The first telegraph was an aerial message and consisted of a signal made by a flash of light. From the earliest times men have used fire signals as a means of sending messages to distant points. When the city of Troy in Asia Minor was captured by the Greeks (about 1100 B.C.) torches flashing their light from one mountain top to another quickly carried the news to the far-off cities of Greece. The ancient Greeks gave a great deal of attention to the art of signaling by fire and they invented several very ingenious systems of aerial telegraphy. The most interesting of these systems is one invented and described by the Greek historian Polybius, who flourished about 150 B.C. When signaling with fire Polybius arranged for using two groups of torches with five torches in each group, and for the purpose of understanding the signals he divided the letters of the alphabet into five groups of five letters each.[24] The torches were raised according to a plan that made it possible to flash a signal that would indicate any letter of the alphabet that might be desired. Thus if the desired letter was the third one of the first group--that is, the letter _k_--one torch would show which group was meant and three torches would show which letter was meant (Fig. 5). In theory this system was perfect, for it provided for sending any kind of message whatever. But in practice it had little value, for it required so many torches and signals that an entire night was consumed in spelling out a few words. [Illustration: FIG. 5.--TELEGRAPHING BY MEANS OF FIRE, 150 B. C.] Although the elaborate system of aerial telegraph proposed by Polybius was not generally adopted, nevertheless for centuries, both in ancient times and during the middle ages, the fire signal was everywhere used for the quick despatch of important news. In the seventeenth century inventors began to devise new systems of aerial telegraphy. In 1663, the Marquis of Worcester, who was always busy with some great invention (p. 178), announced to the world that he had discovered a plan by which one could talk with another as far as the eye could distinguish between black and white, and that this conversation could be carried on by night as well as by day, even though the night were as dark and as black as pitch. But the telegraph of the Marquis was like many of his other inventions--it was chiefly on paper. In 1864, Dr. Robert Hooke of England invented a method by which aerial messages could be sent a distance of thirty or forty miles. His plan was to erect on hill tops a series of high poles connected above by cross-pieces and by means of pulleys suspend from the cross-pieces the letters of the alphabet which would spell out the message (Fig. 6). In order to read the letters at such great distances the eye was assisted by the telescope, an instrument which had recently been invented. [Illustration: FIG. 6.--HOOKE'S AERIAL TELEGRAPH, 1684.] [Illustration: FIG. 7.--CHAPPE'S AERIAL TELEGRAPH, 1793.]