502 L. A. Bauer — Wilde's Explication of the Secular 



magnetic force on the two ends would be unequal and non- 

 parallel. 



(3.) The needles vibrate over about 10 degrees of terrestrial 

 surface, so that when Wilde measures the declination at say 

 London, he is getting it for the whole of England, over which 

 there is a range of about live degrees. 



(1.) The adjustment of the mechanism with reference to 

 maintaining the magnetic pole in latitude 72° with regard to 

 one station alone. It is well known that the magnetic merid- 

 ians require considerable urging to be made to pass through 

 one point. Besides the latitude of the north magnetic pole is 

 uncertain by at least 1° and probably more, no wholly satis- 

 factory determination of its position having ever been made. 

 Ross made a dash at it in 1831-32 and the International Polar 

 Commission in their comprehensive labors forgot all about 

 this interesting point. This will introduce an uncertainty by 

 at least 2° in the polar inclination of the axis of the internal 

 sphere. 



To facilitate the comparison between fact and theory, instead 

 of the table which Wilde gives, we have before us a diagram- 

 matic representation (Diagram A) of all the observations of 

 magnetic declination known for the three stations, London, 

 Cape Town, and St. Helena, and all the results as obtained 

 with the magnetarium, the full curves representing the former, 

 the dotted, the latter. For London our earliest observation is 

 11° 15' East in 1580. The march of the declination here has 

 already been pointed out. For Cape Town or Cape of Good 

 Hoj)e, our series begins in 1605 with the needle pointing - 5° 

 to the East. Marching westward it reaches its maximum 

 westerly elongation about 1870. After hovering here awhile 

 it turned its course and is now diminishing its westerly de- 

 clination at the rate of 2' per annum. At St. Helena the 

 observations date back to 1610, the needle pointing about 7° 

 to the East. Crossing the zero line somewhere between 1680 

 and 1690, it has reached its maximum westerly extreme at the 

 present time and after remaining stationary for a few years 

 will begin to march eastward. The Magnetarium results were 

 ground out separately, beginning in each case with zero declina- 

 tion and continuing until the inner sphere had accomplished 

 one-half of a differential revolution, when the needle pointed 

 again zero, giving thus a half period of secular change. The 

 declination was measured for every 6° of differential motion. 

 The results for London were obtained on the basis of a sym- 

 metrical distribution of terrestrial magnetism. It took 60° of 

 differential motion of the globes to produce the maximum 

 westerly elongation. Now from the observations we find that 

 this covers a period of about 160 years, hence 6° of differen- 



