1825.] Variation of the Marmer*s Compass. S^ * 



we obtain 37' 51'' 8"'*73 for the annual progress of the magne- 

 tic pole as it is derived from the precession of the nodes of 

 Venus and of the earth, being only 2" 2'"*49 less than that 

 derived from the dip and variation observed in 1812. 



This calculation is upon the supposition that it be true, as it 

 is generally believed, that there was no variation at London in 

 the year 1657 ; an opinion upon which there can remain little 

 doubt, when we consider that Mr. Gunter in the year 1622, that 

 is 35 years before there was no variation, observed it to be 

 5° 56i' east, and Mr. Halley in 1692, that is 35 years after 

 there had been no variation, found it 6° west. 



Now from dip and variation observed in London in 1812, that 

 is 155 years after the time of no variation, I find the longitude 

 of the magnetic pole 82° 7' 36" west of London, and as in 1667 

 it was evidently in longitude 180°, it must have proceeded dur- 

 ing the 155 years, 97° 52' 24'^, which being divided by 155, give 

 37' 53'' 11"'*22 for the annual progress of the magnetic pole, 

 as it has been stated above, and the whole revolution = 

 570*1246 years, or 570 years and about 45 J- days. 



If the annual progress be made as tropical revolution of Venus 

 to tropical revolution of the earth, we obtain 36' 54" 42'", 

 which is nearly one minute less than the above, and the whole- 

 revolution would require about 12 years more. If on the con- 

 trary we assume for the annual progress 38' 51" 28"', which is 

 nearly one minute more than the first, the whole revolution will 

 be about twelve years less; namely, 557 years, 21 days, 

 18*^ ll'" 51% forming the astronomical period, called the period 

 of eclipses. Further observations, it is to be hoped, will enable 

 us to decide which of these data comes nearest the truth. Mean- 

 while let us observe that a mean between the four following 

 numbers differs less than half a minute from the annual progress 

 of the magnetic pole as derived from the dip and variation 

 observed in 1812. 



Annual progress deduced from annual preces- 

 sion of the nodes of Venus and the earth . . 37' 00" 53"'-73 

 From tropical revolution of Venus and the earth 36 54 42 



From period of echpses 38 51 28 



From solar and lunar cycles 40 36 05*4 



4)153 23 09-13 



Mean term 38 20 47-28 



From dip and variation of 1812 37 53 11-22 



Difference 27 36-06 



In the calculations of the dip and variation I have taken the 

 dip inversely as the distance from magnetic poles, of which I am 

 persuaded thercj are but two diametrically opposite to one 



b2 



