Abbildungen der Seite

attain perfect accuracy in their observations made during a first exploration. It is not the fraction of a second of time, nor even the fraction of a mile of latitude that is required, but the degree and nearest minute.

All instruments should be in leather or canvas cases, painted white, slung by straps, and " becketed " * sufficiently.

Barometers and other glass instruments are better carried on men's backs (with the upper ends down) than in any other manner, but their bearers must of course be very careful.

If possible, chronometers should be worn night and day, to ensure uniform temperature and care. Motion affects chronometers far less than change of temperature.

The reflecting circle is an instrument highly esteemed (especially by the French), but it cannot measure .an arc larger than 140° satisfactorily, on account of the extreme inclination of the index to the horizon glass. It is heavier and more cumbersome than a sextant of equal radius, and is more difficult to handle, f

A sextant with a doubly graduated arc and an additional horizon glass, will measure 160° satisfactorily.

When the sun is more than 80° high, his altitude cannot be taken in an artificial horizon, because the head of the observer obstructs the sun's rays while endeavouring to observe the altitude.

The roof of the artificial horizon should be portable, fitted to fold together, and, whenever used, should be reversed from time to time, so that half the altitudes should be taken with one end of the roof towards the observer, and the rest with the other. The trough should be smaller than usual, and raised in the roof by a thick bottom, so that an angle may be taken near the horizon. The mercury should be level with the edges of the trough, so that any dross may be scraped off by a piece of paper, or a thin strip of wood, kept in the box. Having the quicksilver level with the edges of the trough facilitates observing a low altitude. The mercury may be kept in a wooden, or in an iron bottle, with a screw-stopper and cap-funnel. To prevent spilling, the trough should have a notch at one corner inside, and be cut away underneath. The bottle should hold more mercury than will fill the trough, and a spare bottle should be carried. If all the mercury should be lost, coloured water will do nearly as well (dark or inky water). The horizon trough and roof should be placed on a flat plate of metal standing on three knobs. This will at once give a place for the horizon where the ground is uneven, wet,

* Having loops or eyes of leather or cord at the sides or corners for slinging or fastening them, when travelling.

f The repeating reflecting circle is much praised by Col. Sabine. See also article upon it iu the Penny Cyclopaedia, under "Sextant."Ed.

grassy, or soft. A leaden plate, about the size of a thin octavo book, with three knobs an inch long underneath, is suitable, because its weight steadies it among grass or in soft places.

The observer should endeavour to sit down on the ground, as near the artificial horizon as he can, in order to steady his arms and body, and avoid being disturbed by any wind. Cross-legged, with the elbows steadied on the knees, is a position as firm as can be maintained.

Rochon's micrometer is frequently useful as a telescope, carried in a case on the back.

Barometers might be made, for the special use of exploring travellers, in a simple manner. The tube should be unconnected.

permanently with the cistern, open at both ends, but r~) capable of being hermetically closed temporarily at one end; it should be large, strong, and graduated on the glass. The cistern should be capable of admitting the tube when required, which should then be supported by a rod of iron screwed to the cistern, and steadying the tube by an arm with a clamp ring. There should be a float in the cistern, and, when not in use, the apertures for the float and tube should be closed by screws. The cistern should be of iron. The mercury should be clean, and as pure as possible, but not boiled.

Such a barometer might not give results strictly accurate, as independent measurements of pressure, but if filled carefully so as to exclude visible airbubbles, duly compared with good standard instruments, and the temperature of the mercury, as well as that of the atmosphere, properly noted at each observation, it is believed that it would give valuable comparative results.

Several tubes might be carried in one strong case, with the baggage, and a spare one in a light metal case, to the place of observation.

Henry Raper, \ „ , _
Robert Fitzroy, f Sub-Committee.


Extracts from a Letter by Bear-Admiral Smyth, addressed to the Secretary.

The first duty of a geographical traveller is the accurate determination of the route, stations, and topography of his journey; and the fewer instruments he is encumbered with the better will they be worked. Skilful mapping of regions little known is an actual boon to science, especially when accompanied by the observations of an intelligent mind. The general elevation of the countries passed over is ever a co-ordinate of importance, since much physical information may be derived therefrom as to the nature of production, modified by geographical position and consequent climate. Such are the appropriate ends of scientific travellers: but it should ever be borne in mind that quality, and not quantity, is the true end.

For he who explores Africa I may, from experience, advise that no accurate part of his instruments—as for instance the barometer-case—be made of wood; and everything should be exposed to a high heat before leaving London, perhaps equal to 120° in the shade, and 160 J in the sun. No levels nor thermometers that will not stand this ordeal should be taken. The instruments should be few and simple, light and well-packed in nonconducting cases; and a portmanteau full of clothing is the very best packing-case for levels and thermometers, since the extremes of temperature will not be experienced therein. Thus, in a tent with the temperature of the day at 95°, a thermometer in a portmanteau was at 60° only.

