Law of Magnetic Declination. 257 



on deck, and in the evening the jolly tars, to their great gra- 

 tification, received each a double allov^ance of grog. 



It was our Commodore's intention to cross the shorter di- 

 ameter of the almost elliptical curve of equal magnetic de- 

 clination, which occurs in this vicinity, with the view, if 

 possible, of ascertaining by observation by what law the 

 " local variation " of the needle is diminishing within the 

 curve of 5°, the latest indicated in the most recent magnetic 

 charts. 



This curve of 5° easterly magnetic declination lies, accord- 

 ing to F. Evans,* between the parallels of 5*" 30' N. and 13'' S. 

 lat., and 120« W. and 134'^ 30' W., north-eastward from the 

 Marquesas Islands. 



The magnetic needle, as is well known, does not point to 

 the geographical poles, but is deflected from the due north 

 and south meridian, in a direction eastward or westward 

 according to locality, at an angle which, in the measure of 

 the easterly or westerly magnetic variation of the plane, is 

 called eastern or western declination or variation, and which 

 not only gradually alters at every place with the lapse of 

 time, but also is universally found to assume different values 

 at different places, so that in certain lines, known as lines of 

 equal declination, the variation remains the same for all places 

 under that line during a certain given period. 



As the compass is the sole reliable guide of the seaman 



* Chart of curves of equal magnetic variations, 1858, by Frederick Evans, Mas- 

 ter, R.N. 



VOL. III. s 



