Press: RareBooksClub.com (March 6, 2012)
Author Name:Clark, Lewis
This historic book may have numerous typos and missing text.
Purchasers can download a free scanned copy of the original book (without typos) from the publisher.
1872 Excerpt: ...or, translated into common language: The magnetic declination, or variation, at any place is equal to the difference of the true and the magnetic bearings of any object; it is east if the number or point which expresses the true bearing is to the right of the number or point which expresses the magnetic bearing; but west if to the left.
From equation (1) we also have A = A + D; or, the variation must be applied to a compass bearing (or course) to the right hand if east, to the left hand if west, in order to find the true bearing (or course).
The same equation also gives A' = A-D; or, the variation must be applied to a true bearing (or course) to the left hand if east, to the right hand if west, in order to find the compass bearing (or course).
To find the variation, it is necessary to determine both the true bearing and the magnetic bearing of some object; at the same instant if the object be in motion.
The true bearing or azimuth of a celestial object may be found--First.--From an observation of its altitude (Prob.
This may be used to the best advantage when the azimuth changes most slowly with the altitude, i.
e., when a given change or supposed error of the altitude produces the least change of azimuth.
The most favorable position of any object is when its azimuth is The numbers or points are supposed to be read from the same compass card, the observer looking at them from the centre.
nearest 90; the unavailable position is on the meridian.
High altitudes and great declinationSj especially if of a different name from the latitude of the place, are to be avoided.
Second.--When it is in the horizon, or its apparent altitude above the sea horizon is 33'+ the dip, (Prob.
) its amplitude, or bearing from the east or west point of the horizon, ...
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