Report on Terrestrial Magnetism. 325 



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 exhi- 

 bit the isoclinal, in conjunction with the isogonal lines, would 

 readily furnish the means of obtaining the correction to be ap- 

 plied to the ship's course by compass, both for the variation of 

 the needle and for the deviation due to the ship's influence upon 

 its compass. Whatever charts of this description may have al- 

 ready 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 in- 

 sure 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 conforming 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 ad- 

 vantage 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 at the northern magnetic 

 poles of convergence and verticity, whether these poles are two 

 different points, or one and the same ; indeed, continued obser- 

 vations at such stations would go far to decide this question, high- 

 ly important in a theoretical 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 establishments in the vicinity of 

 either of these cities. We consider that the most advantageous 

 positions would be, one near the most northerly establishments 

 in Hudson's Bay, and another at or near to Fort Resolution on 

 Great Slave Lake. As, however, observers in such positions 

 would be placed almost beyond the pale of civilization, we fear 



VOL. XXIT. NO. XLIV. APRIL 1837. Y 



