424 



towards the establishment of anything like a theory of terrestrial 

 magnetism, by simultaneous observations made at a few well -chosen 

 fixed stations, than by an almost indefinite multiplication of obser- 

 vations 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 hori- 

 zontal 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 as- 

 certained 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 iso- 

 gonal, lines, would readily furnish the means of obtaining the cor- 

 rection to be applied to the ship's course by compass, both for the 

 variation of the needle and for the deviation due to the ship's influ- 

 ence upon its compass. Whatever charts of this description 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 pro- 

 posed by M. de Humboldt, is the means best adapted to ensure 

 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 se- 

 lected for permanent magnetical observatories in general accord 

 with those expressed by AJ. de Humboldt, we shall, we consider, 

 be only conforming to his wishes, if we point out those stations 

 which, from particular circumstances of position, appear most de- 

 sirable. We consider that it would be of the greatest advantage if 

 two or more permanent magnetical observatories were established 

 in the high latitudes of North America, on account of the proximity 

 of stations so situated to the northern magnetic poles of con- 

 vergence and verticity, whether these poles are two different points 

 or one and the same : indeed, continued observations at such stations 

 would go far to decide this question, highly important in a theore- 

 tical point of view. M. de Humboldt has mentioned Quebec as a 

 desirable station. To this place, and also to Montreal, we conceive 

 that an objection exists, of which possibly M. de Humboldt is not 

 aware: many of the houses in those cities are roofed with tinned 

 iron. This objection .may not, however, exist in some of the esta- 

 blishments i:i the vicinity of either of these cities. We consider 



