SIGNALS.
Colomb and Bolton's flashing signals, adopted in our Army and Navy, and used in many other countries as well, are eminently suited to the wants of an expedition. Anything may be used for signalling, that appears and disappears, like a lantern, or an opened and closed umbrella, or that moves, as a waved flag or a person walking to and fro on the crest of a hill against the sky. Sound also can be employed, as long and short whistles. Their use can be thoroughly taught in two hours, and however small the practice of the operators, communication, though slow, is fairly accurate, while in practised hands its rapidity is astonishing. The proportion of time occupied by the flashes and intervals is as follows. (I extract all the rest of the article from the pamphlet published by the inventors of the system.)
Flashing Signals, with Flags.--Supposing the short flash to be half a second in duration, the long flash should be fully a second and a half. The interval between the flashes forming a figure should be equal to a short flash, and the interval between two figures should be equal to a long flash. After the last figure of the signal is finished, there should be a pause equal to at least one-third of the time taken up by the figures. After this pause, the signal should be again repeated with the same measured flashes and intervals, and so continued until answered by all to whom it is addressed.
Care must be taken never to commence a fresh signal before the answers to the last have ceased; and signals are never to be answered until their repetitions have been observed a sufficient number of times to make an error impossible.
The signalman may work from left to right, or from right to left, as shown in figs. 1 and 2, according to convenience and the direction of the wind. To make a short flash, the flag is waved from a to b, and back to the normal position a. To make a long flash, the flag is waved from a to c, and back to the normal position a.
The numerals 1 to 5 are, therefore, denoted by one to five waves of the flag from a to b, recovering to a.
The numeral 6 by a wave from a to c, recovering to a. The numeral 7 by a wave from a to b, back to a, and then to c, recovering to the normal position a. The numeral 8 is denoted by a wave from a to c, back to a, and then to b, recovering to the normal position a. The numeral 9 is denoted by two waves from a to b, and one from a to c. The numeral 0 by one wave from a to c, recovering again to a, and then two waves from a to b. The other signs are made in the same manner, so that a short motion shall always represent a short flash, and a long motion a long flash.
On the completion of the motions required for each sign, the flag must always be brought to the position a. When the word, or group of figures, is completed, the flag may be lowered in front of the body.
In receiving a message, the flag should always be kept in the position a, except when answering.
In waving the flag, the point of the staff should be made to describe a figure of 8 in the air to keep the flag clear.
Each signal party must consist of not less than two men, whose duties will be as follows:--
In receiving messages: No. 1 works the flag for answering, etc., and refers to the code for the interpretation of the numbers received, and calls out the words to No. 2. No. 2 fixes the telescope and reads from the distant station, calling out the numbers as they are made for the information of No. 1, and writes down the numbers and meaning thereof.
Suppose station "A" in communication with station "B":--No. 1 at "A" on being told by No. 2 that "B" is about to send a message, takes up his position at attention, holding the flag over the left arm and under the right, or vice versa across his body, according to the wind, with the code book in his hand. No. 2 fixes his eyes on the glass, and on receiving the numbers from "B" calls them out to No. 1, who ascertains their meaning from the code, and gives the words to No. 2, who writes them down in his book, and then placing his eye to the glass, tells No. 1 to make the answer. No. 2 does not, however, direct the answer to be made until he is sure of the correctness of the signal received.
Flashing Alphabet, for Use without a Code.--The following alphabet, etc., can be used under circumstances when it is not convenient or possible to have recourse to the Signal Book, and forms in itself a perfect telegraphic system, necessarily somewhat slow in its application, but having the great advantage of requiring very little previous knowledge and practice to work with correctness. The symbols and numbers expressing the alphabet are identical with those forming the alphabet in the Signal Book.
All particulars as to the machines and lanterns used in the Service, for making these flashing signals, and the code, can be procured at W. Nunn and Co.'s Army and Navy Lamp and Signal Works, 65, George Street East, London, E.
