304 HYDROORAPHICAL SURVEYING fniAP. xttt. 



The observer himself registers the time of each observation 

 on the chronograph, using his left hand to make electric con- 

 tact and liis right hand to \\'ork the foot-screw of sextant 

 stand. 



An assistant, roughly noting the time of each observation 

 to the nearest second or two, marks the tape at suitable 

 intervals, as before mentioned. 



The exact time is read off from the tape at leisure, and 



inserted in its proper column in the Sight Book. 



Allowance We should take more observations in our first half of equal 



a^f^gj. altitudes than will be absolutely needed, so as to allow of 



Transit, some losses in the observations after transit, from obscurations 



of the sun. 

 Index At the conclusion of observations it is always well to take 



the index error. It tells us whether our sextant is keeping a 

 steady error, and also, by calculation of semi-diameter there- 

 from, whether the instrument is in adjustment for side error, 

 and also, if we lose the other half of equal altitudes, and decide 

 to work single altitudes instead, we shall have the index error 

 observed at the time. 

 Calculat- After sights before transit, we must calculate the time the 

 for Obser- observations after transit will commence. By far the simplest 

 vations plan, when engaged in observations, is to have an ordinary 

 Transit, watch set to apparent time, which time the ship herself will 

 in many cases be keeping, when, by noting the time by this 

 watch at the last observation, the time of commencement of 

 the first observation after transit will be found by taking the 

 time noted from twelve hours. 



If we do not do tliis, and the sliip be keeping mean time, 

 we must find the mean time of the last observation by apply- 

 ing the approximate Error of the watch. Subtract this from 

 twelve hours, and apply tA\ice the equation of time, subtracting 

 if apparent noon is before mean noon, and adding if vice versa. 

 This AA-ill give mean time of the first observation after transit, 

 \^hich can be re-transferred to the watch by the appHcation 

 of the Error. 



We want the time by the ship's clock, to ensure leaving her 

 at the right time, and the time by our watch, to avoid the 

 chance of being top late, on one hand, and scurry after reaching 

 the observation spot, on the other. 



