As an ill.u.s.tration, let us consider the subject of depth between the cylinder and the escape wheel. As previously stated, 196 degrees of cylinder sh.e.l.l should be employed; but suppose we find a watch in which the half sh.e.l.l has had too much cut away, so the tooth on entering the half sh.e.l.l after parting with the entrance lip does not strike dead on the inside of the sh.e.l.l, but encounters the edge of the exit lip. In this case the impulse of the balance would cause the tooth to slightly retrograde and the watch would go but would lack a good motion. In such an instance a very slight advance of the chariot would remedy the fault--not perfectly remedy it, but patch up, so to speak--and the watch would run.

[Ill.u.s.tration: Fig. 135]

In this day, fine cylinder watches are not made, and only the common kind are met with, and for this reason the student should familiarize himself with all the imaginary faults which could occur from bad construction. The best way to do this is to delineate what he (the student) knows to be a faulty escapement, as, for instance, a cylinder in which too much of the half sh.e.l.l is cut away; but in every instance let the tooth be of the correct form. Then delineate an escapement in which the cylinder is correct but the teeth faulty; also change the thickness of the cylinder sh.e.l.l, so as to make the teeth too short. This sort of practice makes the pupil think and study and he will acquire a knowledge which will never be forgotten, but always be present to aid him in the puzzles to which the practical watchmaker is every day subject.

The ability to solve these perplexing problems determines in a great degree the worth of a man to his employer, in addition to establishing his reputation as a skilled workman. The question is frequently asked, "How can I profitably employ myself in spare time?" It would seem that a watchmaker could do no better than to carefully study matters horological, striving constantly to attain a greater degree of perfection, for by so doing his earning capacity will undoubtedly be increased.

CHAPTER III.

THE CHRONOMETER ESCAPEMENT.

Undoubtedly "the detent," or, as it is usually termed, "the chronometer escapement," is the most perfect of any of our portable time measurers.

Although the marine chronometer is in a sense a portable timepiece, still it is not, like a pocket watch, capable of being adjusted to positions. As we are all aware, the detent escapement is used in fine pocket watches, still the general feeling of manufacturers is not favorable to it. Much of this feeling no doubt is owing to the mechanical difficulties presented in repairing the chronometer escapements when the detent is broken, and the fact that the spring detent could not be adjusted to position. We shall have occasion to speak of position adjustments as relate to the chronometer escapement later on.

ADVANTAGES OF THE CHRONOMETER.

We will proceed now to consider briefly the advantages the detent escapement has over all others. It was soon discovered in constructing portable timepieces, that to obtain the best results the vibrations of the balance should be as free as possible from any control or influence except at such times as it received the necessary impulse to maintain the vibrations at a constant arc. This want undoubtedly led to the invention of the detent escapement. The early escapements were all frictional escapements, i.e., the balance staff was never free from the influence of the train. The verge escapement, which was undoubtedly the first employed, was constantly in contact with the escape wheel, and was what is known as a "recoiling beat," that is, the contact of the pallets actually caused the escape wheel to recoil or turn back. Such escapements were too much influenced by the train, and any increase in power caused the timepiece to gain. The first attempt to correct this imperfection led to the invention and introduction of the fusee, which enabled the watchmaker to obtain from a coiled spring nearly equal power during the entire period of action. The next step in advance was the "dead-beat escapement," which included the cylinder and duplex. In these frictional escapements the balance staff locked the train while the balance performed its arc of vibration.

FRICTIONAL ESCAPEMENTS IN HIGH FAVOR.

These frictional escapements held favor with many eminent watchmakers even after the introduction of the detached escapements. It is no more than natural we should inquire, why? The idea with the advocates of the frictional rest escapements was, the friction of the tooth acted as a _corrective_, and led no doubt to the introduction of going-barrel watches. To ill.u.s.trate, suppose in a cylinder watch we increase the motive power, such increase of power would not, as in the verge escapement, increase the rapidity of the vibrations; it might, in fact, cause the timepiece to run slower from the increased friction of the escape-wheel tooth on the cylinder; also, in the duplex escapement the friction of the locking tooth on the staff r.e.t.a.r.ds the vibrations.

