40 HANDBOOK OF MAGNETIC COMPASS 



magnetic heading of the ship can be observed continuously as the ship 

 swings. Such a procedure would be as follows : 



The forward sight vane is clamped to the dial at the value of 

 the sun's magnetic azimuth, and the sight vanes are then trained 

 so that the sun is reflected in the mirror. As the ship turns, 

 the magnetic heading is always observed under the forward lub- 

 ber's line if the vanes are kept on the sun, and this will serve as 

 a guide for bringing the ship onto any desired magnetic heading. 

 As the desired magnetic course is approached, the compass can be 

 read and corrected, even before that magnetic course is actually 

 obtained ; and a final check can be made when the ship is on the 

 exact course. The pelorus must always be kept in a level position 

 while making observations, particularly of celestial bodies. 



52. A distant object can be used in conjunction with the pelorus^ as 

 with the azimuth circle, in order to place the ship on magnetic head- 

 ings, provided the ship stays within a small area. (See art. 50.) 



53. Methods of determining deviations on compass heading. — 



The deviations on compass headings may be obtained by a comparison 

 of the calculated magnetic aziirmth of the sun and the azimuth as 

 observed on the compass by use of an azimuth circle. Methods of cal- 

 culating sun's azimuths are discussed in chapter VI. The ship is 

 placed on the desired compass heading and an azimuth of the sun is 

 taken on the face of the compass card. The difference in degrees 

 between the observed azimuth and the calculated magnetic azimuth 

 of the sun is the deviation on that compass course. 



54. The pelorus may also be used in conjunction with the sunh 

 azimuth to obtain deviations on compass headings. The ship is 

 brought to the desired compass heading, and the forward sight vane 

 is set on the calculated value of the sun's magnetic azimuth. The 

 sight vanes are then trained on the sun, and the magnetic heading 

 of the ship is indicated under the forward lubber's line of the pelorus. 

 The difference in degrees between the compass heading and magnetic 

 heading of the ship indicated by the pelorus, is the deviation on that 

 compass course. 



55. The azimuth circle or pelorus can be used in conjunction with 

 ranges or a distant object to obtain deviations on compass courses. 

 The procedure is similar to that used with the sun. A range con- 

 sists of any two objects or markers, one in the foreground and 

 the other in the background, which establishes a line of sight 

 having a known magnetic bearing. The true bearing of such a 

 range is determined from a local chart; this true bearing is con- 

 verted to the magnetic bearing by applying the variation, cor- 

 rected for annual change, as given on the nearest compass rose of the 



