Influence of Terrestrial Magnetism on Iron. 211 



ceeded in determining empirically the correction that should 

 be made in the observed deviation, in order to obtain the 

 true magnetic declination of the place where the observa- 

 tion was made. But this process, like the former, requires 

 a series of distinct operations for each vessel in particular, 

 those made for one not being able to be used for another. 

 It is, moreover, not without some practical difficulties. 



M. Poisson, impressed with the importance to navigation 

 of the question upon which we have just been treating, and 

 convinced that it had been only imperfectly resolved by the 

 empirical means that we have pointed out, endeavoured to 

 submit it to analysis, and so to arrive at a general formula 

 of correction. He proposed to determine directly the true in- 

 clination and declination in any given place on the globe, from 

 observations of the compass made on board a vessel, and 

 under the influence of the iron that it contains. The iron 

 being magnetised by the magnetic force of the earth, it is 

 evident that its action upon the needle will be proportional 

 to this force. Further, since the components of this action 

 correspond to three rectangular axes passing constantly 

 through the same points of the ship, they have, for their ex- 

 pressions, linear functions bearing relation to the components 

 of the action of the globe in the direction of these same 

 axes. The magnetic force of the globe, then, is common to 

 all the terms of the equation of the equilibrium of the com- 

 pass, and consequently disappears from it. The formula 

 contains different terms that must be determined ; and, in 

 particular, the quantities dependent upon the total amount 

 and the distribution of the iron contained by the vessel. But 

 for various reasons connected with the distribution of the 

 masses of iron in vessels, which is, in general, symmetric, 

 and with their position, which is in the most part below the 

 horizontal plane drawn through the point of suspension of the 

 compass, M. Poisson succeeded in simplifying the problem. 

 The two unknown terms to be determined, are the true in- 

 clination and declination ; and, for this determination, two 

 data from observation are sufficient : Those required by M. 

 Poisson' s simplified formula are, — the angles of the principal 

 section, or of the axis of the vessel, with the apparent direc- 



