Scientific Lectures. 161 



necessarily follows that there must be two points on the circumference 

 of this disc at which the needle points toward its center. Also, it is 

 said that the earth has two such points, and when in June, 1831, the 

 celebrated Sir James Ross reached the western coast of Boothia Felix, 

 and found the magnetic needle pointing almost directly toward the cen-. 

 ter of the earth, he inferred that he stood on the termination of a line 

 drawn from the earth's center through its magnetic pole to his feet. 

 Thus rewarded for his hardihood, this bold mariner undertook another 

 voyage of discovery in search of a similar point in the southern 

 hemisphere, and in 1841 succeeded in reaching south latitude 76° 12' r 

 on Victoria Land, where the needle made an angle of 88° 40' with the 

 horizon, and he concluded from this and other observations that the 

 position where the needle would be vertical was about 160 nautical 

 miles distant. From these and other discoveries made in the 

 antarctic seas, it is supposed that the pole of the southern hemi- 

 sphere must be somewhere about south latitude 70° and near the meri- 

 dian of 125° E. of Greenwich, which would bring it somewhere on 

 the territory dicovered by our countryman Wilkes. The exact posi- 

 tion of this point, however, is not known, for no explorer has yet 

 reached it. Now, if you will mark on a terrestrial globe the position 

 of these two points, as I have here done, you will see that they are 

 not exactly opposite each other, but are, however, nearer to that rela- 

 tion than is generally taught, and we might be led to infer from an 

 examination of a physicial chart of the world. This is another 

 example of the great importance of using the globe whenever it is 

 possible to do so, and never being satisfied with the information and 

 impression given by a distorted map of the world. Placing the pole 

 of the northern hemisphere at latitude 70° and longitude, west 85° ; and 

 the other pole at south latitude 70° and longitude 125° east, one of these 

 points will be removed from the end of a diameter drawn from the 

 other by only 30° in longitude ; which, on a parallel of 70°, only 

 equals about 600 miles, so that if the southern pole should be moved 

 by this quantity to the west, it would be exactly opposite the pole on 

 the Isthmus of Boothia. But even this want of geographic symmetry 

 has been reproduced on this disc, for I have made its poles distant 

 from each other by the same arc of a sphere's circumference as sepa- 

 rates the magnetic poles of the earth. This I accomplished in the fol- 

 lowing manner. Having calculated the angular distance of the terres- 

 trial poles, I marked this opening by two points on the circumference 

 of the disc, and at these points I placed the conical terminations of the 

 cores of two very powerful electro-magnets. On passing the electric 

 !"Inst.1 11 



