284 Art of Travel.
how much the power of seeing a compass or a watch at night is increased by looking nearly at it through a magnifyingglass. 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., &c., should be solely used for the traveller for his true bearings ; and the degrees, as 25° (or N. 25° 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, &c.; 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 pocketcompass card, just large enough to hide the ordinary rhumbs, but leaving uncovered the degrees round its rim. On this disc 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 triangula