Escapement. An essential part of clocks, it generally indicates the mechanism by which the regulator receives the motion of the last wheel, and suspends or reacts to it, to moderate and regulate the working of the clock. Craftsmen distinguish two kinds of escapement . The origin of one is very ancient and even unknown; the escape wheel acts constantly on the regulator, accelerating and retarding its speed; in the other, it only accelerates the beats and does not retard them, unless by friction. The wheels and hands of clocks having the first kind have a backward motion with each beat; consequently, they are called recoil escapements . Clocks having the second kind have a steady forward motion, although each beat is followed by a brief rest, which has caused them to be called deadbeat escapements . The latter owe their origin to the invention of the spiral spring and the pendulum; they can be applied to any regulator that swings without the aid of a motive force. Their design is such that it cannot be used with regulators which, like the simple pendulum, swing only with the aid of an external power; this will be explained in the following descriptions.

The goal of skilled craftsmen in making an escapement is avoiding the defects that may occur in the regulating force and the power that maintains its motion. For that purpose they design escapements so that a given regulator may be as powerful and as active as possible, and undergo as little friction as possible in its vibrations.

In making their escapements , horologists also consider the kind of regulator they employ; for example, since the small arcs of a pendulum are much more nearly isochronous than large ones, wise craftsmen make the escapement of a pendulum allow very small arcs only; since large arcs take longer than small ones, they try to compensate in this way for errors that could arise from these differences. If the clock is to be moved, they again try to keep its escapement from being affected. If they expect that it will be in various positions, like a watch that is hung, left in its box, or sometimes left face down, they design the escapement to make it impervious to variation in the different positions.

Learned horologists are no less careful to prevent the regulator from wearing the movement. This gives the clock excellent characteristics; it is more durable, the condition of the mechanism remains steadier and more uniform, and consequently it is capable of greater accuracy. These are considerable advantages, and they are found particularly in deadbeat escapements .

The four escapements most used today possess quite fully all the qualities we have just mentioned; we shall limit ourselves to describing them, without giving useless details about all the ones that have been designed or conjectured according to these principles. Although different in appearance from the first four, they are all essentially the same.

Description of the common foliot or verge escapement.

The oldest of escapements , it is also the one most commonly used in watches; it is rightly considered one of the cleverest inventions ever made in mechanics → . The escape wheel (Figure 27) is positioned in such a way that its axis is perpendicular to the balance staff. This staff, called the verge , has two small wings or pallets about 90 degrees apart. They engage the teeth of the wheel, which have an odd number so that when the upper pallet engages one of the teeth, the lower pallet is between two teeth.

Results of this design.

When the watch is wound up, the tip of the tooth pressing against one of the pallets turns it until it escapes, while the other pallet, having no obstacle, moves in the opposite direction into the opposing teeth and encounters the next one at about the same time the first pallet is released; then by its momentum the regulator turns the escape wheel and the rest of the train backwards, until its momentum is spent and it yields to the action of the wheel, which turns it again by acting on the second pallet as it did the first; and so on for the rest of the teeth.

In this design, the regulator lets the wheels move only so far as they give it motion and cause it to swing back and forth. As a consequence of this design, 1) the balance or any other regulator resists the movement, which keeps it from yielding too rapidly to the driving force; and 2) the wheels (disregarding the action of the movement) escape faster or slower according to the mass of the regulator or the number of its vibrations, so the wheels that carry the hands can always be defined to make a certain number of revolutions in a given time. Finally, by the action of this escapement , when the regulator has been set in motion by the motive power, it reacts on the wheels and turns them backwards in proportion to the force it has received; whence there results a kind of compensation in the going rate of watches, apart from the balance spring; a greater driving power from the movement should cause them to gain, but it always followed by a greater reaction of the balance, which tends to make them lose.

Here we could enter into a purely theoretical examination of this escapement , and of the best way to make it, but as with escapements in general and this one in particular, many things are involved which are very difficult, not to say impossible, to determine theoretically, such as the variations arising from friction, resistances, oils, shocks, various positions, etc. In this case, as in all others of the kind where theory is lacking, we must resort to experience. Therefore, while assigning to theory everything that can be, in the other cases we shall rely on what experience has taught horologists.

The most noteworthy property of the common escapement is that the escape wheel imparts motion to the balance through very long levers, whereas the balance acts on the wheel through very short ones, which results in great freedom in the regulator, and greatly increases its regulating power.

