POLYTECHXIC ASSOCIATIOX FbOCEEDINGS. 973 



The only point necessary for us to dotermiue for our present 

 purpose is, to show that which has already been shown, viz : that 

 the magnetic pole is situated at a considerable distance from the 

 North pole, and that being proved, we must now look for the 

 evidence that it revolves about the North pole, which we will pro- 

 ceed to do. 



In the year 1658, as shown by the records at Greenwich, the 

 needle pointed due "north from that position.* It is, then, certain, 

 that in 1658 the magnetic pole^^was situated on the meridian of 

 Greenwich, and between Greenwich and the North pole, or, coin- 

 ciding with the North pole, or in a line beyond it, at 180 deg. west. 

 No other supposition is possible. From 1658 the needle began to 

 have a westerly variation at Greenwich, which continued to increase 

 till the year 1818, a period of one hundred and sixty years, when 

 it had obtained its greatest variation. Now, on the supposition of 

 the revolution of the magnetic pole, it is evident that it would 

 attain its greatest variation when it had passed over 90 deg. in the 

 circle in which it revolves, and that in its progress of revolution 

 the variation would then become less. Accordingly we find that 

 in the year 1818 the westerly variation at Greenwich began to 

 grow less, and from that time to the present has continued to 

 decrease, which is in accordance with the necessity of the case in 

 the supposed revolution. - 



From 1818 to 1868 are fifty years, in which time, supposing the 

 period of one hundred and sixty years to have been the exact time 

 in which the magnetic pole, by its revolution-, passed over ninety 

 degrees of longitude in the circle in which it revolves, in fifty years 

 it would pass over twenty-eight degrees more, which would place 

 it at this time in longitude 118 deg. west, where it is found to be, 

 as nearly as can be determined.! 



Coming now to the longitude of New York, although we have 

 no record here going back to 1658, yet we know the fact that the 

 westerly variation is increasing at New York, while it is decreasing 

 at London or Green^\dch, and this also is a necessity of the case in 

 the supposed revolution; and if the hypothesis of revolution be 

 true, and the period from 1658 to 1818, when it passed from zero 



* Brand's Dictionary of Science, article ''Magnetism." 



f In placing the pole at 118 cleg, west, I give it the same ratio of progress which it 

 appears to have had for one hundred and sixty years, viz : from 1658 to 1818. But if wo 

 consult the present variation as observed at New York, it would appear to be not quite so 

 far west, or ar^ about 116 deg. wesl. 



