In 1816 it was determined to change the Admiralty telegraphs into semaphores constructed on the principle of those used in France, with the improvements 'Ingested by Sir Homo Popham, who had previously done much for the improvement of naval signals. The action of Popham's semaphore is explained by fig. 7, in which dotted lines are added to show the various positions in which the arms may be placed, and nume rals to show the numbers indicated by those positions. Only two arms are em ployed ; but ea they are mounted upon separate pivots, each of them can assume s six different positions, and the two together are capable of affording forty-eight signals ; which number, though less than that given by the six-shutter telegraph, is sufficient to express the letters of the alphabet and the Arabic numerals, and to leave thirteen signals unappropriated, for abbreviations and arbitrary signs. This kind of sema phore (which is minutely described in the ' Society of Arta Transactions,' vol. xxxiv.) was used at the government stations until superseded by the electric telegraph.
The rert cal post of this telegraph was a • hollow hexagonal mast, which, turning on a pivot at its foot, and in a collar where it passed through the roof of the observatory, could be moved so as to display its signals in any direction. The moveable arms were provided with balane•welghts in the form of muses of metal attached to their shorter ends, very near to the pivot, upon which they turned, by which means they could be nerved in any direction with the exertion of a very small force ; and they were made, when out of use, to fall into grooves or meows In the post, so as to become invisible. The movements were effected by means of two near the base of the mast, within the cabin, one for each arm. The winch-handles turned two small bevel-wheels, which communicated motion by means of two horizontal bevel.wheela to long upright shafts or rods, which passed np the inside of the hollow post of the telegraph. At the upper ends of these rods, which were held steady by suitable bearings, were endless screws, working into toothed wheels fixed upon the axes of the arms, and thereby communicating motion to them. In order that the person who worked the machine might know precisely when the arms were brought to the required positions, similar endless screws were added near the lower ends of the vertical rods, to give motion by toothed wheels to indexes, which gave a miniature representation of the motion of the arms. Excepting these indexes and the winch-handles, the whole apparatus was inclosed in the vertical shaft of the telegraph, on the outside of which small blocks were added, to enable a man, with the assistance of a rope from the top of the post, to ascend the machine for the purpose of cleaning and oiling it.
About the same time, Sir Home Popham proposed a modification of the semaphore for marine purposes, which, he conceived, would be found very advantageous for the merchant-service, by superseding the necessity for a costly set of signal-flags, the expense and wear and tear of which formed a serious objection to a system of general telegraphic communication at sea, excepting in the ships of the East India Com pany. His proposed sea-telegraph would not, it is stated, cost more than fifty shillings at first, and its wear and tear would not amount to five shillings a year. As the height of an apparatus resembling the land-semaphore would be objectionable for marine purposes, Popham proposed to use two poets, 12 feet 2 inches high and 6 inches thick, each having fa single arm, 6 feet 4 inches long and 10 inches broad, pivoted to the top. but not falling into a slot in the post, as in the last-described machine. In a small slot at the top of each post is a grooved pulley or sheave, fixed upon the same axis as the arm; and at a convenient height from the bottom of each post another precisely similar pulley is mounted in like way, its axis passing through the post, and carrying a small wheel with four handles at right angles with each other, by which the machine is worked ; the motion of the lower pulley being communicated to the upper ono, and consequently to the arm, by an endless rope, which has two or three turns round each of the sheaves, and passes up by the sides of the post. When the tele graph is in use, the posts may be attached to the sides of a vessel by stepping their lower ends into blocks fixed for the purpose, and lashing them to the bulwarks; or they may be mounted upon trucks, so as to be readily moved from one part of the ship to another. The descrip tion of this machine in Sir Home Popham's communication to the Society of Arts mentions but four positions for each arm, and states that when placed In the four positions diagonally to the post, one arm denotes 1, 2, 3, and 4, and the other 5, 6, 7, and S. This arrangement gives twenty-four distinct signals, and avoids the possibility of mis taking the horizontal for an inclined position of either arm, of which there might, owing to the motion of the ship. be some risk.