An African traveller needs a pocket prismatic azimuth compass, with which he must take rounds of angles—including the sun—at all his stopping places, and on the road also if opportunity offers; but his best instruments should only be unpacked at crucial stations. He should have the most improved measuring tapes, and every opportunity must be taken of multiplying measurements of length for base-lines for filling in between standard positions by what may be deemed dead-reckoning. Still the most important object is an efficient angular instrument for fixing the latitude and longitude, without which the other work is comparatively useless. I alluded to this subject in my last Address to the Society, and have little to add unless a person were under drill. Latitudes, of course, will be measured by altitudes of the sun or stars in the meridian; but, for the determinations of longitude, more practice and attention will be required. Small differences may be measured by a chronometer, yet cannot be trusted, as the goingrate in African travelling is mostly useless: the capital measures made by Admiral Beechey, with two pocket chronometers, when a Lieutenant of the Adventure under my command, round the shores of the greater Syrtis, form a solitary exception. Absolute determinations by the moon, as I have said, must be looked to, not lunar distances—so often mischievous under unpractised hands— but altitudes of the moon and star, getting the right ascension when out of the meridian, or the declination when in the meridian. As to the favourite plan of observing the eclipses of the satellites of Jupiter, besides the uncertainty of instant, the method involves the necessity of carrying a telescope of power, and obtaining corresponding observations.

This is what is to be done ; now how should we do it? A sextant is the most portable of instruments; but you must add thereto an artificial horizon, a heavy, lumbering thing, if good, and if of the best kind, namely, quicksilver, the fluid will assuredly be spilt and lost in the sand, besides some getting out and damaging the instrument. Then again, unpractised persons require a stand for the horizon, or the instrument, or for both; and at best you can only thus measure altitudes above and below a certain height, and so lose a great part of the sky, and that very part in which the sun is placed in Central Africa. A portable altitude-andazimuth instrument, with its stand, would therefore be necessary to good work.

Now such a complication is adverse to the ends of the mere explorer, and therefore it is with equal pleasure and conviction that I recommend the Universal Instrument proposed by my son the Astronomer Royal of Scotland, who has had great experience of these matters in South Africa, during the late mensuration of an arc of the meridian in that country. It is a sextant when used for hand-work; or, if fastened on its stand, will measure any altitude and azimuth angles, give meridian transits on five wires, and the like, more conveniently and quickly than by the usual theodolites and other graduated instruments in use. It is, moreover, simple in construction, inexpensive, and easy to use; and a three-legged stand, for general purposes, is always readily carried, as it requires no care.

It remains to notice the determination of heights, independent of the troubles of levelling or trigonometry. The most accurate of the secondary methods in practice is, of course, by means of the barometer; but the difficulty of transporting this instrument (in my own opinion rather exaggerated) has been so greatly complained of by travellers, that mechanicians have turned their attention to the subject; and miniature barometers, sympiesometers, aneroids, and other substitutes have been the consequence. Yet such are the discrepancies of zero and scale, that, except .the improved Adie's sympiesometer, the boiling thermometer is better than either of them, as keeping its zero constantly. But that method, though affording comparative ranges for a traveller in a new and wild region, is liable to very serious errors, particularly from the boiling taking place under unequal pressure when the steam is confined; nor is it easy to boil properly. The apparatus which I used was made expressly for me by M. Drescler at Palermo, in the year 1813, and consisted of a cylindrical pot, with a lamp appendage for boiling the water; and the thermometers were so mounted that half the scale could be turned up the back by a hinge, leaving only the bulb and part of the stem immersed in the hot water. This is the instrument mentioned in my account of Mount Etna (Sicily and its Islands, p. 145); and it is also described in Baron de Zach's Correspondance Astronomique et Geographique. Various experiments were made with it during my operations in the Mediterranean, merely, however, in comparison with the trigonometrical or barometric points.

But even in this boiling-point process, though on the whole so eligible, there is weight to carry, and trouble to take; and as, in addition to the importance of very frequent observations to get at the section of a country, readings of the barometer and thermometer will be absolutely necessary as an appendage to the astronomical work — it is my own opinion, that a small Adie's sympiesometer is the best instrument for an exploring traveller; and I may further say, that though a chronometer may undeniably be useful, still it is not importantly so, because absolute longitudes by the moon should always be aimed at: a mere seconds' watch will therefore do for intermediate work.

Extracts from a Letter by Rear-Admiral Beechey, addressed to the Secretary.

I Believe that arcs by chronometer may be accurately measured, but with great care, and you may remember that some were done with excellent results, on my plan, by Dr. Colthurst, who went to the Colorado through Sonora. But a track by a pocket compass, well timed and filled into a form of this description, would tell a very good tale if checked by meridian altitudes and observations at the beginning and the end of the day :—


I do not think a transit instrument calculated for an exploring expedition. If the traveller has a caravan, and is going to remain a fortnight or more at a place, and then proceeds on again with the same means of transporting his baggage and instruments, a transit might be taken, as it could be fixed on a stand sufficiently stable for the purpose. But I should have no faith in moon culminations

Vol. xxiv. z

« ZurückWeiter »