Dr. Hooke, the inventor of the balance spring, soon discovered it could be manipulated to isochronism, i.e., so arcs of different extent would be formed in equal time. Of course, the friction-rest escapement requiring a spring to possess different properties from one which would be isochronal with a perfectly detached escapement, these two frictional escapements also differing, the duplex requiring other properties from what would isochronize a spring for a cylinder escapement. Although pocket watches with duplex and cylinder escapements having balances compensated for heat and cold and balance springs adjusted to isochronism gave very good results, careful makers were satisfied that an escapement in which the balance was detached and free to act during the greater proportion of the arc of vibration and uncontrolled by any cause, would do still better, and this led to the detent escapement.

FAULTS IN THE DETENT ESCAPEMENT.

As stated previously, the detent escapement having p.r.o.nounced faults in positions which held it back, it is probable it would never have been employed in pocket watches to any extent if it had not acquired such a high reputation in marine chronometers. Let us now a.n.a.lyze the influences which surround the detent escapement in a marine chronometer and take account of the causes which are combined to make it an accurate time measurer, and also take cognizance of other interfering causes which have a tendency to prevent desired results. First, we will imagine a balance with its spring such as we find in fine marine chronometers.

It has small pivots running in highly-polished jewels; such pivots are perfectly cylindrical, and no larger than are absolutely necessary to endure the task imposed upon them--of carrying the weight of the balance and endure careful handling.

To afford the necessary vibrations a spring is fitted, usually of a helical form, so disposed as to cause the balance to vibrate in arcs back and forth in equal time, _provided these arcs are of equal extent_.

It is now to be taken note of that we have it at our disposal and option to make these arcs equal in time duration, i.e., to make the long or short arcs the quickest or to synchronize them. We can readily comprehend we have now established a very perfect measure of short intervals of time. We can also see if we provide the means of maintaining these vibrations and counting them we should possess the means of counting the flights of time with great accuracy. The conditions which surround our balance are very constant, the small pivots turning in fine hard jewels lubricated with an oil on which exposure to the action of the air has little effect, leaves but few influences which can interfere with the regular action of our balance.

We add to the influences an adjustable correction for the disturbances of heat and cold, and we are convinced that but little could be added.

ANTAGONISTIC INFLUENCES.

In this combination we have pitted two antagonistic forces against each other, viz., the elasticity of the spring and the weight and inertia of the balance; both forces are theoretically constant and should produce constant results. The mechanical part of the problem is simply to afford these two forces perfect facilities to act on each other and compel each to realize its full effect. We must also devise mechanical means to record the duration of each conflict, that is, the time length of each vibration. Many years have been spent in experimenting to arrive at the best propositions to employ for the several parts to obtain the best practical results. Consequently, in designing a chronometer escapement we must not only draw the parts to a certain form, but consider the quality and weight of material to employ.

To ill.u.s.trate what we have just said, suppose, in drawing an escape wheel, we must not only delineate the proper angle for the acting face of the tooth, but must also take cognizance of the thickness of the tooth. By thickness we mean the measurement of extent of the tooth in the direction of the axis of the escape wheel. An escape-wheel tooth might be of the best form to act in conveying power to the balance and yet by being too thin soon wear or produce excessive friction. How thick an escape wheel should be to produce best results, is one of the many matters settled only by actual workshop experience.

FACTORS THAT MUST BE CONSIDERED.

Even this experience is in every instance modified by other influences.

To ill.u.s.trate: Let us suppose in the ordinary to-day marine chronometer the escape-wheel teeth exerted a given average force, which we set down as so many grains. Now, if we should employ other material than hammer-hardened bra.s.s for an escape wheel it would modify the thickness; also, if we should decrease the motive power and increase the arc of impulse. Or, if we should diminish the extent of the impulse arc and add to the motive force, every change would have a controlling influence. In the designs we shall employ, it is our purpose to follow such proportions as have been adopted by our best makers, in all respects, including form, size and material. We would say, however, there has been but little deviation with our princ.i.p.al manufacturers of marine chronometers for the last twenty years as regards the general principle on which they were constructed, the chief aim being to excel in the perfection of the several parts and the care taken in the several adjustments.

Before we proceed to take up the details of constructing a chronometer escapement we had better master the names for the several parts. We show at Fig. 136 a complete plan of a chronometer escapement as if seen from the back, which is in reality the front or dial side of the "top plate."

