WATCH-Work — 6th ed. (1823)

WATCH-Work. There is one part of the movements of clocks and watches of which we have yet given no particular account. This is the method of applying the maintaining power of the wheels to the regulator of the motions, so as not to injure its power of regulation. This part of the construction is called SCAPEMENT, and falls to be described under the present article, to which we have referred from SCAPEMENT.

The motions of a clock or watch are regulated by a pendulum or balance, without which check the wheels impelled by the weight in the clock, or spring in the watch, would run round with a rapidly accelerating motion, till this should be rendered uniform by friction, and the resistance of the air. If, however, a pendulum or balance be put in the way of this motion, in such a manner that only one tooth of a wheel can pass, the revolution of the wheels will depend on the vibration of the pendulum or balance.

We cannot here enter on an historical account of the improvements that have been made on the regulating powers of clocks and watches, nor can we detail the principles on which their action depends. It will be sufficient here to notice the most simple construction of escapements, and then to describe two or three of the most improved constructions that have been applied to time-keepers.

We know that the motion of a pendulum or balance

is alternate, while the pressure of the wheels is constantly exerted in the same direction. Hence it is evident that some means must be employed to accommodate these different motions to each other. Now, when a tooth of the wheel has given the pendulum or balance a motion in one direction, it must quit it, that the pendulum or balance may receive an impulsion in the opposite direction. This escape of the tooth has given rise to the term scapement.

The ordinary scapement is extremely simple, and may be thus illustrated. Let $xy$ , fig. 12. Plate DLXXI. represent a horizontal axis, to which the pendulum $p$ is attached by a slender rod. This axis has two leaves $c$ and $d$ , one near each end, and not in the same plane, but so that when the pendulum hangs perpendicularly at rest, $c$ spreads a few degrees to the right, and $d$ as much to the left. These are called the pallets. Let $afb$ represent a wheel, turning on a perpendicular axis $eo$ in the order $afbe$ . The teeth of this wheel are in the form of those of a saw, leaning forward in the direction of the rim's motion. This wheel is usually called the crown-wheel, or in watches the balance-wheel. See CLOCK and WATCH. It generally contains an odd number of teeth. In the figure the pendulum is represented at the extremity of its excursion towards the right, the tooth $a$ having just escaped from the pallet $c$ , and $b$ having just dropt on $d$ . Now it is evident that while the pendulum is moving to the left, in the arch $pg$ , the tooth $b$ still presses on the pallet $d$ , and thus accelerates the pendulum, both in its descent along $ph$ , and its ascent up $hg$ , and that when $d$ , by turning round the axis $xy$ , raises its point above the plane of the wheel, the tooth $b$ escapes from it, and $i$ drops on $c$ , now nearly perpendicular. Thus $c$ is pressed to the right, and the motion of the pendulum along $gp$ is accelerated. Again, while the pendulum hangs perpendicularly in the line $xh$ , the tooth $b$ , by pressing on $d$ , will force the pendulum to the left, in proportion to its lightness, and if it be not too heavy, will force it so far from the perpendicular, that $b$ will escape, and $i$ will catch on $c$ , and force the pendulum back to $p$ , when the same motion will be repeated. This effect will be more remarkable, if the rod of the pendulum be continued through $xy$ , and have a ball $q$ on the other end, to balance $p$ . When $b$ escapes from $d$ , the balls are moving with a certain velocity and momentum, and in this condition the balance is checked when $i$ catches on $c$ . It is not, however, instantly stopped, but continues to move a little to the left, and $i$ is forced a little backward by the pallet $c$ . It cannot make its escape over the top of the tooth $i$ , as all the momentum of the balance was generated by the force of $b$ , and $i$ is of equal power. Besides, when $i$ catches on $c$ , and the motion of $c$ to the left continues, the lower point of $c$ is applied to the face of $i$ , which now acts on the balance by a long lever, soon stops its motion in that direction, and continuing to press on $c$ , urges the balance in the opposite direction. It is easy to see that the motion of the wheel here must be hobbling and unequal, which has given to this scapement the name of the recoiling scapement.

In considering the utility of the following improved scapement for clocks, we must keep in mind the following proposition, which, after the above illustration, scarcely requires any direct proof. It is, that the natural vibrations of a pendulum are isochronous, or are per-

formed in equal times. The great object of the scapement is to preserve this isochronous motion of the pendulum.

