CLOCK. All instruments for the accurate measurement of time have three parts in com mon. One is a vibrating body, which may be a pendulum swinging under the force of gravity, or a balance wheel vibrating on its axis through the action of an elastic spring. The second feature is a source of power, which may be a weight or a stiff steel spring, called the main spring; the object of either is to keep up the vibrations of the balance wheel or pendulum, which would otherwise soon cease through fric tion and the resistance of the air. The third feature is a system of wheel work by which the power is transmitted from its source to the vibrating body, and by the revolutions of which the time is indicated. The instruments by which these results are brought about may be classified as watches, chronometers and clocks. The watch is a portable form of the in strument, generally so constructed as to be car ried in the pocket and moved about into any position without interfering with its going. This requires that the source of power shall be a spring and the vibrating body a balance wheel. The chronometer (q.v.) is a larger and most accurate watch constructed on the same gen eral principles, but intended, when moved, to maintain a horizontal position. The term is sometimes applied to the most accurate pocket watches; and to distinguish it from them the chronometer proper is sometimes called a ma rine chronometer. The term is chosen because the instrument is used to keep time at sea for the determination of a ship's longitude.
Construction.— In the original construction of the clock, to which this article is especially devoted, the vibrating body was the pendulum and the source of power a weight which could be wound up with a key. But, in order to se cure the advantage of being easily handled and moved about it is now very common to make the smaller class of clocks with either a main spring or a balance wheel or both. But such clocics, unless made in the most expensive way, do not keep time as accurately as those with weight and pendulum; the latter are, therefore, to be preferred in all cases where a clock can be kept in one and the same position, and is wanted to keep fairly good time for consider able periods.
In the earliest instruments of these different kinds the vibrating body was a small horizontal bar, which moved back and forth under the in fluence of the wheel work. This was before the idea of applying a spring to keep up and regulate the vibrations had been- suggested. Such instruments were extremely inaccurate in their results. The possibility of the pendulum clock dates from the time of Galileo, who first showed clearly that a pendulum completed its swings in nearly the same time, whatever its arc of vibration, which time was dependent on its length. But the practical inventor of the pendulum clock was Huyghens, the great Dutch astronomer of the 17th century. He invented the various appliances of the clock, the strik ing apparatus excepted, in substantially the fonn that we have them to-day. lArhat is some
times called the astronomical clock does not differ in principle from the ordinary pendulum clocic, and has nothing distinctive about it ex cept that it is made with the greatest perfection throughout in order to go with the utmost at tainable uniformity from day to day. So suc cessful have clock-makers been in recent times Cat an astronomical clock which varied by one tenth of a second during a day would be re garded as practically useless. A good clock of the kind should keep time within three or four hundredths of a second a day.
The accompanying plate shows in detail the parts of the ordinary clock. Fig. 1 gives a lateral view of the wheel work. An end view of the mechanism is shown in Fig 2. P, Fig. 1, is a weight suspended by a cord which rests in a spiral groove going round and round the main cylinder C. In order that the clock may be wound up at any nme, a ratchet wheel DD, Fig. 2, is used with a grooved cylinder, around which is wound a cord carrying the weight. The manner in which the ratchet acts is too obvious to need description. It is to be re marked that, in the ordinary house clocic, the action of the wheel work stops while the weight is being wound up. As the clock then loses all the time taken to wind it up, the best class of clocks are constructed with a second ratchet which keeps them going while being wound. The action of the weight is transmitted through the train of wheels d, E arid F, to the scape wheel GH, which acts on the pallets IR, con nected with the crutch which acts on the pendu lum. In Fig. 1 By is the pendulum and its crutch vibrating in a plane perpendicular to that of the paper. In the upper part of its length, which passes through the crutch U,. it turns on a horizontal axis which can be seen in the figure extending horizontally from the framework of the clock from just above X to the right Thus, as the pendulum vibrates the arm of the crutch UX vibrates with it. This vibration releases alternately the teeth of the scape wheel GH, a double motion of the pendulum back and forth being required in order that first one tooth and then the other may escape. As shown in the figure the end R of the anchor is moving upwards, the tooth H of the scape wheel being just about to slide off. During the time that the pallet R is holding the. scape wheel, the latter slides and presses upon it, thus pressing the side of the crutch against the pendulum. When the tooth H escapes, the an chor is in such a position that another tooth strikes the lower surface of I and acts on that in the opposite direction while the pendulum is swung the other way. Thus the teeth escape alternately, first one and then the other, in regular alternation, gently pressing the pendu lum during its swing each way.