Chronometer most perfect of the detached escape ments is the chronometer escapement (j/. 6), invented by Pierre he Roy about 1769, and perfected by Thomas Earnshaw and John Arnard about r7So' It seems to be so well adapted to the peculiar movement of a ship on the open sea that it is the only escapement employed for marine time-keepers. Before explaining the action of this escapement we will describe the parts of the mechanism and their relative positions. The balance and hair-spring, not shown in the engraving, are mounted on the arbor d ; the arbor carries a roller S, R., called the "impulse roller," which has an indentation A, q, whose upper part is curved, while the lower part is straight and provided with a stone of highly-polished surface q, a, called the " impulse pallet." The arbor has a second smaller roller (i, 1), with a projection (k), made also of stone and called the "discharging pallet." The escape-wheel S', which is impelled in the direction of the arrow, has inclined pointed teeth—straight on one side and curved on the other—and approaches, in its proper position, the impulse roller just close enough to allow its periphery to move free along the curved side of the teeth; az, ar n, a is a combination of two springs called the "detent;" the longer one to the left is called the "blade of detent," and has at c a projection called the "locking pallet," on which the tooth y of the escape-wheel rests.
The blade of detent rests against the shoulder of a screw-head to limit its approach to the escape-wheel and to adjust the position of the locking pallet c. At the lower end the detent is fastened by two screws in a fixed position, while a small portion of the upper end terminates in a bend (n), called the "horn of detent;" the shorter spring of the detent nz, nz is made of gold, and is attached to the blade of the detest some distance below the pallet c, and runs parallel with the blade a little beyond the horn of detent n, against which it rests.
If we now assume that the balance and rollers are vibrating and the detent pallet k is moving toward the terminal of the detent, the pallet will come in contact with the projecting end of the gold spring nz, thereby bending it to the right and passing it, while the blade of detent is retained in its position by the shoulder of the screw-head r, against which it rests. When the balance returns, it brings the detent pallet again in contact with the end of the gold spring, bending it this time to the left and passing it again. Now, however, the detent blade also will yield and move the lock ing pallet c to the left, thereby releasing the tooth of the escape-wheel, which at that moment moves in the direction of the arrow; the escape-wheel is,how ever, intercepted in its progress by the impulse pallet a, against which the tooth a strikes, and follows until the pallet has moved far enough to set the tooth free. The detent, however, having returned to its normal position, the tooth p will drop on the locking pallet c and stop the wheel again. When the balance returns, the deteut pallet will again pass the gold spring, and the action just described will be repeated. It may be unnecessary to state that the blow struck against the impulse pallet adds to the momentum of the balance for continuing its vibrations; the fact that the balance receives but one impulse at every other vibration makes it a more perfectly detached escapement than the lever escapement. If, however, proper performance
of its functions is expected, it must be made in all its parts and propor tions with the greatest care. For watches it is less adapted, since the movements of the body subject it to certain jerks, which interfere greatly with its proper action. For marine chronometers, however, it fills nearly all wants perfectly, and leaves but little to be desired. Figure 5 (pi. 125) shows a marine chronometer whose train is impelled by two main-springs and two toothed barrels, both being separately wound. This precaution secures to the chronometer an impelling power 9f one spring, in case one or the other of them should break, although the breaking of the spring would interfere with its time-keeping qualities. The rest of the train is hidden from view by a plate which shows the escapement. The hair spring has the form of a cylindrical spiral, and the balance is a compen sating one.
Striking Clocks.—An exceedingly admirable and ingenions mechanism is the striking-work attached to many clocks in general use. The date of this simple invention cannot be ascertained. The oldest clock (Von Wick's) whose details are known is provided with a striking-work whose essential parts are still in use, and which is known as the " locking-plate " strik ing-work (p/. 125, Jic. 1). The train consists of the wheels 11, C, I) and the pinion E, which carries the fan or " fly " for preserving uniformity in the time which elapses between each stroke of the hammer. The great wheel 11 is impelled by a cord and weight bearing on the drum A, and has a series of pins to lift the lever and hammer L which strikes the bell to announce the hour when the minute-hand points to twelve. This is accomplished by the striking mechanism as follows: the wheel C, which .is driven by the great wheel /1, has fixed to its arbor a notched cam called the "locking-cam." The locking-lever J with its bearing at f has also a notch (g), whose outline fits in the notch of the locking-cam, where it rests when the striking-work is inoperative. To prevent the continuous run ning of the train, the pin e on the pin-wheel I) rests against a projection extending upward from the locking-lever J. The dotted line at // indi cates a lever which is gradually raised by the time-keeper just before the completion of an hour. This lever has also a projection extending upward to the same height as the projection on the locking-lever J. The projec tion on J has on its lower portion a horizontal opening or slot which grad ually rises to the pin e. Time moment this opening comes in line with the pin, the pin-wheel D is set free by allowing the pin e to pass through the slot of the projection; but the pin is immediately stopped in its progress by the upright projection of the lever 77, and is detained until the instant the hour is completed, when the lever 7/ drops and releases the pin and the entire train begins to move. The incline of the locking-cam raises to the height of its circumference the locking-lever J until the wheel C has made one revolution; the locking-lever then falls into the notch g of the locking-cam and the pin is again stopped by the projection of the lock ing-lever and the train is arrested. At each revolution of the locking-cam the hammer L strikes one blow.