SHIP'S MAGNETISM. 313 



the fine full lines, and the point where this line cuts the vertical 

 axis corresponds to the true magnetic course of the vessel. To 

 determine the compass reading corresponding to a true magnetic 

 course, start from a point on the vertical axis which corresponds 

 to the true magnetic course, travel parallel to the fine full lines 

 until the curve of errors is reached and then travel parallel to the 

 fine dotted lines until the desired point on the vertical axis (corre- 

 sponding to the actual compass reading) is reached. 



The heavy curve in Fig. 20 represents the actual compass 

 errors on the old British iron-clad Achilles, and the abscissas of 

 the fine sine curves represent the semicircular errors and quad- 

 rantal errors, respectively. The maximum value of the semicir- 

 cular error is 21 15', and the maximum value of the quadrantal 

 error is 6 9'. 



PROBLEMS. 



1. The semicircular error of a compass on board ship is found 

 to have a maximum value of 20 to the east when the ship heads 

 36 west of south. Make a sketch of the outline of the deck 

 of the vessel and draw a line on the deck showing the direction 

 of the horizontal component of the magnetic field at the compass 

 box which is due to the permanent magnetism of the ship, find the 

 value of this horizontal component and find the angle between its 

 direction and the direction of the keel, the earth's horizontal field 

 being equal to 0.2 gauss. Ans. (a) 0.068 gauss, (3) 106 from 

 bow towards port side (left side). 



2. What is the value of the semicircular error of the compass 

 when the ship specified in problem i heads 20 north of east ? 

 Ans. 19 21', west of north. 



3. The only error of a ship's compass is that which is due to 

 the ship's permanent magnetism, the quadrantal error being com- 

 pensated. The semicircular error has a value of 6 to the west 

 when the ship's head is true magnetic north and 4 to the west 

 when the ship's head is true magnetic northeast. On what true 

 headings will the error of the compass be zero ? Ans. 38 minutes 

 south of east, and 38 minutes north of west. 



