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.
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