Part 25 (1/2)
To direct the flash of the Mirror.--There are makes.h.i.+ft 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.
[Sketch of arrangement as described below].
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 a.s.sociation in 1858. I called it a ”hand heliostat.” I always carry one when I travel, for it is a continual source of amus.e.m.e.nt. The instrument is shown in fig. 1 (p. 280), and its principle is ill.u.s.trated 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 s.p.a.ce, sees a bright speck of light in the former, coincident with the point O in the latter.
[Fig 1 and Fig 2, as described in the text].
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 s.p.a.ce, 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-gla.s.s no larger than the finger-nail transmits its signals clearly visible to the naked eye.
I have made a makes.h.i.+ft arrangement on the principle of my heliostat, using the object gla.s.s of an opera-gla.s.s for the lens, and an ordinary looking-gla.s.s: the great size and short focus of the object gla.s.s 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 gla.s.s, one of whose rectangular corners is accurately ground and polished. On looking into the corner when the gla.s.s 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 fas.h.i.+on, 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.
BEARINGS BY COMPa.s.s, SUN, ETC.
Pocket Compa.s.s.--A pocket compa.s.s should not be too small; if one of the little toy compa.s.ses be carried in the pocket, it should be as a reserve, and not for regular use. A toy compa.s.s will of course tell N. from N.N.E., and the like; and that may be very useful information, but the traveller will find that he constantly needs more precise directions. He doubts the ident.i.ty of some hill or the destination of some path, and finds on referring to his map, that the difference of bearing upon which he must base his conclusion, is small: he therefore requires a good sized compa.s.s, to determine the bearing with certainty. One from 1 1/2 to 2 inches in diameter is practically the best. It should have plenty of depth, so that the card may traverse freely, even when the instrument is inclined: it should be light in weight, that it may not be easily jarred by a blow; the catch that relieves the card, when the instrument is closed, should be self-acting and should act well: lastly the movements of the needle should be quick; one that makes slow oscillations should be peremptorily refused, whatever its other merits may be: the graduation of the degrees on the card should be from 0 degrees to 360 degrees, North being 0 degrees and East 90 degrees. I wish some optician would make aluminum cards. The material can be procured as foil, like tinfoil. It can then be stamped and embossed, in which case it retains its shape perfectly, but I cannot satisfy myself as to a good pattern, nor do I see how to make the North and South halves of the disc sufficiently different in appearance.
Compa.s.s for use at night.--The great majority of compa.s.ses are well-nigh useless in the dark, that is, when it is most important to be able to consult them. They are rarely so constructed, that the difference between the north and south sides is visible by moonlight or by the light of a cigar or piece of tinder. The more modern contrivances are very effective; in these the southern half of the compa.s.s card is painted black, the northern being left white. With a very faint light, this difference can be appreciated. In compa.s.ses consisting simply of a needle, the north end of the needle should have a conspicuous arrow-head.
It is extraordinary how much the power of seeing a compa.s.s or a watch at night is increased by looking nearly at it through a magnifying-gla.s.s.
Thus, young people who can focus their vision through a wide range may be observed poring with their eyes close to their books when the light wanes. So again, at night-time, a placard, even in large type, is illegible at a short distance, but easily read on approaching it. It seems, in order that a faint image on the retina should be appreciated by the nerves of sight, that image must have considerable extent.
Moonlight or the light of a cigar may be condensed on the compa.s.s by a burning gla.s.s, or other subst.i.tute for it. (See ”Burning Gla.s.s.”)