Report on Terrestrial Magnetism. 



de Humboldt has not expressly referred, is this : However 

 defective ordinary dipping instruments may be considered to be, 

 there are few persons who have had opportunities either of 

 making observations with the ordinary instruments for determin- 

 ing the variation of the needle, or of comparing those made by 

 others by the usual methods with such instruments, who will 

 not admit that these instruments and methods are fully as de- 

 fective possibly much more so. Thus, however we may mul- 

 tiply the points on the earth's surface at which such observations 

 may be made, still great uncertainty must always rest upon such 

 determinations of these two important elements ; and in all com- 

 parisons of such observations with laws, whether empirical or 

 deduced from theory, it will ever be doubtful whether the dis- 

 cordances which may be found are due to errors of observation, 

 or are indicative of the fallacy of these laws. This source of 

 uncertainty must, in a great measure, if not wholly, be obviated 

 by observations made at fixed stations, with instruments of si- 

 milar construction, which have been carefully compared with 

 each other. And we have no hesitation in stating our opinion 

 that more would be done in determining the positions of the 

 poles of convergence and of verticity on the earth's surface, 

 and other points most important towards the establishment of 

 any thing like a theory of terrestrial magnetism, by simultaneous 

 observations made at a few well-chosen fixed stations, than by 

 an almost indefinite multiplication of observations by the ordi- 

 nary methods. 



That a magnetic chart that should correctly exhibit the several 

 lines of equal variation, Humboldt's " Isogonal Lines," would 

 be of the greatest advantage to navigation, those who are best 

 qualified to judge are most ready to admit. If to these lines 

 were added the isoclinal lines, or lines of equal dip, the value of 

 such a chart would, for the purposes of navigation in particular, 

 be greatly enhanced. Whatever may be the magnitude of the 

 influence of the iron in a ship on its compass needle, the extent 

 of the deviation of the horizontal needle due to that influence, 

 on any bearing of the ship^s head, is a function of that bearing 

 and of the dip of the needle at the place of observation. The 

 extent, therefore, of the horizontal deviations, in various bearings 

 of the ship's head, having been ascertained at any port where 