As the defect of the recoiling scapement was long apparent, several ingenious artists attempted to substitute in its place a scapement that should produce a more regular and uniform motion. Of these, the scapement contrived by Mr Cumming appears to be one of the most ingenious in its construction, and most perfect in its operation. The following construction is similar to that of Mr Cumming, but rendered rather less complex for the purpose of shortening the description.

Let ABC, fig. 13. represent a portion of the swing wheel, of which O is the centre, and A one of the teeth; Z is the centre of the crutch, pallets, and pendulum. The crutch is represented of the form of the letter A, having in the circular cross piece a slit $ik$ , also circular, Z being the centre. The arm ZF forms the first detent, and the tooth A is represented as locked on it at F. D is the first pallet on the end of the arm Zd moveable round the same centre with the detents, but independent of them. The arm $de$ to which the pallet D is attached, lies wholly behind the arm ZF of the detent, being fixed to a round piece of brass $efg$ , having pivots turning concentric with the axis of the pendulum. To the same piece of brass is fixed the horizontal arm $eH$ , carrying to its extremity the ball H, of such size, that the action of the tooth A on the pallet D is just able to raise it up to the position here drawn. ZP $p$ represents the fork, or pendulum rod, behind both detent and pallet. A pin $p$ projects forward, coming through the slit $ik$ , without touching either margin of it. Attached to the fork is the arm $mn$ , of such length that, when the pendulum rod is perpendicular, the angular distance of $nq$ from the rod $eqH$ is just equal to the angular distance of the left side of the pin $p$ from the left end $i$ of the slit $ik$ .

Now, the natural position of the pallet D is at $\delta$ , represented by the dotted lines, resting on the back of the detent F. It is naturally brought into this position by its own weight, and still more by the weight of the ball H. The pallet D, being set on the foreside of the arm at Z, comes into the same plane with the detent F and the swing-wheel, though here represented in a different position. The tooth C of the wheel is supposed to have escaped from the second pallet, on which the tooth A immediately seizes the pallet D, situated at $\delta$ , forces it out, and then rests on the detent F, the pallet D leaning on the tip of the tooth. After the escape of C, the pendulum, moving down the arch of semivibration, is represented as having attained the vertical position. Proceeding still to the left, the pin $p$ reaches the extremity $i$ of the slit $ik$ ; and, at the same instant, the arm $n$ touches the rod $eH$ in $q$ . The pendulum proceeding a hairsbreadth further, withdraws the detent F from the tooth, which now even pushes off the detent, by acting on the inclining face of it. The wheel being now unlocked, the tooth following C on the other side acts on its pallet, pushes it off, and rests on its detent, which has been rapidly brought into a proper position by the action of A on the inclining face of F. By a similar action of C on its detent at the moment of escape, F was brought into a position proper for the wheels being locked by the tooth A. As the pendulum still goes on, the ball H, and pallet connected with it, are carried by the arm $mn$ , and before the pin $p$ again reaches the end.

Watch. end of the slit, which had been suddenly withdrawn by the action of A on F, the pendulum comes to rest. It now returns towards the right, loaded with the ball H on the left, and thus the motion lost during the last vibration is restored. When the pin $p$ , by its motion to the right, reaches the end $k$ of $ik$ , the wheel on the right side is unlocked, and at the same instant the weight H being raised from the pendulum by the action of a tooth like B on the pallet D, ceases to act.

In this escapement, both pallets and detents are detached from the pendulum, except in the moment of unlocking the wheel, so that, except during this short interval, the pendulum may be said to be free during its whole vibration, and of course its motion must be more equable and undisturbed.

5. The constructing of a proper escapement for watches requires peculiar delicacy, owing to the small size of the machine, from which the error of $\frac{1}{250}$ of an inch has as much effect as the error of a whole inch in a common clock. From the necessary lightness of the balance, too, it is extremely difficult to accumulate a sufficient quantity of regulating power. This can be done only by giving the balance a great velocity, which is effected by concentrating as much as possible of its weight in the rim, and making its vibrations very wide. The balance rim of a tolerable watch should pass through at least ten inches in every second.

