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THE HELIOGRAPH.

THE introduction of the heliograph for army and navy signalling adds one more to the many applications of science to war that have been made of late years. The news of the taking of Ali Musjid, in the Khyber Pass, was first conveyed to the Peshawur garrison by sunshine signals made from the fallen fortress itself, while the Commandant of Fort Ekowe, in Zululand, was enabled by the same means to transmit a daily account of the besieged garrison under his orders, to Lord Chelmsford's forces on the Lower Tugela. Colonel Pearson, at Ekowe, strange to say, was unprovided with a properly constructed instrument, and had to do his signalling with an ordinary mirror; and it was not until Lord.Chelmsford's final advance, that the army was thoroughly equipped with proper apparatus. In future, however, the heliograph will be considered as important an item of army equipment as the rifle and the spade, and our army signallers will be provided with the mirror apparatus as a matter of course.

The Mance heliograph-for we are indebted for this simple and efficient instrument to Mr. Henry C. Mance, of the Government Persian Gulf Telegraph Department-was brought to the attention of our Government, or rather that of India, in 1869. Mr. Mance did not, of course, pretend to have invented signalling by sunshine, but only to have devised an instrument whereby such signals could be made use of in a simple and intelligible manner. Indeed, it would be hard to say who was the first to make use of the sun for such a purpose. It is said that the fleet of Alexander the Great was guided along the Persian Gulf by means of mirrors, while by the Indian tribes of North America, the mirror is an implement that has been frequently used for the conveyance of news and intelligence. Mr. Galton, in his "Notes on Travel," alludes to several instances of the kind that came under his notice, and it is but two years since the New York Daily Graphic gave an account of the Nez Percés tribe, by whom the mirror was employed, as smoke has been, as a means of visual telegraphing. The Nez Percés Indians were recently captured by the United States forces and confined in a camp near Leavensworth, where they were permitted to live pretty well as they chose, and where, we believe, they are still located. The chief of the Nez Percés, according to the New

York Daily Graphic, carried with him a looking-glass, "used to direct military manoeuvres in battle by means of reflected rays of light. These various significations, however," explained the American organ, "have never yet been found out by the white man." The light signals employed were likely to have been of the most simple character, but that they were thoroughly understood we can have little doubt.

In the Crimean War there is ample proof that sunshine telegraphy was often used by the Russians. There was a signalling station in Sebastopol itself, of which our officers were perfectly aware, and in the Times of July 11th, 1855, we find in the correspondence from the seat of war, A long train of provisions came into Sebastopol to-day, and the mirror telegraph, which works by flashes from a mound over the Belbeck, was exceedingly busy all the forenoon."

In Spain signalling by sunshine has long been resorted to with success, and at the present moment there is permanent heliographic communication established between Africa and Europe across the Mediterranean. Tangiers, Tarifa, Ceuta, and Algeçiras are all in communication with one another by sunshine signals— a matter of the more importance since great difficulties have occurred in maintaining the telegraph cables in the Straits of Gibraltar in working order. When Admiral Sheriff was stationed at Gibraltar in 1835, he was enabled to connect "the Rock" with Tangiers by a system of optical telegraphy. Admiral Sheriff had a friend at Tangiers, on the African shore, and arranged with the latter a special code of signals. In this case the flashes of light were made with the aid of an ordinary toilet looking-glass, but they were so bright and vivid, that the observer at Tangiers had no difficulty in reading Admiral Sheriff's signals, and instantly answering the same across the sea, a distance of some twenty miles.

Before proceeding to an explanation of the Mance heliograph, and the code of signals that our soldiers employ, it will be well to say a word upon the character of reflected sun rays and the distance at which these can be seen. In the first place, we may remark that the intensity of these sunshine signals can scarcely be imagined by any one who has not seen the heliograph at work, and the distance to which reflected rays may be sent, could suitable stations be provided, is practically unlimited. But two conditions are imperative to the proper working of the signals; these are sunshine and the absence of obstacles along the path

of the rays. For this reason it is that tropical and semi-tropical countries are more favourable to the system than are lands like England, which lie in the temperate zone. Given a bright sun, and sunshine signals may be read without difficulty at a considerable distance. The mirror of the heliograph simply turns aside the sun's rays, or changes their direction, and how far they will travel the philosopher will scarcely take upon himself to say. In the case of the moon, we know, for instance, that we have the sun's rays reflected to us from a distance of 232,000 miles, the moon being nothing more than a huge mirror. But a practical idea may be obtained of the vivid character of reflected sun signals by watching the windows of a house at sundown. Any one who has seen a casement thus glittering in the sun will readily understand how distinctly sunshine signals can be read. One sees the burning spot for miles away, the most striking object in the whole landscape. No wonder, then, that the Indian officers, to whom the heliograph was submitted by its inventor, reported that signals "can be easily read in ordinary weather without telescopes up to fifty miles." The distance at which signals may be seen varies naturally with the size of the mirrors employed, but a ten-inch glass, it may be mentioned, was that experimented with in India, and to which this report refers.

