398 - TERRESTRIAL MAGNETISM. 



of the iiWlcst |)hil()Sophers in Europe and America. Year after year 

 new (lata are amassed as to the cliang-es o-oing on in the distribution of 

 the niaonetisni of the earth, but as yet we have been favored ])y 

 hypotheses only as to the causes of the wondrous changes which the 

 magnetic needle records. 



These hvpotheses were at one time chiefiv based upon a considera- 

 tion of the secular change in the declination, but it is now certain that 

 we must take into account the whole phenomena connected with the 

 movements of the needle if we are to arri\e at any satisfactory result. 

 Besides, it will not suffice to take our data solely from existing fixed 

 observatories, however relatively well placed and equipped, and 

 valuable as they certainh^ are, for it now appears that the secular 

 change is i)aitly dependent upon locality, and that even at places not 

 many miles apart differences in results unaccounted for by distance 

 have been obtained. 



The tendency of observation is increasingly to show that the secu- 

 lar change of the magnetic elements is not a world-wide progress of 

 the magnetic needle moving regularly in certain directions, as if solely 

 caused by the regular rotation during a long series of years of the 

 magnetic poles round the geographical poles, for if you examine Map 

 No. 1,^' showing the results of observations during the years 184()--1.SS(), 

 as regards secular change, vou will observe that there are local causes 

 at work in certain regions, whilst in others there is rest, which must 

 largelv modify the effect of any polar rotation. 



Allow me to explain further. The plain lines on Map No. 1 indicate 

 approximate regions of no secular change in the declination, and the 

 small arrows the general direction (not the amount) in which the north- 

 seeking end of the horizontal needle was moving during those forty 

 3^ears. The foci of greatest change in the declination, with the approxi- 

 mate amount of annual change in the Northern Hemisphere, are shoAvn 

 in the German Ocean and northwestern Alaska, in the Southern Hemis- 

 phere off' the coast of Brazil, and in the South Pacific between New 

 Zealand and Cape Horn. The two foci of greatest annual change in 

 the dip are shown — one in the Gulf of Guinea, where the north-seeking 

 end of the needle was being repelled strongly upward; the other on the 

 west side of Tierra del Fuego, where the north-seeking end of the 

 needle was being attracted strongly downward. 



It is remarkal)le that the lines of no change in the declination pass 

 through the foci of greatest change in the dip. If the needle be repelled 

 upward, as at the Gulf of Guinea focus, it will be found to be moving 

 to the (nistward on the east side of the whole line of no change in the 

 declination from the Cape of Good Hope to Labrador; to the westward, 

 on the west side. If the needle be attracted downward, as at the Tierra 



«Origiiuilly prepared ])y the autlior for the " MaKnetical Results," H. M.S. Cliallenyer. 



