71 



ratives of their voyages a succession of determinations of the Decli- 

 nation, all made at the same spot, namely, at the one anchorage at 

 St. Helena, which extend over a period of 236 years, or from 1610 

 to 1846. The following Table contains eleven such determinations, 

 all from authorities of high repute, which are fortunately so far 

 equably distributed in respect to the years when they were made, as 

 to throw light not only upon the average amount of the secular 

 change of declination during that long period, but also in a consider- 

 able degree upon the regularity, or uniformity with which the change 

 has taken place. By treating these eleven determinations according 

 to well-known methods, we obtain 11 48' as the west declination 

 corresponding to the middle epoch, the year 1763, and 8' - 05 as the 

 most probable rate of the annual increase during the 236 years. 



Declinations observed at the Anchorage at St. Helena. 



Mean Epoch 1763 



Mean Declination +11 48' 



Annual Increase of West Declination 8'-05 



" We have here then a striking example of the magnitude and 

 character of the changes wrought at a particular station by this very 

 remarkable feature of the earth's magnetic force. In less than two 

 centuries and a half, the horizontal direction which a magnet takes at 

 St. Helena by virtue of the terrestrial magnetic force has been found 

 to have changed more than 30, or more than a twelfth part of the 

 whole circle : and when we further examine the facts more closely, 

 we find reason to conclude that this great change has taken place 

 by a steady, equable and uniform progression throughout the whole 

 period. The rate of annual change derived from the eight years 

 during which the observations were maintained by the detachment 