The chronometer escapement consists of four chief or princ.i.p.al parts, viz.: The escape wheel, a portion of which is shown at _A_; the impulse roller _B_; unlocking or discharging roller _C_, and the detent _D_.

These princ.i.p.al parts are made up of sub-parts: thus, the escape wheel is composed of arms, teeth, recess and collet, the recess being the portion of the escape wheel sunk, to enable us to get wide teeth actions on the impulse pallet. The collet is a bra.s.s bush on which the wheel is set to afford better support to the escape wheel than could be obtained by the thinned wheel if driven directly on the pinion arbor. The impulse roller is composed of a cylindrical steel collet _B_, the impulse pallet _d_ (some call it the impulse stone), the safety recess _b b_. The diameter of the impulse collet is usually one-half that of the escape wheel. This impulse roller is staked directly on the balance staff, and its perfection of position a.s.sured by resting against the foot of the shoulder to which the balance is secured. This will be understood by inspecting Fig. 137, which is a vertical longitudinal section of a chronometer balance staff, the lower side of the impulse roller being cupped out at _c_ with a ball grinder and finished a ball polish.

[Ill.u.s.tration: Fig. 136]

[Ill.u.s.tration: Fig. 137]

It will be seen the impulse roller is staked flat against the hub _E_ of the balance staff. The unlocking roller, or, as it is also called, the discharging roller, _C_, is usually thinner than the impulse roller and has a jewel similar to the impulse jewel _a_ shown at _f_. This roller is fitted by friction to the lower part of the balance staff and for additional security has a pipe or short socket _e_ which embraces the balance staff at _g_. The pipe _e_ is usually flattened on opposite sides to admit of employing a special wrench for turning the discharging roller in adjusting the jewel for opening the escapement at the proper instant to permit the escape wheel to act on the impulse jewel _a_. The parts which go to make up the detent _D_ consist of the "detent foot"

_F_, the detent spring _h_, the detent blade _i_, the jewel pipe _j_, the locking jewel (or stone) _s_, the "horn" of the detent _k_, the "gold spring" (also called the auxiliary and lifting spring) _m_. This lifting or gold spring _m_ should be made as light and thin as possible and stand careful handling.

We cannot impress on our readers too much the importance of making a chronometer detent light. Very few detents, even from the hands of our best makers, are as light as they might be. We should in such construction have very little care for clumsy workmen who may have to repair such mechanism. This feature should not enter into consideration.

We should only be influenced by the feeling that we are working for best results, and it is acting under this influence that we devote so much time to establishing a correct idea of the underlying principles involved in a marine chronometer, instead of proceeding directly to the drawing of such an escapement and give empirical rules for the length of this or the diameter of that. As, for instance, in finishing the detent spring _h_, suppose we read in text books the spring should be reduced in thickness, so that a weight of one pennyweight suspended from the pipe _j_ will deflect the detent ". This is a rule well enough for people employed in a chronometer factory, but for the horological student such fixed rules (even if remembered) would be of small use.

What the student requires is sound knowledge of the "whys," in order that he may be able to thoroughly master this escapement.

FUNCTIONS OF THE DETENT.

We can see, after a brief a.n.a.lysis of the principles involved, that the functions required of the detent _D_ are to lock the escape wheel _A_ and hold it while the balance performs its excursion, and that the detent or recovering spring _h_ must have sufficient strength and power to perform two functions: (1) Return the locking stone _s_ back to the proper position to arrest and hold the escape wheel; (2) the spring _h_ must also be able to resist, without buckling or c.o.c.kling, the thrust of the escape wheel, represented by the arrows _p o_. Now we can readily understand that the lighter we make the parts _i j k m_, the weaker the spring _h_ can be. You say, perhaps, if we make it too weak it will be liable to buckle under the pressure of the escape wheel; this, in turn, will depend in a great measure on the condition of the spring _h_.

Suppose we have it straight when we put it in position, it will then have no stress to keep it pressed to the holding, stop or banking screw, which regulates the lock of the tooth. To obtain this stress we set the foot _F_ of the detent around to the position indicated by the dotted lines _r_ and _n_, and we get the proper tension on the detent spring to effect the lock, or rather of the detent in time to lock the escape wheel; but the spring _h_, instead of being perfectly straight, is bent and consequently not in a condition to stand the thrust of the escape wheel, indicated by the arrows _o p_.

