372 HYDROGRAPHICAL SURVEYING [chap. xvi. 



other observations by Sumner's method. The calculation is 

 given in the book quoted in the footnote.* 



The clironometer method is better for observations near 

 the prime vertical, and the rigorous ex-meridian method for 

 observations within the limits applicable to it. It must be 

 remembered that in the latter case the line of position is not 

 the parallel of latitude, but is the line at right angles to the 

 bearing, drawn tlu'ough the point found. 



Weir's diagram is a very convenient method of obtaining 

 the azimuth of a star. 



SHORT EQUAL ALTITUDE. 



In low latitudes, where the motion of the sun in altitude is 

 rapid nearly to the time of transit, a very good longitude may 

 be obtained at noon, by a short equal altitude, taking observa- 

 tions about twenty minutes before and after noon. The change 

 of declination in this short interval will not affect the time, 

 so that the middle time between the observations as shown 

 by the watch can be taken for the time by the watch at 

 apparent noon. All we have to do, therefore, is to take the 

 difference between mean time of apparent noon and the 

 GreenA\ich time, as shown by our clironometer, which gives 

 us longitude directly. 



CIRCUM-MERIDIAN ALTITUDES OF SUN. 



These are of great value, as, when the observations are within 

 the limits of time from noon, the resulting latitude is as correct 

 as from a meridian observation, which may be lost from clouds. 

 They should be worked in precisely the same manner as the 

 shore observations of the same description, and should be ob- 

 tained as near noon as possible. If more than four or five 

 minutes have to be added to the observed altitude, they will 

 not be of much value. 



If Raper's most valuable book is at hand, a short and correct 

 rule, in connection with two of his tables, will be found at 

 p. 232 of the thirteenth edition, which will give the reduction 

 as nearly as requisite for sea work. 



* " Ex-Meridian Altitude Tables and New Navigation," by C. Brent, E.N., 

 A. F. Walton, K.N., and G. Williams, K.N. Geo. Philip & Son, London, 1886. 



