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



near Cape de Verde, emerging near Cape Palmas, and passing on to Cape Town ; leaving 

 Cape Town, it curved upwards into the Indian Ocean near Mauritius, then downwards 

 south of Cape Leeuwin in Western Australia, again upwards through Australia by Adel- 

 aide and Cape York to the vicinity of Hong Kong, and terminating in Siberia in about 

 60° N. and 120° E. for want of data to trace it further. 



The second line passed from Sitka through the western portion of the North 

 American continent, quitting the coast near the meridian of 100° W., then on to the 

 South American coast near Callao, and afterwards following the trend of that coast, 

 reaching the meridian of 80° W. near the entrance to Magellan Strait. 



A third line, much less clearly defined, passed from Sitka in a southerly direction 

 to the equator, and then in a south-westerly direction to New Zealand. 



The next prominent feature in this map of the secular change were the foci of 

 maximum value. The principal focus was found to be approximately situated between 

 the east coast of Scotland and the west coast of Norway, with a value of about 9' annual 

 change, needle moving eastward. A second focus appeared on the east coast of Brazil, 

 extending to about the meridian of 20° west longitude, with a value of 8' in the annual 

 change, needle moving westward. Two minor foci, with a value of 5', were also shown 

 to exist — one in about 45° S. and 130° W., needle moving eastward; the other near 

 Kerguelen Island, needle moving westward. It may be remarked, however, in passing, 

 that for regions south of 50° S. considerable changes are probably proceeding in the 

 earth's magnetism, for which observation has done but little to elucidate. Another focus 

 apparently exists in the western parts of Alaska, but as yet indeterminate in position 

 and value of change, although probably large in the latter respect, the needle moving 

 westward. The general tendency was for the values of the change to decrease gradually 

 from the foci to the lines of no change. 



Now let the results of the map showing equal lines of the secular change of the 

 inclination be considered. Similarly to that of the declination, there are Unes of no 

 change, two principal foci of maximum secular change, but only one minor focus. The 

 lines of no change in the inclination, however, were less clearly defined than those of 

 the declination, in a great measure from want of data ; but that separating the two 

 principal foci of change may be traced as follows : — Passing through Callao in Peru 

 across the South American continent, emerging between Eio de Janeiro and Bahia, 

 touching the focus of maximum change in the declination off the Brazilian coast, and 

 then taking a south-easterly direction. 



The principal focus of change in the inclination was found near the Gulf of Guinea, 

 between Ascension and St. Thome, and for the sake of distinction may be called the 

 Guinea focus ; here the inclination was changing about 15' annually, the north-seeking 

 end of the needle being repelled upwards. The second focus occurred about the 80th 

 meridian of west longitude and the latitude of Cape Horn, and may be called the Cape 



