312 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



JVORTH 



bar into a vertical position in front of, or behind the compass box, 

 and move it towards or away from the compass until the compass 

 is turned through an angle <f> t in a direction opposite to <, the 

 angle <f> t being reckoned from the deflected 

 position of the compass. Then eliminate the 

 VA :> outstanding semicircular error by readjusting 

 the semicircular correctors. 



17. Napier's diagram. After the compass 

 correctors have been adjusted so as to approxi- 

 mately compensate the errors of the compass, 

 it is customary, for the purpose of accurate 

 navigation, to determine the residual errors of 

 the compass and allow for them in the use of 

 the compass. The ship is swung round and 

 for successive actual readings of the compass, 

 the compass error is determined by an inde- 

 pendent determination of the true magnetic 

 heading of the vessel.* Figure 20 shows Na- 

 pier's method for representing the compass er- 

 rors graphically. The successive actual com- 

 pass readings are laid off along the fine vertical 

 line as an axis, and the compass errors are laid 

 off along the fine dotted lines which are in- 

 clined at an angle of 60 to 1 the fine vertical line. 

 To determine the true magnetic course of 

 the ship from the compass reading, start at 

 the point on the vertical axis which corresponds 

 to the actual compass reading, draw a line parallel to the fine 

 dotted lines from the chosen point on the vertical axis to the 

 curve of errors (which is the heavy curve in Fig. 20) ; from the 

 point so reached on the "curve of errors, draw a line parallel to 



* The true magnetic heading is determined by a land-mark, if the vessel is in port, 

 or by observations on the sun or stars if the vessel is at sea, the declination of the 

 compass being known for the place of observation. 





NORTH 



Fig. 20. 



