On the Use of the Sextant and Artificial Horizon.
We do not propose in this work to trespass on the province of books on nautical astronomy. We take it for granted that every traveller using a sextant will also provide himself with an Epitome—Norie’s, Rapers, or Kerigan’s (of course the latest possible edition of either), and with the “Nautical Almanac,” which may be had for three years in advance, by persons contemplating a long journey.
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.
The wooden one of olden date, 9½in. long, 5½in. broad, 5½in. thick, weighs 5⅓lb., cubic measure 287⅓in.
Improved folding roof, &c., all iron, 8in. long, 4½in. broad, 2½in. thick, weighs 6¾lb., cubic measure 90in.
The improvements herein specified are not only its reduced size and weight, but its mechanical arrangements, form, and moderate price.
It consists of two circular disc-like reservoirs, about 2½in. in diameter, and ¾in. in depth, made of iron, at the same casting: one contains the mercury, and the other is the trough, fitted with glass cover for observing.
The discs are connected at their circumference by a narrow neck, and in it is drilled a hole, through which the mercury passes from one reservoir to the other; and this communication is opened or shut off by a stop-cock, on the cone principle, such as is used for water or gas, so that the mercury can be passed from one disc to the other without removing the glass cover, or the risk of losing any mercury.
The mercurial disc, A, is fitted with a cylinder-stopper, D, acting on a spiral spring, by which air can be admitted or allowed to escape.
The trough disc, B, is fitted with two glasses, G and E, which are ground mathematically parallel: one of the glasses is fitted to a frame, and screws on the disc, and is used while passing the mercury in or out of the trough; after which operation it is removed and replaced by the other glass, the edge of which next the stop-cock should be supported by the blade of a pocket-knife and then lowered on the mercury at the opposite side, and by a gentle pressure, force out the intervening air, leaving the glass to float on the surface of the mercury. Without this care, some of the mercury might be pressed over the edge of the disc.
The glass then presents a clear reflecting surface, which is not only protected from the effects of the wind, &c., but also maintains so great a steadiness as to mark a decided improvement over the old triangular glass roof which is placed over the mercury, instead of, as in this case, being on it.
It may be used afloat under favourable circumstances (the observer and artificial horizon being placed on a pendulum table). Another great advantage to this improved artificial horizon is the facility with which altitudes can be observed at 2° elevation, and consequently its adaptation for the measurement of very low stars, as well as the peaks of mountain ranges.
To return the mercury to its reservoir, remove the glass G that floats on the mercury, by lifting it up with the point of a knife, and then screw on the other glass cover, E. It is now only necessary to hold the instrument vertically, the trough end being uppermost (Fig. 4), turn the stop-cock, and press gently downward on the cylindrical stopper, and the mercury will rapidly return to its reservoir.
The following diagrams (half the actual size of the instrument), show the various parts of the instrument, and the method of filling and emptying the reservoir.
Fig. 1 is the instrument complete. A, the mercurial reservoir; B, the observing trough; C, the stop-cock; D, the cylindrical stop.
Fig. 2 is the instrument with the parts of the observing trough removed, which are shown above it. E, rim with glass shade; F, rim without glass shade; G, the glass that floats on the surface of the mercury.
Fig. 3. Position of the instrument while filling the observing trough.
Fig. 4. Position of the instrument while returning the mercury into its reservoir.
In moderate weather the glass G will be quite sufficient protection against the wind, but in gusty weather screw on the rim F, but it must not touch the glass G.
The glass E will protect it from any weather, taking care to level the ground on which the horizon stands.
In filling the observing trough, be careful that the glass cover, E, is screwed on tight; by pressing on the cylindrical stop D (Fig. 1) the mercury flows quickly: the trough half filled, as shown in Fig. 3, is sufficient for ordinary observations; but for very low altitudes the trough must be three-quarters filled or more as found necessary to raise glass G (Fig. 1).
Before returning the mercury into the reservoir, unscrew the short tube near the stop-cock, tapping it smartly at the same time, to shake down the globules of mercury that may remain in the tube; there is a small hole in the screw, which must be brought in sight, then turn the stop-cock, and the mercury will run rapidly into the reservoir. When about to use a sextant and artificial horizon of the common form of construction, our first care is to select a tolerably level, and, if there be wind, a sheltered spot of ground, with a clear view to the north or south, or, if the stars admit of a north and south observation, to see that the view is clear both ways. On this we place our artificial horizon, sometimes on our sextant case, sometimes on a stand (wash leather), but seldom, if we can avoid it, on the bare ground, because then the mercury, if spilled, would be difficult to gather up. The horizon roof we keep near to cover the mercury, in case wind should arise, but we never use it unless in case of necessity; then, sitting either north or south of the horizon, according to the position of the celestial object, we look with the naked eye for its reflected image in the mercury, and so seat ourselves that we can conveniently keep it steadily in view. We set the sextant nearly to zero, and look up without the telescope to the sun or star, and then, gradually moving the index forward, we bring its image down to meet the reflection in the quicksilver; then, screwing on the inverting telescope, which is the simplest and best for observation, we move the index by hand, till the contact is nearly perfect; then fasten the index by the clamping screw, and with the tangent screw complete the contact; and so long as the object is rising, by gradually turning the tangent screw we keep the images together; when they separate more slowly, and at length remain in contact for nearly half a minute, we know that the meridian altitude has been observed; we wait another minute to see them separate in the opposite direction as the body begins to descend, and then read off the observed altitude. Our illustration will sufficiently explain that the sextant is held in the left hand and the tangent screw worked with the thumb and forefinger of the right. Fig. 2 is the method recommended by Captain George: the arc is steadied by the forefinger, and the tangent screw turned by the middle finger and the thumb; a police or bull’s-eye lantern is good to read off by, and the light, of whatever nature, should be placed behind the observer, so that it may not interfere with his work, and yet may be ready for him to use when he wishes to read his altitude.