OBTAINING THE BEST CONDITIONS.

Now the true way to obtain the best conditions is to give the spring _h_ a set curvature before we put it in place, and then when the detent is in the proper position the spring _h_ will have tension enough on it to bring the jewel _s_ against the stop screw, which regulates the lock, and still be perfectly straight. This matter is of so much importance that we will give further explanation. Suppose we bend the detent spring _h_ so it is curved to the dotted line _t_, Fig. 136, and then the foot _F_ would a.s.sume the position indicated at the dotted line _r_. We next imagine the foot _F_ to be put in the position shown by the full lines, the spring _h_ will become straight again and in perfect shape to resist the thrust of the escape wheel.

Little "ways and methods" like the above have long been known to the trade, but for some reason are never mentioned in our text books. A detent spring 2/1000" thick and 80/1000" wide will stand the thrust for any well-constructed marine chronometer in existence, and yet it will not require half a pennyweight to deflect it one-fourth of an inch. It is a good rule to make the length of the detent from the foot _F_ to the center of the locking jewel pipe _j_ equal to the diameter of the escape wheel, and the length of the detent spring _h_ two-sevenths of this distance. The length of the horn _k_ is determined by the graphic plan and can be taken from the plotted plan. The end, however, should approach as near to the discharging jewel as possible and not absolutely touch. The discharging (gold) spring _m_ is attached to the blade _i_ of the detent with a small screw _l_ cut in a No. 18 hole of a Swiss plate.

While there should be a slight increase in thickness in the detent blade at _w_, where the gold spring is attached, still it should be no more than to separate the gold spring _m_ from the detent blade _i_.

IMPORTANT CONSIDERATIONS.

It is important the spring should be absolutely free and not touch the detent except at its point of attachment at _w_ and to rest against the end of the horn _k_, and the extreme end of _k_, where the gold spring rests, should only be what we may term a dull or thick edge. The end of the horn _k_ (shown at _y_) is best made, for convenience of elegant construction, square--that is, the part _y_ turns at right angles to _k_ and is made thicker than _k_ and at the same time deeper; or, to make a comparison to a clumsy article, _y_ is like the head of a nail, which is all on one side. Some makers bend the horn _k_ to a curve and allow the end of the horn to arrest or stop the gold spring; but as it is important the entire detent should be as light as possible, the square end best answers this purpose. The banking placed at _j_ should arrest the detent as thrown back by the spring _h_ at the "point of percussion." This point of percussion is a certain point in a moving ma.s.s where the greatest effort is produced and would be somewhere near the point _x_, in a bar _G_ turning on a pivot at _z_, Fig. 138. It will be evident, on inspection of this figure, if the bar _G_ was turning on the center _z_ it would not give the hardest impact at the end _v_, as parts of its force would be expended at the center _z_.

[Ill.u.s.tration: Fig. 138]

DECISIONS ARRIVED AT BY EXPERIENCE.

Experience has decided that the impulse roller should be about half the diameter of the escape wheel, and experience has also decided that an escape wheel of fifteen teeth has the greatest number of advantages; also, that the balance should make 14,400 vibrations in one hour. We will accept these proportions and conditions as best, from the fact that they are now almost universally adopted by our best chronometer makers.

Although it would seem as if these proportions should have established themselves earlier among practical men, we shall in these drawings confine ourselves to the graphic plan, considering it preferable. In the practical detail drawing we advise the employment of the scale given, i.e., delineating an escape wheel 10" in diameter. The drawings which accompany the description are one-fourth of this size, for the sake of convenience in copying.

With an escape wheel of fifteen teeth the impulse arc is exactly twenty-four degrees, and of course the periphery of the impulse roller must intersect the periphery of the escape wheel for this arc (24).

The circles _A B_, Fig. 139, represent the peripheries of these two mobiles, and the problem in hand is to locate and define the position of the two centers _a c_. These, of course, are not separated, the sum of the two radii, i.e., 5" + 2" (in the large drawing), as these circles intersect, as shown at _d_. Arithmetically considered, the problem is quite difficult, but graphically, simple enough. After we have swept the circle _A_ with a radius of 5", we draw the radial line _a f_, said line extending beyond the circle _A_.