16 THE VOYAGE OF H.M.S. CHALLENGER. 



are agreed in regarding this variation as cosmical in its origin, and they are apparently 

 of opinion that it is caused by some change in the condition of the sun. It seems 

 difficult, if not impossible, to attribute it to anything else, since the terella of Halley 

 cannot be longer regarded as having a physical existence." He then proceeds to give 

 reasons for attributing the secular variation to the result of solar influence of a cumu- 

 lative nature — (l) an influence on a supposed hard iron system of the earth, and (2) a 

 long continued variation of solar power acting cumulatively on the large ice fields 

 round the poles of the earth — the changes in the ice fields acting cumulatively so as to 

 alter the convection currents of the earth, and these again " might in their turn per- 

 ceptibly alter the earth's magnetic system." 



Keeping in view the hypotheses which have thus been advanced, and recalling the 

 chief results of observation during comparatively recent years which have already been 

 discussed, an inquiry may now be made as to how far they accord. 



Observation generally points to the fixity of the magnetic poles — or two limited 

 areas in the earth where the needle is vertical — with respect to the geographical poles ; 

 and accepting this conclusion, the proposition of the revolution of one round the other 

 as the cause of the secular change must be dismissed. Again, observation during the 

 present century tends to show that in Northern Siberia very little change in the 

 magnetic elements can be traced, and therefore there is little or no apparent translation 

 of the point of greatest attraction in that region. Similarly the North American focus 

 of intensity is probably at rest. 



Thus the results are not satisfactory when a comparison is made between the 

 hypothesis of translation either of the magnetic poles of verticity or of the foci of 

 magnetic intensity with the results of observation in recent years. 



To avoid repetition of terms, let Airy's well-known terms of blue and red magnet- 

 ism be adopted, and also let the movements of the red or north-seeking end of the 

 needle alone be considered. 



Now, if a line be traced on a globe from the North Cape of Norway across the 

 Atlantic to Cape Horn, it will pass near some of the foci of greatest known secular 

 change ; and what information does observation give concerning those foci 1 That at 

 the Cape Horn focus of change in the vertical force the needle was moving downwards, 

 or there was the equivalent to a blue pole of increasing power of attraction, the freely 

 suspended needle being attracted towards it over an extended region around. Whilst 

 at the Guinea focus of change in the vertical force the needle was moving upwards, or 

 there was the equivalent to a red pole of increasing power of repulsion, the freely 

 suspended needle being repelled over an extended region of undefined limits. The 

 action of these two poles appears to be strongly marked in the South Atlantic near 

 Brazil, where they apparently combine to produce a focus of considerable angular 

 movement in the horizontal needle. 



