TERRESTRIAL MAGNETISM, ETC. 319 



force required to make the compass correct on the 

 east or west points, is found to be less than at the 

 beginning of the voyage. It is clear that part of 

 the correction made by the magnets ought to have 

 been made by the Flinders bar. But nothing need 

 be done except to diminish the fore-and-aft pull 

 by the magnets, as long as the ship is going to 

 places of weaker vertical force. If without touching 

 or crossing the magnetic equator the ship returns 

 again to places of stronger vertical force, and if it 

 is found that increased longitudinal pull is now 

 required, this should be applied, not by the 

 magnets, but by introducing a Flinders bar or by 

 increasing the bar already in position. 



Generally, for a ship making passages to and fro 

 through regions of considerably different vertical 



amount of the vertical force at any place is calculated by multiplying 

 the value of the horizontal force given by the chart of lines of equal 

 horizontal force of the Admiralty Manual by the tangent of the dip 

 as given by the chart of lines of equal magnetic dip. Thus, for 

 example, the tangent of the dip for the south of England being 2 '44, 

 and the horizontal force there being called unity, the vertical force there 

 is 2-44. The tangent of the dip at Aden is '09, and the horizontal 

 force is 1*95 ; hence the vertical force there is '1755, or about T V of 

 the vertical force at the south of England. 