The Mance heliograph, it seems, was submitted to the war authorities both in this country and abroad at the same time, and fortunate it is that the Indian Army enjoys a certain amount of independence; for while the latter looked upon the instrument with favour, the Home Government would have nothing to do with it. So that if the Horse Guards had been permitted to have their way, we should never have had the benefit of the instrument in the Afghan War, and, by consequence, in the Zulu campaign neither. Thanks, however,

mainly to Colonel Roberts (now General Roberts, who commanded the Kurrum Valley Force) the heliograph was enrolled among our implements of warfare, that officer having recommended that "six or eight instruments be made and stored up with the telegraph train at Roorkee, some men of the Sappers and Miners being kept constantly practised in the use of the instruments." It was these instruments and these practised sappers and miners which stood our army in such good stead when it recently advanced towards Cabul. General Donald Stewart, in command of the Candahar force, maintained tele.

graphic communication by means of the heliograph from the Khojak Pass to Girishk; General Roberts, from Khost, flashed his messages to the fort at Bannu, a distance of sixty miles; and Sir Samuel Browne maintained communication with his base at Peshawur from Jellalabad, through the whole length of the Khyber Pass.

The Mance heliograph is a tripod stand which supports a movable mirror. The mirror is in a frame that permits of a swinging horizontal action, like every toilet looking-glass. But not only can the mirror be moved in a horizontal manner; it can also be twisted sideways in a vertical fashion, the top and bottom turning on a pivot. Under these circumstances it is possible to reflect the sun in any direction, except in that precisely opposite to the luminary, and in this case the difficulty is easily obviated by using a second mirror. The second mirror reflects the rays on to the first, and the first then throws them in the direction required. As in the case of a firearm, it is obviously necessary to aim the flash straight, but this is not so troublesome as may at first sight appear. The horizontal and vertical movements of the mirror permit of its facile adjustment, and in sighting, the observer gets behind the mirror and looks through it. A small portion of quicksilver is removed from the centre of the mirror to enable the observer to do this. The eye, by means of this orifice, is able to look towards the station with which it is desired to communicate, and a sighting rod set up about ten yards in front of the mirror, in the proper alignment, materially aids the observer. A metal stud, answering to the sight of a rifle, is slid upwards or downwards on the rod during the sighting operation, until the centre of mirror (where the observer's eye is), the stud, and the distant station are in a line. The mirror is now reflecting a ray of light to the distant station, and the signaller may rest assured that as long as his reflection strikes the stud in front, the reflection will be seen at the distant station. As the sun gradually works round, it is true, the position of the mirror must be slightly altered, but this offers no difficulty, and, after a proper aim has been taken, the only precaution that the signaller need attend to is to see that every time he reflects, the reflection strikes the stud or sight in front. At the distant station itself the sentinel sees a succession of bright star-like flashes every time the mirror reflects in his direction, or, if the mirror is at rest, then the signal has the appearance of a fixed star.

The way to signal a message is now easy to understand. The

so-called Morse alphabet is used, which is the common language of the electric telegraph. In the Morse alphabet the letters are made up of dots and dashes, or short signals and long signals. Thus, if we make upon paper a short stroke, and another stroke three times as long, we get the letter A, while a long stroke followed by a short stroke signifies N. The short stroke is termed a dot, the long stroke a dash. B is represented by a dash followed by three dots, and C by dash, dot, dash, dot. E, the letter most frequently used, is represented in the Morse code by a single dot, while a single dash stands for T, and so on. Το adopt such a code to sunshine signalling is not difficult. To intimate a dot or short signal, it is necessary merely to reflect the light for a short period, say a second, while a dash is shown by a longer signal, a reflection for the period of three seconds. Thus, if the sentinel sees a momentary flash, he regards it as a dot, and one of longer duration he reads as a dash. In this way he spells out any sentence that may be sent him, and according to experience gained in the Afghan and Zulu campaigns, army signallers would appear to be in a position to transmit from ten to twelve words a minute. The sending of the message, which thus consists of long and short flashes, is brought about by depressing a key attached to the heliograph. The depression of the key slightly alters the inclination of the mirror; pressing the key for a long time or a short time produces long or short flashes.

Colonel Pearson, at Fort Ekowe, who employed the mirror for communicating across an enemy's country to the British troops on the Tugela, in the same way as the French maintained correspondence with besieged Paris by means of balloons and homing pigeons, was not, it has been said, provided with the Mance heliograph, neither had he signallers with him familiar with its plan of working. How, then, we may well ask, could he understand and reply to the flashes that came into his camp from friends many miles away. The answer is, that he could not do so for a long time. The little garrison watched the star-like flashes hour after hour before it comprehended that the bright reflections were meant as signals. At last, the sentinels mounted in the tower of the missionary church at Ekowe, seeing the light appear and disappear continually, concluded that it must be Lord Chelmsford's force signalling to them, and the next thing to be done was to try to interpret the signals. Fortunately, it is a rule in the British Army to send to the School of Military Engineering, at Chatham, men from every regiment to be in