274 HYDROORAPHICAL SURVEYING [chap. xii. 



magnitude, \\ hich can be easily observed on an average night 

 by a practised observer with good instruments. 



A special list of stars is supplied to all surveying ships, 

 which includes all stars in the Greenwich and Cape Cata- 

 logues, and also a diagram with an ingenious method of 

 pairing stars.* 



The German " Jahrbuch," the French " Connaissance des 

 Temps," and the American " Ephemerides," contain many 

 stars, with their apparent place for the day, which do not 

 appear in the " Nautical Almanac." If any of these publications 

 are at hand, it is convenient to make use of the information 

 therein given, instead of calculating the apparent place from 

 the data in the Greenwich and Cape Catalogues. 

 Choosing By observing stars of small magnitude, a sufficient number of 

 pairs can generally be obtained by midnight or a little later. 



The time of meridian passage and approximate altitude of 

 all stars down to the fourth magnitude passing the meridian 

 before that time must be calculated. Only those stars whose 

 altitudes range from 20° to 60° need be considered. 



A general list of stars being prepared and arranged consecu- 

 tively in the order of their passing the meridian, pairs of stars 

 at corresponding altitudes north and south are selected and 

 inserted in the Angle Book, together with the time of the 

 meridian passage of each star, the time that will be shown by 

 the pocket chronometer that is used for taking time, double 

 altitude, magnitude, whether it is north or south of the zenith, 

 and each pair must be numbered. 



The nearer together in point of time the two stars of a pair 

 can be placed, the greater will be the chances of the elimina- 

 tion of the refraction errors, as in a few hours temperature 

 often varies much, dews form, and many differences may arise 

 in the atmospherical conditions. 



Stars over 60° of altitude are not usually good to observe, 

 as, though a sextant will measure over 120°, the image of the 

 star vnM not be sharp when reflected from the index glass at 

 such a large angle unless the glass be unusually good. 



Altitudes of stars selected for pairs should not differ more 



* These are respectively compiled and devised bj' Lieut. H. B. T. 

 Soraerville and Commander Purey Cust, and much facilitate the selection 

 of stars. 