To illustrate this, let us assume that B (Figure 19) is a force moving in a constant direction, B E, pushing a pallet, C P, which turns around point C. I say the efforts of this power to turn the pallet will be to each other, in the various positions C P, as the square of lines C E to C p , expressing the distances from points p and E to the center.

To demonstrate this, let us imagine that the force acts perpendicularly at E and travels a very short distance, like E G; let us then imagine the pallet and the force reaching p, and let us assume that the force travels a distance t p equal to distance E G. The arc described by radius p will be p d. The arcs described by these two points of pallets p and E, in these different positions, will therefore be like lines p d and E G, or its equal, p t ; but because of similar triangles E C p and t p d , we see that these lines are to each other as C E and c p ; these arcs will therefore be like these lines. Now we know from one of the first principles of ← mechanics that the forces of a power are in inverse proportion to their speed; these forces at points p and E will therefore be in inverse proportion to C E and C p , which express the speeds at points P and E. Therefore they will be as C p to C E, but in addition they will be applied to levers which will be in the same proportion; the total force at points E and p will therefore be as the square of E C is to the square of p C.

It follows that the greater the angle p C E formed by the pallet and the perpendicular to the direction of force, the greater the force of that power.

It is easy now to apply that proposition to what we have said about the property of the common escapement. For this purpose, imagine that Figure 24 represents the plane drawing of an escape wheel and the pallets of a balance. Teeth a and b will be the ones nearest the eye in the projection, and d e f the ones farthest away; L c P will represent the projection of the pallets. But we may see the motion of the teeth a and b in direction G M as not very different from their circular motion, like that of teeth d e f in the opposite direction, from M to G. It is clear, from what we demonstrated above, that as the wheel swings the pallet its force increases, and it is greatest when it is about to release, as at P, because then the angle of the pallet with the perpendicular to the direction of the wheel is the greatest, and contrary to tooth d , which is about to encounter the other pallet L t , pushes it with much less force, since angle M c t , formed by this pallet and the perpendicular to the direction of the wheel, is much smaller. This proves, therefore, what we have advanced about the property of this escapement , to wit: the escape wheel has much more force to drive the balance than it has to resist it when it reacts on the wheel. This force would be as the square of the levers the wheel acts on at points P and t if the wheel moved in a straight line, as we assumed for ease of demonstration; but since it moves in a circle, the force increases in a greater proportion, for the lever of that wheel by which it acts on the pallet decreases as the angle of the pallet increases, since this lever is only the sine of the complement of the angle formed by the radius of the wheel, which ends at the tip of the tooth, and the one parallel to the axis of the verge, an angle that increases constantly as the tooth pushes the pallet. The length of this lever must then enter into measuring the action of the escape wheel on the pallet; the more the lever of a wheel decreases, the more its force increases. It follows that the forces with which the escape wheel acts on the pallet it releases and the one it encounters are in compound proportion to the direct of the square of the pallet levers that perform that action, and inverse to the sines of the complements of the angles formed by the radius ending at the tip of the tooth in these various positions, and the radius parallel to the axis of the verge.

This characteristic of the escapement was too advantageous to be neglected by clever clockmakers, therefore they did not fail to bring the escape wheel as close to the balance arbor as they could, in order to achieve the greatest difference between the forces at points P and t (Figure 24), thereby automatically increasing angle M c P and decreasing the other one, M C t. But they soon learned that this practice entailed great disadvantages: 1) the balance described too great an arc with each beat, which made it tend to reverse and lock; and 2) narrow pallets were required, which made the watch too susceptible to disorder in different positions, since the play of the pivots in their holes was much greater in relation to narrow pallets than to wide ones.

So after a very great number of attempts and experiments, changing the length of the pallets, the angle between them, and the distance of the balance arbor from the escape wheel, it was found that 90 degrees was the best angle for the pallets, and that the escape wheel should be close enough to the balance arbor that when a tooth engages a pallet after releasing another, it should swing the pallet through an arc of 40 degrees before releasing it in turn. Reflecting on this issue, it might be thought more appropriate that the angle between the pallets were greater than 90 degrees, because then the total arc would be described by a shorter lever. But since inevitable changes decrease the size of the vibrations, since the escapement cannot be perfectly precise, and since there is always a short drop to the pallets when the balance starts to react, clockmakers shorten the lever acted on by the escape just after release, which they cannot do without lengthening the one formed at the end of the reaction. These two levers become approximately equal after the watch has gone a certain time, the swings steadily decreasing.