Reflecting the Sun with a Mirror.--To attract the notice of a division of your party, five or even ten miles off, glitter a bit of looking-glass in the sun, throwing its flash towards where you expect them to be. It is quite astonishing at how great a distance the gleam of the glass will catch the sharp eyes of a bushman who has learnt to know what it is. It is now a common signal in the North American prairies. (Sullivan.) It should be recollected that a passing flash has far less briliancy than one that dwells for an appreciable time on the retina of the observer; therefore the signaller should do all he can to steady his aim. I find the steadiest way of holding the mirror is to rest the hand firmly against the forehead, and to keep the eyes continually fixed upon the same distant object. The glare of the sun that is reflected from each point of the surface of a mirror forms a cone of light whose vertical angle is constant, and equal to that subtended by the sun. Hence when a flash is sent to a distant place, the size of the mirror is of no appreciable importance in affecting the size of the area over which the flash is visible. That area is the section of the fasciculus of cones that proceed from each point of the mirror, which, in the case we have supposed, differs immaterially from the cone reflected from a single point. Hence, if a man watches the play of the flash from his mirror upon a very near object, it will appear to him of the shape and size of the mirror; but as he retreats from the object, the edges of the flash become rounded, and very soon the flash appears a perfect circle, of precisely the same apparent diameter as the disc of the sun: it will, in short, look just like a very faint sun. The signaller has to cause this disc of light to cover the person whose notice he wishes to attract. I will proceed to show how he can do so; but in the mean time it will be evident that a pretty careful aim is requisite, or he will fail in his object. The steadiness of his aim must be just twice as accurate, neither more nor less, as would suffice to point a rifle at the sun when it was sufficiently obscured by a cloud to bear being looked at: for the object of the aim is of the same apparent size, but a movement of a mirror causes the ray reflected from it to move through a double angle.
The power of these sun-signals is extraordinarily great. The result of several experiments that I made in England showed that the smallest mirror visible under atmospheric conditions such that the signaller's station was discernible, but dim, subtended an angle of only one-tenth of a second of a degree. It is very important that the mirror should be of truly plane and parallel glass, such as instrument-makers procure; the index glass of a full-sized sextant is very suitable for this purpose: there is a loss of power when there is any imperfection in the glass. A plane mirror only three inches across, reflects as much of the sun as a globe of 120 feet diameter; it looks like a dazzling star at ten miles' distance.
To direct the flash of the Mirror.--There are makeshift ways of directing the flash of the mirror; as, by observing its play on an object some paces off, nearly in line with the station it is wished to communicate with. In doing this, two cautions are requisite: first, the distance of the object must be so large compared to the diameter of the mirror that the play of the flash shall appear truly circular and exactly like a faint sun (see preceding paragraph): secondly, be careful to bring the eye to the very edge of the mirror; there should be as little "dispart" as possible, as artillerymen would say. Unless these cautions be attended to very strictly, the flash will never be seen at the distant station.
An object, in reality of a white colour but apparently dark, owing to its being shaded, shows the play of a mirror's flash better than any other. The play of a flash, sent through an open window, on the walls of a room, can be seen at upwards of 100 yards. It is a good object by which to adjust my hand heliostat, which I describe below. Two bits of paper and a couple of sticks, arranged as in the drawing, serve pretty well to direct a flash. Sight the distant object through the holes in the two bits of paper, A and B, at the ends of the horizontal stick; and when you are satisfied that the stick is properly adjusted and quite steady, take your mirror and throw the shadow of A upon B, and further endeavour to throw the white speck in the shadow of A, corresponding to its pin-hole in it, through the centre of the hole in B. Every now and then lay the mirror aside, and bend down to see that A B continues to be properly adjusted.
Hand Heliostat.--Some years ago, I took great pains to contrive a convenient pocket instrument, by which a traveller should be able to signal with the sun, and direct his flash with certainty, in whatever direction he desired. I did so in the belief that a signalling power of extraordinary intensity could thus be made use of; and, I am glad to say, I succeeded in my attempt. I at last obtained a pretty pocket instrument, the design of which I placed in the hands of Messrs. Troughton and Simms; and upon the earlier models of which I read a paper before the British Association in 1858. I called it a "hand heliostat." I always carry one when I travel, for it is a continual source of amusement. The instrument is shown in fig. 1 (p. 280), and its principle is illustrated by fig. 2. The scale is about 2/3.
