496 HYDROGRAPHICAL SUR^^YING [chap. xxi. 



results, a comparison of all the bearings derived from the 

 different sources when referred to two comnion points, with the 

 aid of the sextant angles connecting them, will enable us to 

 determine the best value to adopt for the bearing and sca^e, 

 after due consideration has been given to the conditions apper- 

 taining to each position. If the conditions are favourable, and 

 proper care has been taken, the difference should never amount 

 to much. 



It may be here remarked that at any ship's position it is 

 possible we may have : 



(a) A bearing actually observed. 



(6) A bearing calculated from the station pointer fix. 



And thus, (c) the comj)arison of (a) and (b) with similar data 

 obtained independently from the other vessels engaged, all 

 of which are referred in the most dii'ect way to a common hne 

 of bearing joining two suitable points. 



SECTION XVI. 



ASTRONOMICAL OBSERVATIONS AND THE CONSIDERATIONS 

 ATTACHING TO THEM. 



If entirely dependent upon astronomical observations, it is 

 weU to arrange the daily programme of sounding so as to 

 obtain solar position hues, a.m. and p.m., which should include 

 ex. meridian altitudes, du-ect and supplemental when possible, 

 on opposite lines of bearing by each ship on successive days, 

 taken near the extreme beacons embracing the triangulation 

 which are the best situated for obtaining the scale. 



To reduce the possibilities of error, it is best when observing 

 to maintain the ship so that the f)osition hne then derived 

 will also pass approximately close to the beacon. 



Twilight stars, a.m. or p.m., taken on two or more successive 

 nights at suitable positions near the extremes of the triangu- 

 lation, will give the best results, and from this the scale can be 

 derived. 



A reUable bearing is best obtained by means of the ships 

 disposed at night as in Section XIII., or in a manner that the 

 third ship is situated so as best to connect the two extreme 

 ships should they not be inter visible. 



