The principle of construction adopted by Huygens, from the peculiar action of the levers and spindle, required a light pendulum and great arcs of oscillation ; and although, to secure isochronous vibration in these large arcs, the ingenious device of constraining the motion in a cycloidal curve was resorted to, yet the consequence was, as has been remarked, that " Huygens's clock governed the pendulum, whereas the pendulum ought to govern the clock.' About ten years afterwards, the celebrated Dr. Hooke invented an escapement, which enabled a less maintaining poN'ver to carry a heavier pendulum. The pendulum, too, making smaller arcs of vibration, was less resisted by the air, and therefore performed its motion with greater regularity. This device is called the crutch or anchor escapement. It was brought by Hooke before the notice of the royal society in 1666; and was practically introduced into the art of clockmaking by Clement, a London clockinaker, in 1680. It is the form still most usually employed in ordinary clocks. It regulates the motion as follows: The pendulum is fixed at A, and hangs down behind the pallet-wheel (the last of the train of wheel-work), which revolves in the direction of BC, under the action of the weight; B and C are the pallets. When the pendu lum swings to the left, AC rises, and a tooth escapes from C, while another falls on the outside of B, and, owing to the form of the pallet B, this latter recoils during the remainder of the swing.
The same thing occurs on the pendulum's return; the arm AB rises, a tooth escapes from B, and another falls on the inside of C, and is pushed backwards by it during the remainder of the swing. The revolution of D is thus regularly retarded, one tooth being allowed to escape for every two oscillations—i.e., every two seconds—and as the wheel contains 30 teeth, it performs one revolution per minute (the seconds hand is fixed on the extremity of the axle of this wheel). During a portion of each contact between the pallets and teeth, the onward pressure of the wheel give's an additional impetus to the pendulum, so as to counteract the retarding effects of the resistance of the air and friction; which would otherwise bring it to a stand.
The only defect of this escapement is the recoil, and various modifications have been devised to obviate this. The first and most successful was made by George Graham, an English watchmaker in the begin ning of the 18th c., and his improved form is called the dead 'scapement or dead-beat escapement (Fig. 3). Here the outer sur face of B and inner of C are arcs of circles, whose center is A, and a little consideration will show that there can be no recoil. This escapement is adopted in time-keepers when great accuracy is required. Other inventions, as the detached escapement, the pin-wheel escapement in various forms, and the gravity escapement (described below), though very efficient, have not come into • general use.
In the great clock in the new houses of parliament at Westminster, the pendulum is upwards of 13 ft. long, to beat 2 seconds, and its bob weighs 6 ewts. The motion is
kept up by a remontoir or gravity escapement. On each side of the pendulum-rod a small metallic hammer is hung upon a peg. " The swinging of the pendulum first draws out a little bolt, that stopped the turning of a wheel; the wheel then goes round, under the influence of the weight, lifting one of the little hammers as it does so, until it is caught by anodic'. bolt. The hammerhead next falls by its own gravity, and strikes the pen dulum-rod just as it is in the act of descending, communicating the force of its blow to quicken the movement; the same thing is afterwards repeated on the opposite side of the vibration, and then again on the same side; so going on alternately." The push thus gives is evidently unvarying. The wheel has three stops and cogs on it, and goes once round in three beats of the pendulum, or in six seconds. With this contrivance "it is found that all the teeth of the several wheels may be rough, just as turned out from the casting, and the clock will nevertheless keep better time than it would have done with the most perfectly finished teeth under other arrangements." The gradual perfection of the clock required also improvements in the pendulum (see P.E.snumuu).
The improvements in the escapement and the pendulum above referred to bring the mechanical perfection of the clock, as a time-keeping instrument, to the point which it has •attained at the present day. But the art of horology would be incomplete unless there was some standard, independent of individual mechanical contrivances, to which all may be referred, and by which the erross'of each—which must exist in the most perfect human contrivances—may be correctaid. The movements of the heavenly bodies are still, as of old, the only standard for a general measurement of time, affording as they do marks of unvarying certainty, to be read by all alike; and clocks and other meAanical contrivances are individual and imperfect measures of the intervals, to be trusted only until there is a new opportunity of comparing them with the certain and public signals of the heavens. These signals can, however, only be accurately read by persons furnished with the proper apparatus, and instructed sufficiently in its use. This is done iu observatories, and there are in most parts of this country now sufficient oppor tunities of setting clocks by a communication more or less direct with these establish ments. When these are not to be had, the sun-dial may still be used with advantage, as a means of approximation to the correct time. The time which a clock ought to mark mean tiine, the definition of which will be found in the article Day (q.v.). The mean place depends on the longitude. Supposing a clock to be set to Greenwich mean time, a clock keeping mean time of any place will be 4 minutes faster for every degree of longitude east of Greenwich, and 4 minutes slower for every degree west. Since the introduction of railways, clocks are usually set, within Great Britain, to Green wich mean time.