1883.] On Deviations of the Standard Compass. 79 



vertical induction in soft iron, changing directly as the tangent of the 

 dip. 



Quadrantal Deviation. 



This deviation, caused by induction in horizontal soft iron sym- 

 metrically placed, does not change with a change of magnetic latitude. 

 Time alone appears to produce a gradual change in its value during 

 the first two or three years after the ship is launched, when it becomes 

 nearly permanent. 



The diminution of the mean directive force of the needle, which is 

 common to all modern vessels of war, improves slowly at first by lapse 

 of time, and finally assumes a permanent value. 



Relative Proportions of Hard and Soft Iron. 



It has been found that the relative proportions of the hard and soft 

 iron affecting the standard compasses of twenty-five vessels examined 

 differ considerably, even in ships of similar construction. 



This difference may be accounted for by the compasses not being 

 placed in the same relative position in the ships considered as magnets 

 of various forms, and containing numerous iron bodies introduced 

 during equipment. 



General Conclusions. 



The following general conclusions have special reference to the 

 standard compass of the six classes of vessels previously men- 

 tioned : — 



1. A large proportion of the semicircular deviation is due to 

 permanent magnetism in hard iron. 



2. A large proportion of the semicircular deviation maybe reduced 

 to zero, or corrected for all magnetic latitudes, by fixing a hard steel 

 bar magnet or magnets in the compass pillar in opposition to and of 

 equal force to the forces producing that deviation. 



3. A very small proportion of the semicircular deviation is due to 

 sub-permanent magnetism, which diminishes slowly by lapse of time. 



4. The sub-permanent magnetism produces deviation in the same 

 direction as the permanent magnetism in hard iron, except when 

 temporarily disturbed, (1) by the ship remaining in a constant 

 position with respect to the magnetic meridian for several days, (2) 

 by concussion, (3) or by both combined, when the disturbance is 

 intensified. 



5. To ascertain the full value of changes in the sub- permanent 

 magnetism, observations should be taken immediately on the removal 

 of the inducing cause. 



6. In the usual place of the standard compass the deviation caused 



