The sextant and its applications; including the correction of observations for instrumental errors, and the determination of latitude, time, and ... on land and at sea, with examples and tables

by William Simms

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. Not indexed. Not illustrated. 1858 ... Now as the greatest value of q between w = 0 and 125 can never exceed l-5, and as the value of this quantity is very little affected by a variation in that of /S, whereas the value of p may be as great as 20 when co is large, and in this case only will vary considerably with a variation of /3, the greatest value of p moreover occurring with the smallest of q, it is evident that it will not be necessary to consider the dependence of the value of r upon that of /3, except in the case in which the term q. sin2 7 is small compared with p. cos2 7, or that in which 7 is not nearly equal to 90, p and its variations being at the same time considerable, so that r=p. cos 7 nearly. But on computing the values of p, we find that whereas at co = 125 the value is 17-6 if /3=20 it amounts only to 106 if /S=15, so that the ratio 176: 106 represents very nearly the advantage gained at a s =125 by the reduction of /3 from 20 to 15, an index-glass which with /3=20 would pi-oduce a double image with an angular separation of 17"-6 producing with /3= 15 a separation of only 10"-6, and for smaller separations in the same proportion. The angle between the optical axis and the perpendicular to the plane of the horizon-glass ought therefore, in the construction of the instrument, to be made as small as a due regard to the free action of the several parts will permit, a conclusion o'herwise evident on inspection of the tabulated values of the coefficient B. CHAPTER I. GENERAL TREATMENT OF THE SUBJECT. The Sextant may be employed in every operation that involves the measurement of an angle, the objects subtending the angle to be measured being so far distant that the direct rays from each which fall upon the eye may, for all practical purposes, be considered parallel to tho...

40 pages, Paperback

Published January 1, 2012