Experience has also shown horologists that a watch regulator must have a certain relation to the motive force, otherwise either it is not powerful enough to correct variations in the force, or it offers too great a resistance, which makes the watch susceptible to stopping. The method shown by practice for giving the regulator a power that avoids both extreme equally is to make watches go without a balance spring, and to give the balance a mass such that it lets the hand cover 27 minutes per hour, and adding a balance spring speeds the hand by 33 minutes per hour. Note, however, that in order to go 27 minutes in an hour without a balance spring, the watch must be of good quality, for various defects in the movement may increase or decrease the power, so that mediocre watches may go more than 27, like 28 or even 30, while very good ones may go only 26.

When designed with the greatest care, the common escapement has three considerable qualities: simplicity, ease of execution, and little friction in all of its parts. It is unfortunate that with all its advantages it cannot compensate adequately for the irregularities of the movement; this disadvantage comes, as we have just said, from the fact that watches go 27 minutes per hour without the aid of a balance spring, by the sole power of the motive force. By doubling the power in a watch, it is made to gain about one hour in 24.

The verge escapement has several more defects. The pivot carrying the escape wheel bears all the effort of the going train, and all the action and reaction of the pallets; the reaction is the greater because it takes place beyond the pivot. Moreover, for reasons that will be set forth below, it cannot be used in clocks, for which reason they are usually fitted with the twin-pallet escapement , or the one owed to the sagacity of Dr. Hook.

Another recoil escapement , which really differs from the preceding in name only, is the conical or "pirouette" escapement . Here is a brief description. 1) The teeth of the last wheel are like those of a crown wheel, meshing with a pinion on the balance arbor. 2) The arbor of the last wheel (in the preceding example, the contrate wheel) is a staff with pallets that are driven in alternation by the teeth of a crown wheel shaped like those of a contrate wheel.

From this brief account, it may readily be seen that this escapement does not differ from the preceding one except that instead of acting between the escape wheel and the balance, it acts between the crown wheel and the escape, which being geared to the pinion on the balance, drives the regulator for several rotations on each beat.

The purpose of this design was to slow the vibrations of the balance to about a second, while retaining the same motion. M. Sulli says ( Règle artificielle du temps , p. 241) that he has seen this kind of watch with no balance spring, which took two seconds for each vibration. It appears, the same author says, "that this design was invented the better to imitate the vibrations of a seconds pendulum clock, which was then a new and unfamiliar invention. It may also be," he adds, "that M. Huyghens's first balance spring watches had this kind of escapement , and certain craftsmen hostile to this novelty, whose character they did not understand, imagined that conical- escapement watches owed their accuracy to the slowness of the beat rather than the application of the spring, which they tried to leave out."

Description of Dr. Hook's escapement, or anchor escapement.

In this escapement , there are two branches or arms on the pendulum arbor (Figure 25) that embrace the escape wheel; one ends in a convex face turned outwards, and the other in a concave face turned inwards. When the escape wheel frees itself from the former, the second, located on the other side of the axis, must engage the teeth facing it; soon freed from them, it makes the other one in turn engage with the escape wheel, and so on. Thus the losses of pendulum motion are restored; this will be shown more fully in the précis of M. Saurin's paper ( Mémoires de l'Académie Royale des Sciences, 1720 ), which we reproduce.

"Everyone says that the driving force maintains the motion of the pendulum, but how does it do so? That is a question no one has ever thought to ask. Experience has led clockmakers to give the escapement the design necessary for this purpose, but very few know the whole secret of this design; they would be baffled by the problem I propose: find the cause of the duration of the vibrations . It will be solved by the explanation I shall give.

Figure 25 shows an escape wheel and an anchor with its pendulum, the latter being at rest. It is vertical and the anchor horizontal; a straight line AA, joining the ends of the escapement faces, would be perpendicular to the vertical line C B [1]. On one side, a tooth of the wheel rests against point B of one of the curves, portion A B being engaged with the tooth. On the other side, a similar portion A B extends between two teeth and is distant from them by about the same amount.

When the driving weight is wound up, it is far from having the force necessary to set the pendulum in motion. To start, it must be swung aside and released; then falling of its own weight and accelerated in its fall by tooth H, which is assumed to push it as far as A, it swings to the other side. Then tooth N encounters the anchor at F and is forced to recoil slightly by the momentum of the pendulum, which swings back by gravity, and again accelerated by the tooth that recoiled, it returns to the side from which it swung initially. Then it encounters the next tooth, which recoils like the other one and follows it, hastening its swing as before.

