C. GENERAL THYSICS. 149 



ism. About the year 1841 his attention was drawn to this 

 problem by his friend Major Sabine, and he shortly after 

 published a solution of the questions connected with this 

 subject in a form far more readily understood than had pre- 

 viously been given by Poisson. The formulae of Mr. Smith 

 supplied the means of a sufficiently exact calculation of all 

 the laws affecting the accuracy of the compasses on board 

 of any wooden vessel, and, when reduced to simple tabular 

 forms, they were at once adopted by the Admiralty, and have 

 ever since continued to be used in the royal navy. As the 

 use of iron in vessels increased, and the weight of the arma- 

 ment of ships of war added to the irregularities of the com- 

 pass to an inconvenient degree, an entire revision of the 

 old Admiralty instructions became necessary, and the mathe- 

 matical part of the work was intrusted to Mr. Smith. That 

 gentleman reduced the complete expression given by Pois- 

 son, including the effect of both the permanent and the in- 

 duced magnetism of the iron portions of the vessel, to a few 

 simple and easily applied formula?; and this last edition of 

 the Admiralty Compass Manual has been translated into nu- 

 merous languages, and adopted by the United States, and 

 the Russian, German, Portuguese, and French governments. 

 The rule given by Smith for placing the needles on the com- 

 pass-cards has been adopted by every nation. It reads thus : 

 " When there are two needles used, they should be placed 

 with their ends on the compass-card at 60, on each side of 

 the ends of a diameter ; and when there are four needles, they 

 should be placed with their ends at 15 and 45 distance from 

 the ends of the diameter." 



The reasons given for this rule by Smith, when it was orig- 

 inally proposed, were afterward supplemented by various im- 

 portant merits which were found to be incident to this con- 

 struction of the card. The general tenor of Mr. Smith's re- 

 searches, as well as the result of all the experience of the 

 British navy, seems to show that compass needles (which 

 have sometimes attained to the monstrous length of fifteen 

 inches, or even more, in some of the great modern passenger 

 steamers) ought by no means to be so large as they are gen- 

 erally made ; that, in fact, seven inches long should be the 

 limit in length, and that if shorter than this the errors of the 

 needle will be still more perfectly corrected. In fact, when the 



