As long ago as 333 years before Christ, Alexander the Great employed mirrors to convey signals by the light of the sun. Since the time of the great warrior the idea has been reduced to a science and called " heliog-raphy." The heliostat, an instrument invented in Holland early in the eighteenth century, and the heliograph, invented by Manse in 1875, have both been used by the British army in their eastern campaigns. The instruments mentioned differ somewhat in construction, but the results are the same, no matter which instrument is used. In both signals are produced by causing a reflected ray of the sun to appear and disappear alternately at a distant point, the intervals of appearance and obscuration being carried in lengths so as to produce the combination of long and short signals known as the Morse alphabet. In these instruments the reflecting body is a glass mirror, which varies in size according to the distance to which it is desired to signal. A five-inch mirror has given under favorable atmospheric conditions distinct signals that could be read sixty miles away.

The heliograph has also been found to be of great service in defining distant points of large surveys and was used to a fine advantage in verifying the arc of the meridian by the astronomers at the Cape of Good Hope a few years ago.