6. In considering the most proper escapements for watches, we may assume the following principle, viz. that the oscillations of a balance urged by its spring, and undisturbed by extraneous forces, are isochronous.

7. In ordinary pocket watches, the common recoiling escapement of clocks is still employed, and answers the common purposes of a watch tolerably well, so that, if properly executed, a good ordinary watch will keep time within a minute in the day. These watches, however, are subject to great variation in their rate of going, from any change in the power of the wheels.

8. The following is considered as the best construction of the common watch escapement, and is represented by fig. 14. as it appears when looking straight down on the end of the balance arbor. C marks the centre of the balance and verge; CA represents the upper pallet, or that next the balance, and CB the lower pallet; F and D are two teeth of the crown wheel, moving from left to right; E, G, are two teeth in the lower part, moving from right to left. The tooth D appears as having just escaped from the point of CA, and the tooth E as having just come in contact with CB. In practice, the escapement should not be quite so close, as by a small inequality of the teeth, D might be kept from escaping at all. The following are thought the best proportions: The distance between the front of the teeth (that is, of G, F, E, D), and the axis C of the balance, is $\frac{1}{2}$ of FA, the distance between the points of the teeth. The length CA, CB of the pallets is $\frac{1}{3}$ of the same degrees, and the front DH or FK of the teeth makes an angle of $25^\circ$ with the axis of the crown wheel. The sloping side of the tooth must be of an epicycloidal form, suited to the relative motion of the tooth and pallet.

It appears from these proportions, that by the action of the tooth D, the pallet A can throw out till it reach $a$ , $120^\circ$ from CL, the line of the crown-wheel axis. To this if we add $BCA=95^\circ$ , we shall have $LCa=120^\circ$ . Again, B will throw out as far on the other side.

Now, if from $240^\circ$ , the sum of the extent of vibration of both pallets, we take $95^\circ$ the angle of the pallets, the remainder $145^\circ$ will express the greatest vibration which the balance can make, without striking the front of the teeth. From several causes, however, this measure is too great, and $120^\circ$ is reckoned a sufficient vibration in the best ordinary escapement.

9. Of the improvements on the escapements of watches, Graham's one of the most important is that by Mr George Graham, which we shall proceed to describe. DE, fig. 15, represents part of the rim of the balance wheel; A and C, two of its teeth with their faces $be$ formed into planes, inclined to the circumference of the wheel in an angle of about $15^\circ$ , so that the length $be$ of the face may be nearly quadruple of its height $em$ . Let a circular arch ABC be described round the centre of the wheel, and through the middle of the faces of the teeth. The axis of the balance will pass through some point B of this arch, and the mean circumference of the teeth may be said to pass through the centre of the verge. On this axis is fixed a portion of a thin hollow cylinder $bcd$ , made of hard tempered steel, or of some hard and tough stone, such as ruby or sapphire. By this construction the portion of the cylinder occupies $210^\circ$ of the circumference. The edge $b$ , to which the tooth approaches from without, is rounded off on both angles. The other edge $d$ is formed into a plane, inclined to the radius about $30^\circ$ . Now, suppose the wheel pressed forward in the direction AC, the point $b$ of the tooth, touching the rounded edge, will push it outwards, turning round the balance in the direction $bcd$ . The heel $e$ of the tooth will escape from this edge when it is in the position $h$ , and $e$ is in the position $f$ . The point $b$ of the tooth will now be at $d$ , but the edge of the cylinder will be at $i$ . The tooth therefore rests in the inside of the cylinder, while the balance continues its vibration a little way, in consequence of the impulse it has received from the action of the inclined plane. When this vibration is ended, by the opposition of the balance spring, the balance will return, and the tooth now in the position B, rubbing on the inside of the cylinder, the balance comes back into its natural position $bcd$ , with an accelerated motion by the action of its spring, and would of itself vibrate as far as the other side. It is, however, assisted again by the tooth, which presses on the edge $d$ , pushes it aside till it attain the position $k$ , when the tooth entirely escapes from the cylinder. At this instant the other edge of the cylinder, having attained the position $l$ , is in the way of the next tooth, which is now in the position A, while the balance continues its vibration, the tooth resting and rubbing on the outside of the cylinder. When this vibration is finished, the balance, by the action of the spring, resumes its first motion, and as soon as the balance gets into its natural position, the tooth begins to act on the edge $b$ , pushes it aside, escapes from it, and drops as before in the inside of the cylinder. In this construction the arch of action or escapement is $30^\circ$ = twice the angle which the face of a tooth makes with the circumference.

