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. 1892 Excerpt: ...the result must be subtracted from twenty-four hours to give true mean time, or from twelve hours for civil time. If the hour angle be large, and the time be required with great accuracy, the equation of time for the day must be computed for the hour. If the body be a star, the calculated angle i is the interval of sidereal time between the observation and the star"s upper culmination; and the true time must be deduced therefrom. An example of this method of finding the true mean time is given in Appendix IV. Convergence Op Meridians. Convergence denotes the change in bearing of a line, running straight over the earth for some considerable distance, due to the fact that the meridians to which its bearings are referred, converge to the pole. Thus, if ABC (Fig. 17) represents such a line, which is an arc of a great circle on the earth, the only line a theodolite properly set up and levelled can run, as the telescope, on being transited, would point to the centre of the earth when downward. EP, QP, are meridians, P is the pole. The convergence on the line AB is the angular Fig. 17. difference EAB-QBC or EAB Equatoh--ABP. As the exterior angle-e Q EAB of the triangle ABP is always greater than the interior alternate angle ABP, the true bearingof an easterly line, such as ABin thesouthernhernisphere, is constantly though slightly diminishing, as its distance from A increases. The formula used for ascertaining this angular difference is as follows:--Let c = the convergence in seconds; P = difference of longitude in seconds between the two extremities of the line AB, or the angle APB Fig. 17); f = the latitude of A; 0"=the latitude Psm± of B; then c =-i 77 When A and B are in the same, or 0--0 008-2--nearly the same latitude, that is when the line runs quite,...