Fig. 18 represents the telegraph fitted up for making nocturnal signals. One lantern C, called the central light, is fixed to the same pivot, upon which the arms move. Two other lanterns are at tached to the extremities of the arms. A fourth lantern I, used as an indicator, is fixed on the same horizontal level, with the central light, at a distance from it equal to twice the length of one arm, and in the same plane nearly in which the arms revolve. Hence the whole apparatus consists of two fixed and of two movable lights, four in all.
The number of telegraphic signs, combinations, or changes, which this telegraph is capable of ex hibiting, are only 28, but these are amply sufficient for every purpose of telegraphic communication, whether by the alphabetical method, or in reference to a telegraphic dictionary of words and sentences. These signs are represented in Fig. 21, showing the appearance of the same combinations both by day and night.
In some few of the nocturnal signs, it will be ob served, that one of the lights is marked black. This only happens when one of the movable lanterns is supposed to be in its quiescent position, hanging vertically down below the centre light. In this case, as the lantern may be exhibited on either side of the post, in may sometimes be seen, and some times not, by the distant observer. At first I pro posed to interpose a couple of screens, one on each side of the post, to hide the lanterns altogether when in this position. Afterwards that idea was abandoned, it having been found, in practice, that it made no difference in regard to the clearness of the signs alluded to, whether the movable lanterns were seen or obscured, when in the position denoted by the black circles.
The indicator, both by day and night, being merely a mark and nothing more, which, when once seen, requires no farther attention to be paid to it; and the central light by night, and the post by day, being also merely guides to the eye; the signs of this telegraph are, in reality, composed of the combinations of two movable bodies only by day, and of two movable lights only by night, being the smallest number of parts, with which an effi cient telegraph can possibly be formed: and in this diminution of the number of combinable parts, as well as in the unity of plan, consists the superior simplicity of this telegraph, as compared with other efficient telegraphs that have been proposed.
The arms and the indicator for the day signals are made of wood, framed and pannelled, for the sake of lightness. The indicator plays in a mortise,
cut in the upper part of the post, and is let down into its horizontal, and raised into its vertical po sition, by means of a small rope, and a small pul. ley. The arms must be fixed externally, one on each side of the post, and must be exactly counter poised, by means of light frames of open iron work, which become invisible by day, at a little distance, and which, even when viewed closely, do not impair the clearness of the telegraphic signs. This pre caution is absolutely necessary, otherwise the arms will not remain in any given position, without being held by the hand, or stopped by some mechanical contrivance, which would be a very great inconve nience in the practice of signal making.
'Motion may be communicated to the telegraphic arms, by means of an endless chain, passing round, and acting upon a couple of pulleys; one of which is fixed to the arm itself, and turns upon the same pivot, whilst the other moves upon a pivot, fixed to the lower part of the post. The chain consists alternately of single and double plates of an oblong form, and rivetted together at the ends, on the principle of a watch chain. The two pullies at the top and bottom being finished with great care, per fectly equal, and having projecting teeth, or studs, fixed in a groove in each, to engage the double or open parts of the chain, the telegraphic arm above will always follow to a hair's breadth, the move ments of an index, or lever, below, attached to the lower pulley, which has a dial plate opposite to it, marked on the post, for the guidance of the opera tive signal man.
In the field, or on board ship, a leathern strap or a rope may be substituted in lieu of the chain, for the sake of economy; but as these expedients are incapable of the same accuracy as the former, the signal men, in working by them, must not trust to the indices, but must regulate the positions of the arms chiefly by the eye. The surface of the pulleys, when intended for a strap, must be moderately con vex, those for the rope moderately concave, and both should be broader than when a chain is to be used. The leathern strap requires an extra pulley of a smaller size, for pressing in one side, and tighten ing it, when the telegraph is to be used. This pul ley is fixed to a small lever attached to the middle of the post, and is thrown into action by a string.