974 Transactions of the American Institute. 



to its greatest Avesterly variation at Greenwich, be perfectly accu- 

 rate, then it will continue to decrease at Greenwich till the year 

 1978, when the needle will again point due north at Greenwich; 

 while at New York (longitude 74 deg. west) the westerly variation 

 will continue to increase, and will not attain to its greatest point 

 till the year 1950. And corroborative of all this, supporting the 

 hypothesis of revolution, it is known, that about the year 1790 the 

 needle pointed due north from New York, as it should do, while 

 to-day it has a westerly variation of nearly eight degrees, which 

 again is in perfect accordance with, and indeed an absolute neces- 

 sity in, the supposed revolution. The period from 1658 to 1790 is 

 one hundred and thirty two years, being the pro rata time neces- 

 sary to pass over seventy-four degrees of longitude; and here it 

 will be apparent that in 1790, when the needle pointed due north 

 at New York, the magnetic pole was then situated on the meridian 

 of New York, and it is now 44 deg. west of it; thus showing the 

 progress of revolution. 



Going still farther west to the islands in the Pacific ocean, we 

 lind there, that the variation is easterly and growing less, still fully 

 uupporting the hypothesis of revolution. 



I will mention but one other fact in proof of my position. Dr. 

 Bowditch in his Navigator; vaention'B that in 1580 (a period seventy- 

 eight years earlier than any yet mentioned), the needle at London 

 then pointed eleven degrees and some minutes east. The magnetic 

 pole must then have been situated in about latitude seventy-six 

 degrees north and forty-five degrees east from Loudon, and between 

 that time and 1658 it had moved up to the meridian of London. 

 Now, if we add to this the facts already proved, that from 1658 to 

 the present time, the magnetic pole has moved from the meridian 

 of London to one hundred and eighteen degrees west of it, and 

 having passed over the meridian of New York, the proposition, 

 that the magnetic pole revolves about the North pole once in about 

 six hundred and forty years, is then, I think, fully demonstrated; 

 and there is nothing lacking in the demonstration but the exact 

 time of the revolution, which, as I have said before, must be deter- 

 mined by future careful and accurate observation. It no doubt 

 may be determined with nearly as much accuracy as the periodical 

 return of an eclipse.* 



• The only evidence which I have seen going to disprove my hypothesis of revolution, is 

 a remark of Dr. Bowditch in his Navigator; while at the same time giving some dozen or 

 twenty statements of the variations of the compass at different times and different places. 



