Importance of Reliable Iso- Magnetic Charts. 259 



with a rule for empirically finding the amount of variation, for 

 short periods at least, according to which the magnetic needle 

 is found to vary from year to year at every spot along cer- 

 tain given lines, which it has been found possible to delineate 

 upon the charts; thus showing at a glance the amount of 

 variation to be allowed for at any given spot. As this is suf- 

 ficient for all practical purposes of navigation, the seaman is, 

 in most cases, relieved of the necessity of making for himself 

 these observations and calculations, if only he can ascertain 

 with anything like accuracy the position of his ship on the 

 earth's surface, and has determined the amount of local varia- 

 tion on board. 



These iso-magnetic lines are, however, susceptible of great 

 improvement, and if they are ever to become practically 

 and universally useful, repeated observations must not be 

 neglected by such navigators as have the means and the 

 requisite scientific knowledge to pursue such investigation. 



On board the Novara not a single sunshiny day was suf- 

 fered to pass without the variation being frequently deter- 

 mined, or such observations repeated as related to the deter- 

 mination of local attraction on board. 



Under such circumstances, an unusual value attached to 

 our ascertaining and following up so far as practicable the 

 decrease in declination of the magnetic needle till it reached 

 the zero point assigned to it, and comparing our own ob- 

 servations with the amounts stated on the charts. 



It was, however, at least as regarded nautical matters, of 



s 2 