If the pendulum moved in a vacuum, we know that aside from friction, it would always return to the same point; again, excluding the action of the two opposing teeth, it is clear that its vibrations would always remain the same and would never end. Now let us add the force of the two opposing teeth of the escape to that of gravity; since the latter acts on the pendulum equally on either side and thus is canceled out, the vibrations will again remain the same, never diminishing or stopping, since we assume that nothing prevents the pendulum from always returning to its starting point. But it is obvious that it must be hindered by the resistance of the air; the vibrations will diminish and finally stop.

What then is the cause of constant vibrations in our clocks? It lies precisely in the design of the escapement , which is such that when the pendulum is at rest, part A B of one of the faces is engaged with tooth H, which touches it not at point A but at point B, and an equal portion, A B, of the other curve is advancing between the two teeth N Q, with a clearance such that with the pendulum in motion, when tooth H escapes at point A, tooth N encounters the face opposite point F, which means that BF equals BA. Likewise, when tooth N is released, tooth H encounters the other face at a similar point, F; in other words, distance AF is equal, and twice AB, on both faces.

It must be observed that when tooth H is at point F, the weight of the pendulum is at L to the left, and when tooth N is at the similar point, F, on the other side, the weight of the pendulum is at L to the right; so both teeth acting in succession from F to B, they accelerate the pendulum in its swing from L to D, and still acting on the face from B to A, they again accelerate it in the whole arc it describes from D to L. So the force the tooth gives to the pendulum does not abandon it at point D but keeps acting on it as far as point L, and this is the extra effort on the swing from D to L that causes the duration and constant evenness of the vibrations, as can readily be seen.

Let us assume that arc S D S is the one described by the pendulum in its constant vibrations, swinging from S to D. If there were neither air resistance nor friction, the acceleration of its motion caused by gravity and by the tooth following its swing would easily give it sufficient speed to reach the other side at S against the effort of the opposite tooth, which it only encounters at L; but obviously friction and air resistance having decreased the speed during the downward swing, and decreasing it further as the pendulum swings upwards, it cannot reach point S without extra help. If therefore it reaches S, the help comes from the action of the tooth, prolonged from D to L. Point S is such that that the effort added from D to L is exactly equal to the loss caused by friction and air resistance throughout the arc S D S.

If in starting the pendulum it had been raised to a point, I, higher than S, since the effort of the tooth from D to L is not great enough to offset the loss, on the other side the pendulum would only swing below I, and its vibrations would keep diminishing until it reached point S, where the added effort equals the loss. The same would be true if it had been raised less far than S; the pendulum would swing past the point where it started, and its vibrations would keep increasing until they reached point S."

What M. Saurin says concerning the pendulum and the anchor escapement applies to the other regulators and all kinds of escapements ; in every one there is always a part with pallets or curves as at A B, engaging the escape wheel; this is the part intended to restore the motion lost by the regulator to air resistance and friction. This seems quite clear to me from the preceding, so I shall not pause to point out the same thing in the descriptions to follow.

We shall return to the anchor. It has several fine properties: its curves, as my father discovered and M. Saurin has demonstrated, must be nearly circular, whereby they compensate perfectly for unevenness in the motive force, because in the greatest oscillations the escape wheel acts with greater mechanical advantage. Another property of this escapement is that the pendulum arcs may be very small, and consequently nearly isochronous, and the pendulum bob very heavy.

Two considerable drawbacks greatly reduce all these advantages: the friction of the escape teeth on the curves, and the difficulty of giving the latter the required precision. For these two reasons, the twin-verge escapement is usually preferred, which has the same advantages and is much less subject to friction.

The twin-verge escapement . The most ingenious and useful things are often abandoned and completely forgotten. That happened to the escapement we describe here; it is very old, but it was hardly used except when my father recognized all its properties and undertook to put them to use.

This escapement consisted of two wheel sectors (Figure 20) meshing with each other, each one on an arbor with a pallet. One of the arbors also carried the crutch, and when the escape wheel moved one of the pallets, the gearing moved the other one in the opposite direction to engage the escape, and so on. My father made several changes in the way the two pallets were linked; he replaced the sectors with a cylinder or roller turning in a fork, as shown in Figure 26. After several attempts and experiments, he also succeeded in making it compensate precisely for unevenness in the power. Let us try to reveal how that is achieved, as surprising perhaps as it is difficult to detail.

