BEARINGS BY COMPASS, SUN, ETC.
Pocket Compass.--A pocket compass should not be too small; if one of the little toy compasses be carried in the pocket, it should be as a reserve, and not for regular use. A toy compass 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 identity 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 compass, 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.
Compass for use at night.--The great majority of compasses 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 compass card is painted black, the northern being left white. With a very faint light, this difference can be appreciated. In compasses 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 compass or a watch at night is increased by looking nearly at it through a magnifying-glass. 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 compass by a burning glass, or other substitute for it. (See "Burning Glass.")
True and Magnetic Bearings.--The confusion between true and magnetic bearings is a continual trouble, even to the most experienced travellers. Sir Thomas Mitchell's exploring party very nearly sustained a loss by mistaking the one for the other. I recommend that the points of the compass, viz. North, N.N.E., etc., should be solely used for the traveller for his true bearings; and the degrees, as 25 degrees (or N. 25 degrees E.), for his magnetic. There would then be no reason why the two nomenclatures should interfere with one another, for a traveller's recollection of the lay of a country depends entirely upon true bearings--or sunrise, sunset, and the stars--and is expressed by North, N.N.E., etc.; but his surveying data which find no place in his memory, but are simply consigned to his note-book, are necessarily registered in degrees. To give every facility for carrying out this principle, a round of paper should be pasted in the middle of the traveller's pocket-compass card, just large enough to hide the ordinary rhumbs, but leaving uncovered the degrees round its rim. On this disk of paper the points of the compass (true bearings) should be marked so as to be as exact as possible for the country about to be visited.
Errors in Magnetic Bearings.--The compass-needle is often found to be disturbed, and sometimes apparently bewitched, when laid upon hill-tops; even when they consist of bare masses of granite. The disturbance is easily accounted for by the hornblende in the granite, or by other iron-bearing rocks. Explorers naturally select hills as their points of triangulation; but compass observations on hill-tops, if unchecked by a sextant observation of the sun's bearings, are never so reliable as those taken on a plain.
Bearings by Sun and Stars.--It requires very great practice to steer well by stars, for, on an average, they change their bearings even faster than they change their altitudes. In tropical countries, the zodiacal stars - as Orion and Antares--give excellent east and west points. The Great Bear is useful when the North Pole cannot be seen, for you may calculate by the eye whereabout it would be in the heavens when the "pointers" were vertical, or due north; and the Southern Cross is available in precisely the same way. The true North Pole is about 1 1/2 degree or 3 diameters of the full moon, apart from the Pole star; and its place is on a line between the Pole Star and the Great Bear. An almanac, calculated to show the bearing, and the times of moonrise and moonset, for the country to be travelled over, as well as those of sunrise and sunset, would be a very great convenience; it would be worth while for a traveller accustomed to such calculations to make one for himself.
Diagram.--The diagram (preceding page) is intended to be traced in lines of different colours, when it will be found to be far less confused than at present.
Its object is to enable a traveller to use the sun, both as a rude watch and as a compass. The diagram is calculated for the latitude of London, but will do with more or less accuracy for the whole of England. A traveller going to other countries may easily draw up one for himself, and on a larger scale if he prefers it, by using the Azimuth tables and the Horary tables of Lynn.