Scientific Instruments.
Instruments for mapping the route.
If the traveller aims at exploring and approximately mapping the country he passes through, astronomical instruments are indispensable, and of these none are more useful than the compass; the sextant, with artificial horizon; the note book, conveniently ruled, for recording observations; and the protractor, the scale, and the dividers, for laying them down upon the map. If merely a pleasure excursion in a sufficiently known country is contemplated, a pocket compass will be all that is needed; and even that is dispensed with by hunters and traders, who push farther every year into the wilderness without fear of either mistaking their way or being unable to return upon their own tracks.
A great amount of detail may be filled in with the following simple outfit:—A pocket compass, not only showing the points as in common use, but graduated on the outer circle with degrees, reading uninterruptedly from zero all round to 360: this will give the direction of the road, the bearings of any two objects, and the angle between them. A waistcoat-pocket ivory 6in. folding rule: this will serve the purpose both of scale and protractor; the eighths of an inch may be conveniently taken to represent miles, and by laying the rule upon the compass, so that its joint coincides with the centre on which the needle turns, and opening the legs to the degrees marked upon the circumference, the required angle may be approximately transferred to the note book. For observing latitudes a sextant is indispensable. If great accuracy is not required, a pocket or box sextant of from three to four inches diameter, and reading to half miles, will answer; but for more precision one of at least 8in. radius should be taken, framed entirely of metal, as wood will shrink and warp; it should read to 15 or even to 10 seconds, or sixtieths of a mile. There are many forms of artificial horizon, but of these the mercurial is the best, and, in fact, the only one we can confidently recommend. The trough should not be less than five inches long by three broad, and we prefer an oval form, with a convenient spout for pouring off the quicksilver when done with. A glass roof is used to protect the surface of the mercury, should there be any wind, and this may be made to fold into a very small compass, if desired. Six, or at least four, pounds of mercury should be provided; and this should be kept in an iron bottle, with screwed stopper and cover, serving as a funnel, which should be further protected by a piece of washleather tied over it. We have used as a substitute a common stoneware ink bottle, with leather securely tied over the cork, but wooden bottles are sure to split and leak when taken to hot countries.
With this equipment, a superior compass, for the more accurate determination of bearings, will be required. A prismatic compass is very useful, but we have used, with great convenience and accuracy, a flat one with a card of three inches diameter, divested of everything but the slit and hair line sights, which are used just as those of a rifle are, and protected only by a stout glass, which saved the trouble of removing and replacing the cover. A small pouch on the waist belt was appropriated exclusively to this. The note book may be of good non-metallic writing-paper, such as is in common use. This may be written on very conveniently with a H. or HH. drawing pencil, which is practically indelible. It would be convenient to have lines ruled along the side of the page for the courses and time or estimated distance; £. s. d. columns will do very well for this. For mapping, paper may be purchased ruled with squares of almost any desired size; the inches are marked with strong lines, and the subdivisions, eights or tenths, with fainter. This should be cut to fit one of the quarto sketching folios, with folding frame to confine the sheet in use, and pocket for spare paper as commonly used by artists. The instruments absolutely necessary for plotting the result are a semicircular, or, still better, a circular protractor, marked like the compass with degrees from 0 to 360, and made of brass, or preferably of some transparent material. A 6in. scale, with the usual divisions, and a good pair of compasses or dividers, with points as fine as possible, but somewhat obtuse, to prevent the possibility of their piercing the paper and breaking off in it. For heights of mountains, the simplest and most reliable instrument is the hypsometrical or boiling-point apparatus, which, though not so accurate as the mountain barometer, is sufficiently so for ordinary purposes, and has this great merit—it cannot easily be put out of order. The rainfall, should the country be blessed with any, may be measured by a Casella rain gauge, which we have also used very successfully on a pinch as a funnel for drawing off rum from a barrel. Thermometers reading up to boiling point ought to be carried, and in addition to these the traveller may provide himself with a self-registering maximum and minimum, and a wet and dry bulb thermometer.
One of the greatest difficulties that an observer working on shore with the artificial horizon can meet with is—that the actual angle to be observed is doubled by reflection in the quicksilver. Few sextants read higher than 120° or 130°, consequently, when the sun is 70° high, it is beyond the reach of ordinary instruments. To meet this, Captain C. George, R.N., of the Royal Geographical Society, has invented a very beautiful little instrument, in the form of a double box sextant, and the object of which is either to take two angles at one observation, by referring two distant objects to a common centre, and completing at once a perfect triangle, or, by the increased power of the instrument, to take any required angle that may be too great for those in ordinary use. The instrument is best described as being a special arrangement of two sextants placed one over the other. Each sextant is complete in all its essential details, and, if so required, can be detached and separately used.
The “Improved Double Sextant” is capable of being applied to the following uses:—
(1.) To the measurement of angles of nearly double the arc which can be measured by the ordinary sextant.
(2.) To the simultaneous measurement of two angles.