Every free pendulum ( see Pendulum) describes wide arcs in a longer time than smaller ones, so since excess power produces greater arcs in a pendulum applied to a clock, this increase inevitably slows the oscillations. On the other hand, it also causes advance, because the greater force of the escape wheel makes greater resistance to the reaction of the pallets and transmits to them part of the increased speed the power tends to impart to them. So if it is possible to make the latter cause of acceleration equal to the slowing caused by the greater arcs, whether the power increases or diminishes, the period of the vibrations will always remain the same. Now ( see Pendulum) the slowing caused by wider oscillations is less, the smaller the initial arcs. When the pendulum moves little from its point of rest, the slowing becomes negligible, and experience has shown that with the preceding escapement the influence of the clock's driving force on the pendulum could be small enough that the clock would lose if it were increased. In other words, the gain caused by an increase in power would be smaller than the loss caused by the wider arcs produced by the increased power. In addition, by virtue of the escapement the latter cause of losing can be increased or decreased at will, the arcs being given the width desired, the action of the motive force remaining constant. It must be concluded that for any pendulum there is an arc in the vicinity of which the causes of gaining and losing stated above will compensate each other perfectly.

We know that the power being the same, the longer the pallets of the escapement , the smaller the arcs described by the heavy regulator; on the contrary, the shorter the pallets, the wider the arcs of the light regulator. That is readily seen, since in the latter case the escape wheel drives at points that are closer to the axis of rotation.

Now since the action of a motive force is always in the same relation on pendulums of the same length, for the preceding reasons if the bob is light it describes wider arcs, and the escape wheel acts with less mechanical advantage. It follows that there is a certain length for the pallets at which the pendulum fitted to the clock describes an arc in the vicinity of which the losing caused by the greater arcs and the gaining caused by the greater power cancel each other, and consequently there is compensation for any unevenness in power. This is confirmed by experience. For a seconds pendulum, this length is half the diameter of the escape wheel, when it has thirty teeth.

Before he used the preceding method, my father had tried that same compensation with the verge escapement . His guiding principle was always to resort to complication only when simplicity did not suffice, but he soon realized that with pallets of the required length, the contrate wheel could not provide deep enough engagement, because driving in one direction it acts (as we have seen) as if its motion were in a straight line.

I shall not dwell on the advantages of the preceding design or on the accuracy that can be expected from it; I would fear lest my testimony be thought suspect. I shall simply report what M. de Maupertuis says in his book Sur la Figure de la Terre , p. 173. Here are his words: "We had an excellent instrument, a clock by M. Julien le Roy, whose accuracy seemed miraculous in all the observations we made using it."

Deadbeat escapement. Description of the watch escapement by M. Graham.

This escapement consists of a hollow cylinder, A C D, Figure 23, cut away as far as the axis of the balance on which it turns, and an escape wheel (A C, Figure 22), parallel to the plates, whose teeth, raised on one of its sides, are opposite the middle of the cylinder; the teeth are very nearly as long as its inner diameter, and they are spaced apart by its whole outer diameter. Their curvature must be such that their force in pushing the edges of the cylinder increases in proportion to the resistance of the regulator, and the swing or the arc described by the balance when driven by these curves is about 36 degrees. Here is how they operate.

When cylinder D E K (Figure 22) stands in the interval between two teeth and the watch is wound, one of them, A P for example, pushes aside one of the faces until it has driven it through an arc of 18 degrees; point A has reached D and point P is at K. Then edge K advances on the wheel by an amount equal to 18 degrees of the cylinder arc K D. When point A reaches point D, the tooth escapes, and its tip P drops into the cylinder, leaving an arc of 18 degrees between itself and face K; the regulator continues to vibrate with no obstacle other than friction on the cylinder and the pivots. But after it has covered an arc of about 72 degrees, its momentum is absorbed by said friction and the balance spring, whose resistance has steadily increased; the spring reacts and turns the cylinder the other way, and with it the opening; then the tooth pushes the second face as it did the preceding one, which cannot happen without the outer circumference of the cylinder stopping the next tooth, B, until the opening returns and the tooth produces the same result as the preceding one. And so on.

This escapement has a great advantage over the one used in ordinary watches, which is to compensate much better for unevenness in the motive force and the wheel train. This excellent property comes from the fact that when the teeth of the escape wheel rest against the cylinder and in the opening, they leave the regulator almost free, so that increase or decrease in the power merely increase or decreases the arc of vibration without materially changing its duration. So the isochronicity of the action of the balance spring, or the pendulum swinging in a cycloid, can undergo no other variations that those caused by friction on the cylinder and in the opening; this friction changes according to the motive force. But these errors are not comparable to the ones produced by the same causes in watches that have recoil escapements.

