258 Voyage of the Novara. 



while traversing the ocean, and it is of the utmost importance 

 to investigate and accurately lay down the ship's course for 

 the port which is her object to make, it appears necessary to 

 explain to the uninitiated how the local variation of the mag- 

 netic needle is determined, as thereby one can readily find the 

 precise angle at which the magnetic meridian of any place is 

 deflected from the true meridian. 



The determination of this divergence is effected by means 

 of observations of the sun, by the aid of which one can calcu- 

 late at any moment its actual bearings, as seen from the deck 

 of the ship, and this, compared with the true position of the 

 sun, gives the amount of variation. 



This apparently simple method of determination encounters 

 in practice, owing to certain local influences, a variety of obsta- 

 cles, for it is executed on board of a ship, which frequently 

 contains within itself, at a greater or less distance from the 

 binnacle, large superficies of iron, operating less or more pre- 

 judicially upon the needle, by deflecting it from the direction 

 which it would actually have but for these masses of iron. 

 Hence the variation is not even the same in all parts of the 

 ship, nor does it follow the same direction, but varies according 

 to certain laws, founded upon the intensity and direction of 

 the magnetic attraction of the earth. It is therefore neces- 

 sary to make allowance for these local deflections of the 

 needle, in order to find the true variation of the needle. 



So far as regards the last-named, many thousand observa- 

 tions, both by land and sea, have resulted in furnishing us 



