16 



HANDBOOK OF MAGNETIC COMPASS 



accompanying roll are maximum on north and south compass head- 

 ings, and those accompanying pitch are maximum on east and west 

 compass headings. 



The horizontal B amd C components of 'permanent magnetism 

 cause varying deviations of the compass as the ship swings in azimuth 

 on an even keel. Plotting these deviations against compass headvng 

 will produce sine and cosine curves^ as shown in figure 8. These 

 deviation curves are called semicircular curves because they reverse 

 direction in 180°. 



Easterly 

 (t) 



Degrees of 



Compass 



Deviation 



Athwartship Permanent 

 Magnetic "C" Deviations 



Fore-and-Aft Permanent 



Westerly Magnetic "B" Deviations 



(-) 



Ship's Compass Heading - Degrees 



FiGDEE 8. — Permanent magnetic deviation effects. 



17. The permanent magnetic semicircular deviations can be illus- 

 trated by a series of simple sketches, representing a ship on successive 

 compass headings, as in figures 9 and 10. 



18. The ships illustrated in figures 9 and 10 above are pictured on 

 cardinal compass headings rather than on cardinal magnetic headings, 

 for two reasons : 



(1) Deviations on compass headings are essential in order to 

 represent sinusoidal curves which can be analyzed mathematically. 

 This can be visualized by noting that the ship's component mag- 



