CHAP. XIX.] SWINGING SHIP 419 



or, if the object be far enough off, can be steamed round a 

 circle small enough to make no difference in the bearing. 



If steamed round, it is well to repeat the operation, tm-ning 

 in the opposite direction, as the compass may partake of the 

 swing of the ship, which will introduce error. The mean of 

 the two will then be the bearing to use. 



The second method is perhaps the one generally employed, By re- 

 and is very convenient with a theodolite at hand. BeariiTgs, 



An officer is landed with azimuth compass and theodolite. 



He obtains the bearings with the compass of some well- 

 defined object, and setting up his theodolite, takes it for his 

 zero. 



In arranging the theodolite on zero, it saves calculation to 

 point the degree and minute of the magnetic bearing to the 

 zero instead of 360°. Thus, if the zero bears by compass 

 S. 44° 20' E. (supposed to be unaffected), set the vernier to 

 135° 40'. The angles read to the ship will then be the angle 

 east of the magnetic north. 



A flag on a long staff is held behind the theodolite, when all 

 is ready. 



The ship, under steam, and with a flag placed exactly over 

 the standard compass, steams slowly round, hoisting a large 

 flag close up to the masthead just before the sliip's head comes 

 to each point, which is dipped at the moment of observation, 

 when the bearing of the shore station is taken. 



The flag on shore is dipped, to show that the angle of the 

 flag over the compass has been obtained by the theodolite, and 

 is again shown as a response, when the flag is mastheaded for 

 the next observation. 



The time of each observation is taken by previously com- 

 pared watches. 



In this case, too, the ship should be swung in the opposite 

 direction, if it is deemed necessary. 



The difference of the reciprocal bearings is the deviation at 

 each observation. 



If more than one observation at any or all poins thas been 

 obtained, the results are meaned for the accepted deviation. 



It is usual to observe at every point of the compass, for the 

 ship's head, but in some vessels it may be necessary to sub- 

 divide this. 



27—2 



