3. _Night Communication upon the Entire Horizon._--When we wish to correspond at a short distance, say two miles, and make signals visible from the entire horizon, the mirror, A, is put in place, so that it shall reflect the luminous fascicle vertically. The fascicle, at a distance of about fifty feet, meets a white balloon which it renders visible from every point in the horizon. The maneuver of the occultator brings the balloon out of darkness or plunges it thereinto again, and thus produces the signs necessary for aerial communication. [Illustration: FIG. 24.] These ingenious arrangements, which depend upon the state of the atmosphere, do not appear to have been imitated outside of the navy. CAPT. GAUMET'S OPTICAL TELEGRAPH. The system of optical communication proposed by Capt. Gaumet, and which he names the _Telelogue_, is based upon the visibility of colored or luminous objects, and upon the possibility of piercing the opaque curtain formed by the atmosphere between the observer's eye and a signal, by utilizing the difference in brightness that exists between such objects and the atmosphere. It is a question, then, of giving such difference in intensity its maximum of brightness. To do this, Capt. Gaumet proposes to employ silvered signals upon a black background. He uses the simple letters of the alphabet, but changes their value. His apparatus has the form of a large album glued at the back to a sloping desk. Each silvered letter, glued to a piece of black cloth, is seen in relief upon the open register. A sort of index along the side, as in commercial blank-books, permits of quickly finding any letter at will. Such is the manipulator of the apparatus. The receiver consists of a spy-glass affixed to the board that carries the register. For a range of two and a half miles, the complete apparatus, with a 12x16 inch manipulator and telescope, weighs but four and a half pounds. For double this range, with a 20x28 inch manipulator and telescope, the total weight is thirteen pounds. The larger apparatus, according to the inventor, have a range of seven miles. For night work the manipulator is lighted either by one lamp, or by two lamps with reflector, placed laterally against it. This apparatus, although well known, and having been publicly experimented with, has not, to our knowledge, been applied practically. From a military standpoint, its short range will evidently not permit it to compete with optical telegraphic apparatus, properly so called. Perhaps it might rather be of service for private communications between localities not very far apart, since it costs but little and is easily operated. OPTICAL SIGNALING BETWEEN BODIES OF TROOPS. Optical communications by signals, during day and night, with experienced men, may, in the absence of telephones, telegraphs, and messengers, render important service when the distance involved is greater than two thousand feet.