It was not till near the close of the seventeenth century that a really practical system of visual signalling from hill to hill was introduced by Dr Hooke, whose attention had been turned to the subject at the siege of Vienna by the Turks. He erected on the top of several hills having a sky-line background three high poles or masts, connected at their upper ends by a cross-piece. The space between two of these poles was filled in with timbers to form a screen, behind which the various letters were hung in order on lines, and, by means of pulleys, run out into the clear space between the other two, when they stood out clear against the sky-line. The letters were thus run out and back again in the required order of spelling, and were divided into day and night letters--the former being made of deals, the latter with the addition of links or lights; besides which there were certain conventional characters to represent such sentences as, 'I am ready to communicate,' 'I am ready to receive.' In his description of the device, read before the Royal Society on the 21st of May 1684, Dr Hooke, after claiming for it the power of transmitting messages to a station thirty or forty miles distant, said: 'For the performance of this we must be beholden to a late invention, which we do not find any of the ancients knew; that is, the eye must be assisted with telescopes, that whatever characters are exposed at one station may be made plain and distinguishable at the other.' A cipher code was subsequently added by an ingenious Frenchman named Amontons. In 1767 we find Mr Richard L. Edgeworth, the father of Maria Edgeworth, employing the sails of a common windmill for communicating intelligence, by an arranged system of signals according to the different positions of the arms. The signals were made to denote numbers, the corresponding parties being each provided with a dictionary in which the words were numbered--the system in vogue for our army-signalling till 1871, when the Morse alphabet was substituted for it.