APPENDIX B. 



SHIP'S MAGNETISM AND THE COMPENSATION OF THE 

 COMPASS.* 



7. The ship's compass. The style of ship's compass which is 

 now almost universally used is that which is due to Lord Kelvin. 

 The card of this compass is shown in Fig. 8. The points of the 



compass and the circle divi- 

 sions are printed on a paper 

 ring to which is attached a 

 light rim of aluminum which 

 keeps it in shape. Radial 

 threads connect the ring to a 

 central disk which contains a 

 sapphire cap by which the 

 compass is supported on an 

 iridium point. Eight small 

 magnets of glass -hard steel 

 are tied to the radial threads 

 four on either side of the 

 jewel cap, as shown in the figure. The entire weight of the card ; 

 including the magnetic needles, is 170^ grains, and this extreme 

 lightness combined with the relatively large moment of inertia due 

 to the distribution of the mass, insures a long period of free vibra- 

 tion and therefore great steadiness. The lightness of the card also 



* A good discussion of this subject is given in Gray's Treatise on Magnetism and 

 Electricity, Vol. I, pages 85-100, Macmillan and Company, 1898. For full details, 

 the reader is referred to Lord Kelvin's Instructions for Adjusting the Compass, to be 

 obtained from James White, of Glasgow. The practice in the United States Navy 

 concerning the matter of compass errors and compass adjustments is given in several 

 small pamphlets which are published by the United States Navy Department, and in 

 a book entitled A Treatise on Navigation, by Commander W. C. P. Muir, U. S. 

 Navy, Annapolis, 1906. The practice in the British Navy is given in the Admiralty 

 Manual of Deviations of the Compass. 



298 



Fig. 8. 



