When a rope is used, three turns of it arc taken round each pulley, hauling it tight at the same time, alter which the two ends, being previously prepar pared with thimbles, or eycsplices, arc brought to wards each other, and made fast, by a lanniard, or smaller rope, passing through the eyes.
When the strap or rope is used, the lower pulley instead of having one short lever only, serving as an index, may have four such levers, so as to re semble a small windlass.
At the end of each arm, two light pieces of iron, meet in an angle of 45 degrees, forming an open triangle, to the vertex of which the movable lantern L is attached, by means of a pin. A cylindrical weight w must be fixed at the same time to the end of the iron counterpoise, to restore the proper equilibrium of the arms, which is, of course, de ranged by the addition of the lantern. As the lan terns and weights, and in short, every addition ne cessary for exhibiting the nocturnal signals, are fixed at dusk, and removed by daylight, it becomes necessary, at permanent stations, that the roof of the signal house, over which the telegraph stands, shall be formed with a small flat terrace, accessible by means of a ladder or staircase.
In the intermediate stations of a permanent tele graphic line on shore, two lanterns are required to do the duty of the centre light, one on each side of the telegraphic post, because one lantern can, of course, be seen in one direction only, owing to an intervention of the post. These two, as well as the two movable lanterns, are fixed externally, at a sufficient distance from the plane of the arms, to prevent them from striking, as in Fig. 20, in which e o are the central lanterns, L 1. the movable lan terns, and w w the weights, added to counterpoise them.
The indicator light I may either be fixed to a separate post, as represented in Fig. 18, or it may be attached to a rod r, strengthened by a brace b, and guy ropes g g, as in Fig. 19, which is an ele vation of the universal telegraph, fitted up for night signals, on a scale larger than that of the former explanatory figures. The apparatus now alluded to, having only one lantern to support, may be made extremely light. The end of the rod drops into a small open mortise at the head of the post, and has a semicircular groove on its lower surface, which is engaged by a horizontal bolt, driven through the sides of the post. A small rope fixed
to the end of the rod, but omitted in Fig. 19, for the sake of clearness; is made fast to a cleat upon the post below, to prevent the rod from moving. The foot of the brace is secured to the post by a plate and stud.
This apparatus, which entirely depends upon the telegraphic post, and turns with it, may be fixed, or disengaged, in a moment, and is peculiarly adapted for ships, and for field service, in which the length of the telegraphic arm does not exceed from five to six feet. But at permanent stations on shore, where larger telegraphs would probably be used, the apparatus for supporting the indicator lamp should be a permanent fixture, to save the trouble of continually shipping and unshipping it. At such stations, if the signals were required to be made in various lines or directions, the pole for supporting the indicator lamp should be fixed to the post at bottom, so as to stand out from it obliquely, like a ship's bowsprit, with lifts, or ropes, to sup port it, leading to the top of the post, and a couple of guys to secure it from lateral motion. Hence one oblique spar only would be used, instead of the two pieces (namely, the rod and brace) before de scribed. But as there may be many stations in a telegraphic establishment on shore, in which the signals require to be exhibited in one invariable line only, at all such stations, the indicator lantern should be fixed to its own separate post, which may either be placed vertically (as in Fig. 18), or obliquely, as may be considered most expedient.
Lamps for burning oil have recently been brought to such perfection, that a light of sufficient inten sity, for any distance suitable for telegraphic pur poses, may easily be obtained. In regard to form, if night telegraphs be adopted on shore, square lamps like those of mail coaches, but having the two glass sides opposite to each other, so as to show light in two directions only, would be the most pro per. But for sea service the pattern called the "globe lamp," which has of late been generally adopted to the Royal Navy, in lieu of their former signal lanterns, appears to be decidedly the best. In this, the light is exhibited in every direction through a very strong globular glass, to which are fitted a copper top and bottom, pierced with air holes.