The cylinder escapement has a further considerable advantage: thanks to it the wheel train, the spring and the whole watch are less subject to wear; since the escape wheel does not recoil, there is much less friction on the pivots, the wheel teeth and the pinions.

Several defects darken all these fine properties, as it were, and make this kind of watch, and all those designed according to the same principle, lose the accuracy they have after a cleaning. First, as I have said, there is friction on the cylinder, which causes wear, and consequently variations in accuracy. True, to lessen this friction the cylinder is oiled, but that makes the watch susceptible to all the variations to which this fluid is subject.

My father devised a method for partly remedying this trouble; he located the curved teeth so as to meet the circumference of the cylinder and the faces of the opening at different heights, more or less remote from the plane of the balance wheel, so that (Figure 23) if one tooth strikes at A, for example, the next one strikes at C, another at D, etc. Thereby, if the escape has thirteen teeth, inaccuracy caused by wear can be reduced by thirteen to one, but it must be admitted that this makes the wheel more difficult to produce.

M. Graham's escapement for seconds clocks.

We have seen ( article Cycloid) that small arcs of a pendulum are more nearly isochronousin addition than wide ones, and at the same time less subject to disturbance by unevenness in the motive force. To benefit from these advantages, M. Graham lengthens the anchor arms considerably, encompassing about half the escape wheel, and provides some distance, AB (Figure 21), from the circumference of the escape wheel to the axis of the anchor; in addition, parts CD and EF are arcs described from axis point B.

For example, when the wheel has pushed aside oblique face DP on the arm that stopped it, the other arm presents arc EF, so since the tooth rests in succession on points that are always equidistant from axis B of the anchor's motion, the pendulum can complete its vibration with no recoil in the wheel train, as it does with Dr. Hook's anchor.

The favorable testimony given to M. Graham's clock by the Academicians who have traveled north leaves no doubt that this escapement is one of the best, although it seems subject to much friction. One might criticize the designer for eliminating the compensating curves made on the faces of the conventional anchor. He would no doubt reply that since the arcs are extremely small, those curves are superfluous. In fact, M. de Maupertuis observed that by reducing the power in this clock by half, reducing the arcs from four degrees twenty minutes to three degrees, those great differences cause a gain of only three and a half to four seconds a day, so this curve may be considered rather useless, and utterly impossible to construct with precision.

Having described these various watch and clock escapements, mentioning the advantages and drawbacks of each one, this would be the point at which to determine which ones are the best and should be employed in preference to the others. But if this is easy where clocks are concerned, since M. Graham's escapement and my father's twin pallet design both readily meet all that can be required of the best escapement, this is not the case for watches. Although the verge escapement and M. Graham's cylinder design possess several advantageous features, they are still far from the required perfection; their advantages and drawbacks seem so completely to offset each other that if one seems preferable to the other, it is not because it gives a watch greater accuracy, but that its accuracy is longer-lasting and more constant.

In fact, it cannot be disputed that watches with cylinder escapement are very accurate, and even more accurate than verge watches, when newly cleaned and with fresh oil on the cylinder, because then they are subject only to irregularities (disregarding the action of heat on the balance spring) other than those caused by unevenness in the motive force; as we pointed out above, this escapement has the property of compensating for it. The accuracy of cylinder watches does not last, because the friction in this escapement, on the faces of the cylinder as well as its convex and concave surfaces, increases as soon as the oil begins to dry up, and it produces variations that soon diminish the accuracy of the watch. As they increase, these frictions give rise to wear, and as it progresses and the oil dries out, the variations increase, sometimes to a point where cylinder watches have been seen to gain or lose five or six minutes and more in 24 hours, and regulating them is impossible.

