The construction of the escapement is of the utrnost inmortance when the best per formance of a clock is required. If one exam ines the second hand of an old-fashioned house clock he will noticeethat, at the moment of each beat, the second. hand moves forward to one second and then has a slight motion bacicsvard before the next beat. This backward motion takes place immediately after one tooth has escaped, and continues until the pendulum has sprung to its jp-eatest distance on one side. This bacicward. action throwing extra action on the pendulum tends to accelerate its motion. hence, good clocks are provided with a °dead beat° escapement invented by Graham. In this form the pallet on which the teeth of the scape wheel first impinges is at right angles to the radius from the centre.of motion of the anchor to the pallets themselves. The action of the scape wheel then does not begin until the pendu lum reaches a certain point of its downward swing, which. point is the same whether the swings are wide or narrow.
Under the system just described, the pendu lum is kept in continuous vibration and the wheel work in continuous motion from the time the clock is wound up until it is run down. But it is also necessary that the clock shall, at every moment, show the time on its face. This is brought about by the wheels 0 and N,. Fig. 1, one of which is. arranged to revolve in an hour, the other in 12 hours. The result is too familiar to need any description. If the wheel work which moves the hand were rigidly connected with that of the clock, the latter could not be set without moving the whole of the clock work. The system of wheel work which carries the hands is, therefore, so ar ranged as to be held on the axis c, by friction, thus admitting of its being turned at pleasure, while it is held by the axis so long as the hands are untouched. The minute hand is rigidly attached to its axis in order that it may be used as a hand in turning the wheel work of the two hands: Generally, but not always, the hour hand is held on its axis only by friction so that it can be turned around at pleasure without interfering with the other hand or with the going of the clock.
Regulation.— It is, of course, necessary to the correct going of a clocic that the length of the pendulum should correspond to the number of the teeth in such a way that, as the pendulwn goes through its natural vibrations, the wheels shall go according to the time of day. In the old-fashioned house clocic, and in most good modern clocics, when it is desired to show seconds as well as hours and minutes, the pendulum talces one second to oscillate in each direction. It is then called a °seconds pendulum.* There are then 30 teeth in the scape wheel, the latter turtling through one sixtieth of the revolution as each alternate tooth escapes. The seconds pendulum is about 39% inches in length. (See PENDULUM ). It
is not easy to move a clock about when the pendulum is so long. Hence, in ordinary clocics for household use either a balance wheel or a shorter pendulum is used. If the latter is one fourth the length of the seconds pendulum, it will vibrate in half a second. But, as clocks are constructed, the pendulum is not always made to vibrate in any simple fraction of a second. In this case, although a second hand may be put on the clock, and may go through its revolution in a minute, its motions will not beat seconds.
The rate of going of ordinary clocks varies with the temperature, the clock going faster the lower the temperature. In the case of the pendulum clocic this is because the pendulum rott contracts with cold and expands with heat. If a balance wheel is used, not only does the balance contract with cold but the spring which moves it becomes stronger; so that for a double reason the clock goes faster in cold weather. Hence if accurate time is to be kept the pendulum or balance wheel must be com pensated for changes of temperature. To show how this is done in the case of the balance wheel, see CHRONOMETER. For the two methods of compensating a pendulum, see PENDULUM. Between the extremes of temperature to which the house clodc is ordinarily subjected the dif ferences of rate will only be about five or six seconds a day. As few people are able to regu late their clocks to this degree of precision it is only in very good clocics that compensa tion is attempted. The effect of heat and cold is much greater on the balance wheel than on the pendulum and may amount to half a minute of more a day.
Striking The striking mech anism of the clock is brought about by a sepa rate train of wheel work with its separate weight or spring, as shown in Fig. 2. There are two systems of making a clock strike the hour, one of which may be called the con tinuous system, and is used in clocks of Amer ican construction, while the other is mostly used in French clocks. The continuous system is one in which the strilcing goes on in its regular order, two always following one, three follow ing two, and so on, no matter how the hour hand may be situated. In this system if the clock begins striking wrong through ,running down or from any other cause it will continue to strike wrong until a proper adjustment is made. This is effected by touching a spring or pulling a wire in the clock by which the strik ing mechanism is set in action and the clock made to strike around until it comes to the right hour. The French system is arranged with an eccentric cam on the axis of the hour wheel which stops the striking mechanism when the hammer of the clock has struck a number of strokes corresponding to the position of the hour hand. On this system the striking mech anism needs no adjustment to strike correctly always.