It is necessary to explain how the cylinder is connected with the verge, so as to make such a great revolution round the tooth of the wheel. The triangular tooth $e b m$ is placed on the top of a little pillar fixed into the end of the piece of brass $m D$ formed in the rim of the wheel. Thus the plane of the wedge tooth is parallel.

parallel to the plane of the wheel, but at a small distance above it. The verge is represented at fig. 16. and consists of a long hollow cylinder of cast steel, having a great portion of the metal cut out. If spread out flat, this cylinder would assume the form of fig. 17. and if we conceive this flat piece rolled up till the edges GH and G'H' unite, we shall have the exact form. The part acted on by the point of the tooth is denoted by the dotted line b d, and the part D, I, F, E serves to connect the two ends.

This scapement of Mr Graham is called a horizontal scapement, because the balance is parallel to the other wheels.

Another scapement of a superior construction was contrived by M. Lepaute of Paris, and is of such a singular form as to render it extremely difficult to illustrate it by a figure. The representations at fig. 18. and 19. will, however, give general readers some idea of its mode of action, and a skilful artist will easily see how the several parts may be adapted to each other. ABC fig. 18. represents part of the rim of the balance wheel, having the pins 1, 2, 3, 4, 5, &c. projecting from its faces; the pins 1, 3, 5, being on the side next the eye, and the pins 2 and 4 on the opposite side. D is the centre of the balance and verge, and the small circle round D represents its thickness. But the verge in this place is crooked, that the rim of the wheel may not be intercepted by it. To it is attached a piece of hard tempered steel a b c d, of which the part a b c is a concave arch of a circle, having D for its centre. It wants about $30^\circ$ of a semicircle. The rest c d is also an arch of a circle having the same radius with the balance-wheel. In the natural position of the balance, a line drawn from D, through the middle of the face c d is a tangent to the circumference of the wheel. But if the balance be turned round till the point d of the horn come to d', and the point c come to 2 in the circumference in which the pins are placed, the pin pressing on the beginning of the horn or pallet, pushes it aside, slides along it, and escapes at d, having generated a certain velocity in the balance. Let another pallet similar to that now described be placed on the other side of the wheel, but in a contrary position, with the acting face of the pallet turned away from the centre of the wheel. Let it be so placed at E, that the moment the pin 1 on the upper side of the wheel escapes from the pallet c d, the pin 4 on the lower side of the wheel falls on the end of the circular arch e f g of the other pallet. Now, if the pallets be connected by equal pulleys G and F on the axis of each, and a thread round both so that they shall turn one way; the balance on the axis D having received an impulse from the pin 1, will continue its motion from A towards i, and will carry the other pallet with a similar motion round the centre E from h to k. The pin 4 will therefore rest in the concave arch e f g as the pallet turns round. When the force of the balance is spent, the pallet c d returns towards its first position. The pallet e f g turns with it, and when the point of the first has arrived at d, the beginning g of the other arrives at the pin 4; and, proceeding farther, this pin escapes from the concave arch e f g, and slides along the pallet e f g, pushing it aside, and of course urging the pallet round the centre E, and the balance on the axis D round at the same time, and in the same direction. The pin 4 escapes from the pallet e f g, when h arrives at 3; but while the

pin 4 is sliding along the yielding pallet e f g, the pin 3 is moving in the circumference BDA; and the instant that the pin 4 escapes from h at 3, the pin 3 arrives at 2, where the beginning c of the concave arch c b is ready to receive it. It therefore rests on this arch, while the balance continues its motion, and this may continue till the point b of the arch comes to 2. The balance now stops, its force being spent, and then returns; and the pin 3 escapes from the circle at c, slides along the yielding pallet c d, and when it escapes at 1, another pin on the lower side of the wheel arrives at 4, and finds the arch e f g ready to receive it. And thus the vibration of the balance will be continued.