Now watches with a verge escapement , if well made, are free from such deviations; their accuracy is more lasting, and they are less affected by cold and heat. The result is that although their accuracy, as we said, is not as great as that sometimes found in good cylinder watches, it may be said that in a given period, provided it is quite long, they will go better than the latter; in other words, the sum of their variations will be less, for nothing is more common than seeing verge watches go for two or three years without cleaning, which is very rare for cylinder watches, since their accuracy does not last that long; they may even begin to vary in a shorter period of time. Some are seen that six months after cleaning have already lost all their accuracy, which usually happens when the escapement is not well made, or the cylinder is not as hard as it could be; then it wears and is cut, and the watch is no longer reliable. The verge escapement has the further advantage of being easy to make, and watches employing it are easy to repair. Cylinder escapement , on the other hand, is very difficult to make, there are very few watchmakers capable of making it to the degree of perfection required, and consequently a very small number are able to repair watches to which it is fitted; being untutored in what can make this escapement more or less perfect, they are incapable of remedying defects that may arise and the alteration that wear or some other cause may produce. In fact, there are so few watchmakers capable of properly repairing cylinder watches that a great number of M. Graham's watches have been spoiled by treatment from unskilled hands. In consequence of this, watches with a verge escapement are more serviceable in general than cylinder, and the latter should be the choice only of astronomers or persons who need a watch that goes very accurately for a time, who are able to have it cleaned often and repaired by a skilled watchmaker; furthermore, to obtain all the accuracy we have mentioned, the watch must be very well made.

Such, then was situation of the cylinder escapement in 1750, when we wrote that article; all things considered we thought it better in general to make use of the verge escapement . Since then, in 1753, M. Caron the younger [2] improved it, or rather invented a new one that so well remedies one of the drawbacks criticized in it that we feel obligated to add a description here.

In this escapement, as in the cylinder, the escape wheel is parallel to the plates. The wheel may have any number of teeth, usually thirty. The teeth are shaped like those of an ordinary wheel, but a bit longer and thinner. At their tips they carry pins perpendicular to the surface and in alternation between surfaces, so that there are fifteen on one side of the wheel and fifteen on the other. The balance staff is a hollow cylinder, cut in such a way that it appears made up of two plain portions of a cylinder joined by a small stem located very close to the convex surface. The stem carries a pallet shaped like a comma, with two parts: one is circular and concave following the concavity of the cylinder, and here the pins on the escape wheel rest; the other is straight and serves as an impulse lever to the pins, for the vibrations of the balance. At the point diametrically opposite the stem, a post carries a comma or crescent similar to the first, so placed that the escape wheel passes between the two pallets and encounters them alternately with the opposite pins.

From this brief description it is readily seen how this escapement works. For example, one pin of the escape acting on the lever turns it from outside in; hence the pin escapes, and the next one encounters the concave portion of the other crescent, where it rests until the end of the vibration, when it escapes and falls on the lever of this crescent, turning it from inside out, and so on. It is clear from the nature and construction of this escapement that it compensates for any unevenness in the movement and the power, as does M. Graham's or the cylinder, and (which makes it much better than the latter) the levers are not subject to wear, like the faces of M. Graham's cylinder. Since wear is one of the greatest disadvantages of his escapement , it will be easy to discover the cause of the superiority in this new escapement by observing that since the wear is produced only by the repeated action of the escape teeth on the faces of the cylinder, it cannot occur in the escapement we have just described; since the pins travel the whole length of the lever, the friction at each point along the lever as the wheel turns is to the friction on the face of the cylinder in the same rotation as the rubbing of the surface of the pins on the lever is to the surfaces of the teeth of the same wheel. Now since the pins can be very thin, and the surface only a fortieth of the tooth surface on the faces of the cylinder, the resulting wear will be negligible. This escapement has a further advantage over M. Graham's; the rests occur at the same distance from the center, since they occur on the concave surface of the cylinder, whereas in the famous horologist's design they occur at different distances from the center, the teeth resting in alternation on the concave and the convex surface of the cylinder.

It could be objected that in this escapement, as has been done, since the inner diameter of the cylinder must be equal to the interval between two pins plus one pin, it becomes larger in relation to its wheel than in Graham's escapement . The answer is that the size of the cylinder is not determined by the nature of the new escapement , and it can be made smaller (which is a further advantage), as has in fact been done since its invention.

It was very pleasing for a watchmaker to have invented such an escapement , but the more reason he had for self-congratulation, the more reason he had to fear lest someone take from him the honor of this invention, as nearly happened to M. Caron. However, the comte de Saint-Florentin asked the Académie royale des Sciences for its judgment in the controversy between him [Caron] and another clockmaker who claimed the invention of the new escapement , and on February 24, 1754, after the report by Messrs. Camus and de Montigny (appointed to examine the various credentials of the litigants), the Académie decided that M. Caron was the true inventor, and that the party disputing the credit for the invention was merely an imitator. This, I believe, is the first judgment of this kind the Académie has pronounced, and it would be most desirable that it should settle similar disputes more often, or that there were in the world of Letters a similar tribunal to restrain plagiarists in their desire to appropriate the inventions of others, thereby encouraging minds truly capable of inventing by ensuring them ownership of their discoveries.

