In the correcting systems already described the sub-clocks are complete in themselves, so that they work quite independently, except at the instant of correction. For hotels, schools, and other large buildings requiring clocks at a number of different points, a simpler arrangement is adopted. Only one complete clock is used, this being the master-clock, which may be wound either electrically or by key. The sub-clocks are dummies, having only a dial with its hands, and an electro-magnetic arrangement behind the dial for moving the hands. The sub-clocks are electrically connected with the master-clock, and the mechanism of this clock is arranged to close the circuit automatically every half-minute. Each time this occurs the magnet of each sub-clock moves forward the hands half a minute, and in this way the dummy clocks are made to travel on together by half-minute steps, exactly in unison with the master-clock. CHAPTER XVI THE TELEGRAPH We come now to one of the most important inventions of the nineteenth century, the electric telegraph. From very early times men have felt the necessity for some means of rapidly communicating between two distant points. The first really practical method of signalling was that of lighting beacon fires on the tops of hills, to spread some important tidings, such as the approach of an enemy. From this simple beginning arose more complicated systems of signalling by semaphore, flags, or flashing lights. All these methods proved incapable of dealing with the rapidly growing requirements of commerce, for they were far too slow in action, and in foggy weather they were of no use at all. We are so accustomed to walking into a telegraph office, filling up a form, and paying our sixpence or more, that it is very difficult for us to realize the immense importance of the electric telegraph; and probably the best way of doing this is to try to imagine the state of things which would result if the world’s telegraphic instruments were put out of action for a week or two.