.'JOG HYDROGRAPHICAL SURVEYING [chap. xvi. 



caoh lino of position is brought up to the patent log reading at 

 the time of observation of the last star of the series. 

 Selection The selection of stars sufficiently bright for observation 

 f ^ ^oh^ ^® governed mainly by the angle at which the lines of position 

 vation. derived from one pair will cut those derived from the other. 

 The accuracy of a line of position is independent of the hour 

 angle, wliich need not be considered, when working by the 

 " new navigation." 

 identifica- A conipass bearing of a star when its altitude is observed 

 s^ °^ Avill enable it to be identified with certainty, either by means 

 of the armillary sphere, or by the table in the pamphlet " What 

 Star is it ?" supplied in ail chart boxes. 

 Venus by The planet Venus can, when near quadrature, be observed 

 ^*y- all day. The altitude can be calculated, when she will easily 



be found in the field of the telescope. This observation is but 

 too little used, as it is a most valuable one. 

 Different As observations must be carried on tliroughout the day, in 

 of^obt^ain- Order to get as many positions as we can, we now come to 

 ing Posi the different methods of obtaining latitude and longitude other 

 ^^°^ than by the ordinary means of longitude by chronometer and 



latitude at noon. 



There are three methods of finding latitude and longitude 



at the same time — viz., by Ivory's rule for double altitude ; by 



Sumner's method; and by a short equal altitude; and for latitude 



only, we have circum-meridian altitudes. These are aU of service 



under different circumstances, Avhich Avill be hereafter described. 



Daily When dealing with a large number of clironometers, after 



Ej^o^Qf making the daily morning comparison with the standard, the 



standard error on G.M.T. of each chronometer should be worked up, 



meter*on using the harbour rates. Applying the daily comparisons, 



GMT. the performances of each separate chronometer are thus referred 



from Har- directly to the standard, and a mean error for the standard is 



^0^ adopted for each successive day, which is noted in red ink in 



all Chro the Comparison Book. On arrival in harbour, observations 



nometers. being obtained, the sea rate for each clironometer is found, 



Correc- and their errors on G.M.T. are worked up to the time of the 



tionstobe (jaily comparisons for each day on the passage. Applying the 



ascertain- daily comparisons to the errors of the different clironometers, 



Rates^ a daily mean error for the standard is obtained depending 



on the sea rate of all the chronometers. The difference between 



