THE HELIOGRAPH (sun-writer) is an instru ment designed for signaling by reflected sun flashes. The United States Army 'field kit' contains two 4-inch mirrors, two tripods, a ment to a post, stump, or other firm base instead of tripods. In setting up and the posi tion of the sun is the guide for determining whether one or two mirrors should be used. \Then the sun is in front of the operator, that is, in front of a plane through his position, at right angles to the line joining the stations, the sun mirror only is required ; with the sun in rear of this plane both mirrors should be used, although a single mirror may be used to advantage with the sun well back of the operator. In the former case the rays of the sun are reflected from the sun mirror di rect to the distant station; in the latter they are reflected from the sun mirror to the station mirror, thence to the distant observer. Under fa vorable atmospheric conditions the range of the heliograph is great. The greatest ranges (100 to 125 miles) ever attained with this instrument are credited to the United States Army during the course of experi ments in April and May, 1'800, in Arizona and New Mexico, during which, by using intermediate stations, communication was maintained con nectedly for about two weeks between points 2000 miles apart.
Other signaling devices are used in addition to the standard apparatus above described as occasion justifies, among which are disks (single or double) made of white canvas stretched on rings or hoops of wire and attached to a light staff, and semaphores consisting of a post with arms movable by ropes, each position represent ing a letter or number. These may be used at
night by attaching lanterns to the arms.
Signal flags on halliards, stationary or by motions, are also frequently used, as is discussed under SIGNALS, MARINE.
At night signals may be made by candle bombs, which are pasteboard shells charged with bril liant stars, fired from bomb guns or mortars or signal rockets, which under favorable circum stances can he used up to ranges of about eight miles. Rockets are most efficiently employed as c/n•onoscmic or time interval signals. Signal composition hires are pyrotechnic compositions which burn with great intensity of light and color, generally red, white, and green. To observe all these visual signals it is necessary to employ powerful and portable telescopes. The nal telescopes for use at long ranges magnify about 30 times and have a focal length of 26 inches. The glass is strong. Binocular glasses are also useful, as they combine a low magnifying power with a large field. The new 'porro prism' glasses are now issued to the United States Signal Corps.