From the above description we may deduce the proper dimensions of the parts of the pallet. Thus, the length of the pallet c d or e f g, must be equal to the interval between two succeeding pins, and the distance of the centres DE, must be double of that interval. The radius Dc or Eg, may be as small as we choose. The concave arches c b a and e f g, must be continued so far as to allow a pin to rest on them during the whole excursion of the balance. The angle of scapement, in which the balance remains under the influence of the wheels, is obtained by drawing the lines Dc and Dd, and we shall find that this angle c D d is here about $30^\circ$ , though it may be made either greater or less than this.

Fig. 19. explains how the two pallets may be combined on one verge. KL is the verge with a pivot at each end. It is bent like a crank MNO, to admit the balance wheel between its branches. BC represents this wheel, seen edgewise, with its pin alternately on different sides. The pallets are also represented by b c d and e f g, sized to the inside of the branches of the crank, fronting each other. The position of their acting faces may be seen in the preceding figure, on the verge D, where the pallet e f g is represented by the dotted line 2i', as situated behind the pallet c d. The remote pallet 2i' is so placed, that when the point d of the near pallet is quitted by a pin 1 on the upper side of the wheel, the angle formed by the face and the arch of rest of the other pallet is just ready to receive the next pin 2, which lies on the lower side of the rim. It is plain that the action here will be the same as if the pallets were on separate axes. The pin 1 escapes from d, and the pin 2 is received on the arch of rest, and locks the wheel, while the balance continues in motion. When the balance returns, 2 gets off the arch of rest, pushes aside the pallet 2i', escapes from it when i gets to 1, and then the point c is ready to receive the pin 3, &c. The vibrations may be increased by giving a sufficient impulse through the angle of scapement, but they cannot exceed a certain quantity, otherwise N, the top of the crank, would strike the rim of the wheel. The vibrations may be easily increased to $180^\circ$ , by placing the pins at the very edge of the wheel; and by placing them at the points of long teeth, so that the crank may get in between them, the vibrations may be carried to a much greater extent.

The construction just described is exceedingly ingenious; and if the machinery be well executed, this scapement will excel the horizontal scapement of Graham, both as it has but two acting faces to form, and as it admits of making the circle of rest extremely small, without lessening the acting face of the pallet. The construction is, however, very delicate and difficult, and must require a very nice workman.

An excellent escapement of much more easy construction, is that commonly called Duplaie's escapement, and with this we shall conclude our account of watch-work.

Fig. 20. represents the essential parts somewhat magnified. AD a portion of the balance-wheel, having teeth f, h, g, at the circumference. These teeth are for producing the rest of the wheel, while the balance is making excursions beyond the escapement. This is effected by an agate cylinder spg, on the verge. This cylinder has a notch o. When the cylinder turns round in the direction opg, the notch easily passes the tooth B which is resting on the cylinder surface; but when it returns in the direction bpo, the tooth B gets into the notch and follows it, pressing on one side of it till the notch comes into the position o. The tooth being then in the position h, escapes from the notch, and another tooth drops on the convex surface of the cylinder at B. The balance-wheel is also furnished with a set of flat-sided pins, standing upright on its rim represented by aD. There is likewise fixed on the verge a larger cylinder GEC above the smaller one opq, with its lower surface clear of the wheel, and having a pallet C, of sapphire, firmly indented into it, and projecting so far as to keep clear of the pins on the wheel. The position of this cylinder, with respect to the smaller one below it, is such that the tooth b being escaped from the notch, the pallet C has just past the pin a, which was at A while B rested on the small cylinder; but it moved from A to a, while B moved to b. The wheel being now at liberty, the pin a exerts its pressure on the pallet C in the most direct manner, and gives it a strong impulsion, following and accelerating it till another tooth stops on the little cylinder. The angle of escapement depends partly on the projection of the pallet, and partly on the diameter of the small cylinder, and the advance of the tooth B into the notch. Independent of the action on the small cylinder, the angle of escapement would be the whole arch of the large cylinder between C and x. But a stops before it be clear of the pallet, and the arch of impulsion is shortened by all the space described by the pin while a tooth moves from B to b. It stops at d.

For an account of other escapements we must refer our readers to the Memoirs of the Academy of Sciences at Paris for 1748, Cummin's Elements of Clock and Watch-work, a French work entitled Machines apprêvées par l'Académie des Sciences, and Young's Lectures on Natural Philosophy, vol. i. p. 193, and Plate 16, vol. ii. p. 193.