We have related this anecdote concerning M. Caron's escapement because we believed it would not be out of place in a work like this one, devoted not only to the description of arts and crafts, but also to the history of the discoveries made in them, insuring in so far as possible fame to the true originators.

* Escapement of M. Caron the younger, correction.

Since the dispute between Messrs. Caron and le Paute on the invention of the double comma escapement, another has arisen concerning an improvement, between the inventor and M. de Romilly, a skilled horologist. The new dispute has also been referred to the Académie des Sciences. Here is a summary of M. de Romilly's claims. 1) In M. Caron's escapement, the balance staff has a cylinder that when invented had as an inner diameter the interval between two pins. The two rests of the double comma escapement occur on this concave circumference. The cylinder is divided in two by a slot perpendicular to its axis, and only a small stem joins the two cylinders. M. de Romilly claims to have reduced the inner diameter of the cylinder so as to admit only one pin. 2) At each end of the gap there is a pallet shaped like a comma forming an angle whose apex is on the concave circumference of the cylinder, the pallets separated from each other by the thickness of the wheel. M. de Romilly claims to have brought the apex of the angle formed by the two planes nearer the center, reducing the concave circumference. 3) The wheel has pins planted on the tips of its teeth, perpendicular to its two surfaces. M. de Romilly claims to be the first to make the wheel with each tooth carrying two pins in a single piece, which allows him to undercut the sides of the tooth in order to accommodate wide arcs. 4) In the going of a watch with double comma escapement, as originally invented, according to M. de Romilly the greatest arcs could not be more than 150 to 180 degrees; he claims instead that in the improved escapement the smallest oscillations are always greater than 240 degrees, and the greatest over 300, whence M. de Romilly concludes that there is less friction, more efficient use of power, wider oscillations in the improved escapement, etc., advantages that are no doubt quite real, otherwise M. Caron, content to be the inventor, would not claim title to the improvement, sed adhuc sub judice lis est [but the case is still before the court]. This is apparently what determined M. le Roy, the author of the excellent article above, to leave this annex to us. The learned academician judiciously commented that it would not be fitting for him to precede the company of which he is a member in deciding a question of fact brought before it, therefore we shall not decide it, but simply announce it in this excerpt from the statement M. de Romilly submitted to the Académie. If the Académie judges this new dispute and we have the opportunity to report its judgment, we shall not fail to do so.

Legend, Plate V, key K

• Figure 19. Geometrical demonstration.
• Figure 20. Twin lever escapement.
• Figure 21. Graham's deadbeat escapement for seconds clocks.
• Figure 22. Graham's deadbeat escapement for watches.
• Figure 23. [(legend omitted) Deadbeat (cylinder) balance, side view.]
• Figure 24. Verge escapement.
• Figure 25. Dr. Hook's anchor escapement.
• Figure 26. Twin verge or lever escapement, by M. Julien le Roy.
• Figure 27. Foliot, or old-style escapement.

Escapement, or hammer escapement.

Small pallet or lifter with a pipe that fits a square or is pinned to the hammer stems in repetition watches or clocks. The escapements allow the teeth of the quarter-hour piece to actuate the hammers, lifting them to strike.

Beat, or escapement beat.

Adjusting the beat of a watch or clock is clockmaker's term for positioning the balance by means of the balance spring, or the pendulum by means of the position of the clock, in such a way that the swings ( see Impulse arc) of the balance and the pendulum on either side of the rest point are equal. We have just seen by the description of various escapements for watches and clocks that the teeth of the escape wheel always act on pallets, planes or curves, to impart vibrations to the balance or the pendulum. Thus, putting a watch or a clock in beat is simply positioning the balance or the pendulum so that as the teeth of the escape wheel act alternately on the pallets or curves, when they escape they have swung the balance or pendulum an equal arc on either side of the rest point. This position of the balance or pendulum is most important, for otherwise if one or the other side is a little too heavy compared to the driving force, the watch or clock will tend to stop; on the side where the arc is greater, the regulator opposes the motion of the wheel with greater force, and the slightest irregularity in the movement will no longer be able to overcome the resistance of the regulator, and the clock will stop.

1. The references do not always correspond to the figure (translator's note).

2. Pierre-Augustin Caron de Beaumarchais (translator's note).