1837.] 



Report on Terrestrial Magnetism. 



231 



obviated by observations made at fixed stations, with instruments of 

 similar construction, which have been carefully compared with each 

 other. And we have no hesitation in stating our opinion thai 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 observa- 

 tions by the ordinary 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 observa- 

 tion. The extent, therefore, of the horizontal deviations, in various 

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

 the dip of the needle is known, their extent at any other place, however 

 distant, at which the dip is also known, may readily be calculated. 

 Consequently, a chart which should correctly exhibit the isoclinal, 

 in conjunction with the isogonal lines, would readily furnish the means 

 of obtaining the correction to be applied to the ship's course by com- 

 pass, both for the variation of the needle and for the deviation due to 

 the ship's influence upon its compass. Whatever charts of this de- 

 scription may have already been constructed, and whatever materials 

 may exist for the construction of more accurate ones, it is well known 

 that great discrepancies exist among the data requisite for such con- 

 structions. And it appears to us that such a careful inquiry into the 

 whole of the phenomena of terrestrial magnetism as is proposed by M. 

 de Humboldt, is the means best adapted to insure the accuracy which 

 would be of such inestimable advantage in this most useful application 

 of scientific knowledge. 



Although our views with regard to the stations proper to be selected 

 for permanent magnetical observatories in general accord with those 

 expressed by M. de Humboldt, we shall, we consider, be only conform- 

 ing to his wishes, if we point out those stations which, from particular 

 circumstances of position, appear most desirable. We consider that 

 it would be of the greatest advantage if tw r o or more permanent mag- 

 netical observatories were established in the high latitudes of North 

 America, on account of the proximity of stations so situated at the 



