Everybody must have noticed that the numerous public clocks in a large town do not often agree exactly with one another, the differences sometimes being quite large; while even in one building, such as a large hotel, the different clocks vary more or less. This state of things is very unsatisfactory, for it is difficult to know which of the clocks is exactly right. Although large clocks are made with the utmost care by skilled workmen, they cannot possibly be made to maintain anything like the accuracy of a high-class chronometer, such as is used by navigators; and the only way to keep a number of such clocks in perfect agreement is to control their movements from one central or master clock. This can be done quite satisfactorily by electricity. The master-clock and the various sub-clocks are connected electrically, so that a current can be sent from the master-clock to all the others. Each sub-clock is fitted with an electro-magnet placed behind the figure XII at the top of the dial. At the instant when the master-clock reaches the hour, the circuit is closed automatically, and the current energizes these magnets. The minute hands of all the sub-clocks are gripped by the action of the magnets, and pulled exactly to the hour; the pulling being backward or forward according to whether the clocks are fast or slow. In this way all the clocks in the system are in exact agreement at each hour. The same result may be attained by adjusting all the sub-clocks so that they gain a little, say a few seconds in the hour. In this case the circuit is closed about half a minute before the hour. As each sub-clock reaches the hour, its electro-magnet comes into action, and holds the hands so that they cannot proceed. When the master-clock arrives at the hour the circuit is broken, the magnets release their captives, and all the clocks move forward together. It is possible to control sub-clocks so that their pendulums actually beat exactly with the pendulum of the master-clock; but only a small number of clocks can be controlled in this way, and they must be of the best quality. The method is similar to that used for hourly corrections, the main difference being that the circuit is closed by the pendulum of the master-clock at each end of its swing, so that the pendulums of the sub-clocks are accelerated or held back as may be required.