The most important instrument is the sextant itself, and in the selection of this the greatest care should be used. Ebony or other wood may do for the frames of such as are to be used at sea in temperate climates, but for tropical use, even for sea service, we recommend a brass or gun-metal frame, and for observing on land no other should be used.
The quadrant, which is quite equal to the observation of altitude at sea, will take an angle of 90°, but should read up to 10° or 20° more; but the sextant, or sixth of a circle, is made to take in an angle of twice 60°, or 120°, and should also read 10° or 20° higher. When we were at the Zambesi mouth, in 1858, Dr. Livingstone’s sextant reached only 127°, while our own would read to 137° or 140°, which, when the altitude of the sun was increasing daily, gave us the advantage of observing a week or ten days longer than he could. We left the instrument in the care of the Portuguese Commandant of Tette, but have no hope of ever seeing it again, the town having been burnt and the inhabitants massacred by Landeens, Banzai, or other ferocious savages.
The sextant we generally use—and which we have tested by many years’ constant use—is brass framed, reads to 126° 56´, is of 8 inches radius, and has its arc and vernier on which the figures are engraved of gold, which has a soft lustre, exceedingly agreeable to the eye under a tropical sun, and is equally pleasant to read by lamp light; a screen of ground glass is placed before it to soften the light still more, and prevent annoying glitter and reflection; the degrees are divided into sixths of ten minutes each, and the minutes likewise into ten seconds; the microscope travels on a fixed frame, and a small milled-headed screw brings it to the figures to be read. A small lamp fixed on the axis of the index arm, with a reflector to shed the light upon the arc and vernier, is sometimes made use of.
For nearly all angular measurements that an explorer is likely to require, a really good sextant will be found sufficient, but some, for the sake of still greater power and accuracy, provide themselves with a repeating circle; but although this possesses many advantages, we doubt whether the expense of such an instrument will not place it beyond the reach of the generality of travellers, while the extra care required will constitute too great a claim upon the time of any who cannot devote themselves entirely to astronomical observations, but must, perhaps, give the greater part of their time to other avocations. The double sextant invented by Captain George, R.N., of the astronomical department of the Royal Geographical Society, described at page 27 of this work, will be found a portable and most convenient instrument for the use of explorers.
The theodolite has also many advantages, especially in taking a round of angles; but from what we have seen of it in practice we should be inclined to think that an explorer, with his sextant and compass, is more independent, and can do more than he could possibly effect with the theodolite.
The artificial horizon is, as its name imports, intended to obviate the difficulties caused by the fact that the real, or sea horizon, may be at times invisible, obscured by fog or clouds, or that the observer may be absent from it, and often far inland, where the unevenness of the earth’s surface prevents anything like a reliable real horizon being found. Of the artificial horizons used at sea we have not much to say, the unsteadiness of the vessel forbidding the use of any instrument that can be disturbed by motion. The best we have seen is Captain Becher’s pendulum horizon, a little frame swinging near the object-glass of the sextant, and carrying a couple of horizontal wires, so arranged that when they appear in one to the eye of the observer they ought to be on a level with the horizon and parallel with it; a small lamp is so fitted to the sextant as to render these wires visible at night if the altitude of the moon or a star is required; but we do not think the latitude deduced from such an observation can be more than approximately true.
It is on shore, and most of all in the far interior of the great continents of the world, that the artificial horizon is most needed, and that it renders the truest service to the explorer; and therefore it is of the greatest importance that the instrument should be at once simple in construction, easy of management, not easily put out of order, and, above all, perfectly reliable in the result obtained from it. The first requisite is a perfectly flat and horizontal reflecting surface, in which, when the observer looks down upon it, the image of the sun or star may be distinctly seen. Now, it is easy to find flatness: a disc of silvered glass, or polished metal, or even a bit of crown glass, painted black on the under side, or a common round shaving-glass would do if this only were required; but this flat surface must also be perfectly horizontal, and to attain this various arrangements of tangent screws and spirit levels have been invented, all of which require great care in levelling, and have the defect that the slightest accidental touch while they are in use may alter the level, and so vitiate the observation. By common consent, therefore, observers almost universally trust to fluid mirrors, which must be perfectly level if they are sufficiently quiescent to reflect a perfect image. Water, darkened with any colouring matter—ink; water with a little treacle, to render it less liable to be agitated by the wind, or thin tar will do; but all these have disadvantages which render them only fit to be looked upon as substitutes when mercury cannot be obtained; in fact, long ago we were told by the late Captain Washington, Hydrographer to the Admiralty, to use nothing but mercury.
The horizon trough, as it is called, is simply a block of wood of oblong form, about 6in. long, 4in. wide, and 1in. thick; this is hollowed to the depth of about ⅜in. or ½in., leaving a sufficient rim to retain the mercury which is poured into it. Sometimes a hole is pierced in the rim, and is continued in the solid wood under the hollow, so that the mercury, being poured into a small funnel fitted in the rim, runs underneath and rises like a fountain in the centre of the trough. The various arrangements of this kind and others more complicated are called fountain horizons, but they are not really necessary, their principal object being to insure the perfect purity of the mercurial surface by forcing it to flow downward first through the funnel, and so to leave the scum behind; but this object may be just as well attained by inverting the bottle, so that all the impurities may float upon the surface, and allowing the pure mercury to run through the perforated stopper into the trough. The bottle is generally of iron, and has the perforated stopper already mentioned, which, when the mercury has to be returned to it, serves also as a funnel. Wooden bottles are also made, but no traveller ought to depend upon them, as in hot climates they shrink and split; and we have found in Namaqualand all our mercury adrift in a tinned box, forming an amalgam which did not at all improve it. We have therefore, for many years, kept it in a common stoneware ink bottle, with a bit of washleather tied over the cork, and have found this to answer admirably; in pouring out the mercury, having removed the cork, we stop the mouth of the bottle with the forefinger, completely invert it, and then, slightly moving the finger, leave an opening sufficient for a stream of pure mercury to flow into the horizon. Our trough is round, and about 4in. across, which is quite large enough. In perfectly calm weather we prefer to observe on the plain surface of the uncovered mercury; but if wind comes on, as it often does in Africa and Australia about noon, we cover it with a roof of the usual form, i.e., two small panes of glass fixed in a frame so as to form an angle of 45° each with the horizon, or 90° with each other, and, standing like a roof over the mercury, allow the rays from the heavenly body to pass down to it, and be reflected to the eye of the observer. Various methods of rendering this roof as portable as possible have been tried; our own is figured in the first chapter of this work.
Captain George’s new artificial horizon, however, bids fair to supersede all the old forms of arrangement, as its portability, strength, and simplicity of adjustment stand unrivalled. Captain George’s horizon may be made sufficiently large to equal the surface of the one now used; but the portable or pocket form here alluded to is of the following dimensions:—
Self-replenishing, 6in. long, 2½in. broad, ¾in. thick, weighs 1¼lb., cubic measure 11¼in.