E is the eye of the signaller; M the mirror; and L, S, fig. 2, a tube containing at one end, L, a lens, and at the other, S, a screen of white porcelain or unpolished ivory, placed at the exact solar focus of L: a shade, K, with two holes in it, is placed before L. Let R, r, be portions of a large pencil of parallel rays, proceeding from any one point on the sun's surface, and reflected from the mirror, as R' r' (fig. 2). R' impinges upon the lens, L, through one of the holes in K, and R' goes free toward some distant point, O. Those that impinge on the lens will be brought to a focus on S, where a bright speck of light might be seen. This speck radiates light in all direction; some of the rays, proceeding from it, impinge on the lens at the other hole in the shade K, as shown in fig. 2, and are reduced by its agency to parallelism with r' and R', that is, with the rays that originally left the mirror: consequently E, looking partly at the edge of the lens, and partly into space, sees a bright speck of light in the former, coincident with the point O in the latter.
What is true for one point in the sun's disc, is true for every point in it. Accordingly, the signaller sees an image of the sun, and not a mere speck of light, in the lens; and the part of the landscape which that image appears to overlay, is precisely that part of it over which the flash from his mirror extends; or, in other words, it is that from any point of which a distant spectator may see some part or other of the sun's disc reflected in the mirror. There is no difficulty in signalling when the sun is far behind the back, if the eye-tubes are made to pull out to a total length of five inches, otherwise the shadow of the head interferes. For want of space, the drawing represents the tubes as only partly drawn out. The instrument is perfectly easy to manage, and letters can be signalled by flashes. Its power is perfectly marvellous. On a day so hazy that colours on the largest scale--such as green fields and white houses--are barely distinguishable at seven miles' distance, a looking-glass no larger than the finger-nail transmits its signals clearly visible to the naked eye.
I have made a makeshift arrangement on the principle of my heliostat, using the object glass of an opera-glass for the lens, and an ordinary looking-glass: the great size and short focus of the object glass is a great convenience when using a mirror with a wide frame.
Professor W. H. Miller, the Foreign Secretary of the Royal Society, has since invented a yet more compact method of directing the flash, which he has described in the Proceedings of the Royal Society for 1865. It consists of a plate of silvered glass, one of whose rectangular corners is accurately ground and polished. On looking into the corner when the glass is properly held an image of the sun is seen, which overlays the actual flash. Beautifully simple as this instrument is, I do not like it so much as my own, for the very fact of its requiring no "setting" is its drawback. With mine, when the image of the sun is lost it is immediately found again by simply rotating the instrument on its axis; but with Professor Miller's the image must be felt for wholly anew.
Fire Signals.--Fire-beacons, hanging up a lantern, or setting fire to an old nest high up in a tree, serve as night-signals; but they are never to be depended on without previous concert, as bushes and undulations of the ground will often hide them entirely. The sparks from a well-struck flint and steel can be seen for much more than a mile.
Smoke Signals.--The smoke of fires is seen very far by day; and green wood and rotten wood make the most smoke. It is best to make two fires 100 yards apart, lest your signalling should be mistaken for an ordinary fire in the bush. These double fires are a very common signal to vessels in the offing, on the African coast.
Other Signals.--By Sight.--A common signal for a distant scout is, that he should ride or walk round and round in a circle from right to left, or else in one from left to right.
Mr. Parkyns, speaking of Abyssinia, describes the habits of a caste of robbers in the following words:--"At other times they will lie concealed near a road, with scouts in every direction on the look-out; yet no one venturing to speak, but only making known by signs what he may have to communicate to his companions or leader. Thus he will point to his ear and foot on hearing footsteps, to his eyes on seeing persons approach, or to his tongue if voices be audible; and will also indicate on his fingers the numbers of those coming, describing also many particulars as to how many porters, beasts of burden or for riding, there may be with the party."
A kite has been suggested as a day signal; and also a kite with some kind of squib, let off by a slow-light and attached to its tail, as one by night. (Colonel Jackson.)
Sound.--Whistling through the fingers can be heard at considerable distances: the accomplishment should be learnt. Cooing in the Australian fashion, or jvdling in that of the Swiss, are both of them heard a long way. The united holloa of many voices, is heard much further than separate cries. The cracking of a whip has a very penetrating sound.
Smells.--An abominable smell arrests the attention at night.
Letters carried by Animals.--In short reconnoitring expeditions made by a small detachment from a party, the cattle or dogs are often wild, and run home to their comrades on the first opportunity; in the event of not being able to watch them, owing to accident or other cause, advantage may be taken of their restlessness, by tying a note to one of their necks, and letting them go and serve as postmen, or rather as carrier-pigeons.