertical and carries near its upper end two arms _D D_, which are each connected by a cord with a pendulum. The two pendulums oscillate constantly in the inverse sense the one to the other.
[Ill.u.s.tration: Fig. 154]
[Ill.u.s.tration: Fig. 155]
ANOTHER TWO-PENDULUM ESCAPEMENT.
We show another escapement with two pendulums in Fig. 153. These are fixed directly upon two axes, each one carrying a pallet _P P"_ and a segment of a toothed wheel _D D_, which produces the effect of solidarity between them. The two pendulums oscillate inversely one to the other, and one after the other receives an impulse. This escapement was constructed by Jean Baptiste Dutertre, of Paris.
Fig. 154 shows another disposition of a double pendulum. While the pendulum here is double, it has but one bob; it receives the impulse by means of a double fork _F_. _C C_ represents the cycloidal curves and are placed with a view of correcting the inequality in the duration of the oscillations. In watches the circular balances did not afford any better results than the regulating rods or rules of the clocks, and the pendulum, of course, was out of the question altogether; it therefore became imperative to invent some other regulating system.
[Ill.u.s.tration: Fig. 156]
[Ill.u.s.tration: Fig. 157]
It occured to the Abbe d"Hautefeuille to form a sort of resilient mechanism by attaching one end of a hog"s bristle to the plate and the other to the balance near the axis. Though imperfect in results, this was nevertheless a brilliant idea, and it was but a short step to replace the bristle with a straight and very flexible spring, which later was supplanted by one coiled up like a serpent; but in spite of this advancement, the watches did not keep much better time. Harrison, the celebrated English horologist, had recourse to two artifices, of which the one consisted in giving to the pallets of the escapement such a curvature that the balance could be led back with a velocity corresponding to the extension of the oscillation; the second consisted of an accessory piece, the resultant action of which was a.n.a.logous to that of the cycloidal curves in connection with the pendulum.
CORRECTING IRREGULARITIES IN THE VERGE ESCAPEMENT.
Huygens attempted to correct these irregularities in the verge escapement in watches by amplifying the arc of oscillation of the balance itself. He constructed for that purpose a pirouette escapement shown in Fig. 155, in which a toothed wheel _A_ adjusted upon the verge _V_ serves as an intermediary between that and the balance _B_, upon the axis of which was fixed a pinion _D_. By this method he obtained extended arcs of vibration, but the vibrations were, as a consequence, very slow, and they still remained subject to all the irregularities arising from the variation in the motive power as well as from shocks. A little later, but about the same epoch, a certain Dr. Hook, of the Royal Society of London, contrived another arrangement by means of which he succeeded, so it appeared to him at least, in greatly diminishing the influence of shock upon the escapement; but many other, perhaps greater, inconveniences caused his invention to be speedily rejected. We shall give our readers an idea of what Dr. Hook"s escapement was like.
[Ill.u.s.tration: Fig. 158]
[Ill.u.s.tration: Fig. 159]
On looking at Fig. 156 we see the escape wheel _R_, which was flat and in the form of a ratchet; it was provided with two balances. _B B_ engaging each other in teeth, each one carrying a pallet _P P"_ upon its axis; the axes of the three wheels being parallel. Now, in our drawing, the tooth _a_ of the escape wheel exerts its lift upon the pallet _P"_; when this tooth escapes the tooth _b_ will fall upon the pallet _P"_ on the opposite side, a recoil will be produced upon the action of the two united balances, then the tooth _b_ will give its impulse in the contrary direction. Considerable a.n.a.logy exists between this form of escapement and that shown in Fig. 153 and intended for clocks. This was the busy era in the watchmaker"s line. All the great heads were pondering upon the subject and everyone was on the _qui vive_ for the newest thing in the art.