Dec. 5, 1889] 



NATURE 



107 



Newfoundland, to the West Coast of Africa, near Cape 

 de Verde, emercjing near Cape Palmas, and then to Cape 

 Town ; thence curving upwards near Mauritius, down- 

 wards south of Cape Leeuwin, again upwards through 

 Adelaide and Cape York to the vicinity of Hong Kong. 

 A second line passed from Sitka through the western 

 portion of the continent of North America, striking South 

 America near Callao, then following the trend of the coast 

 to a point near the western entrance to Magellan Strait. 



The foci of maximum value of change were found : 

 (i) between Scotland and Norway, change about 9' an- 

 nually, needle moving eastward ; (2) on the east coast 

 of Brazil, needle moving westward about 8'. Minor foci 

 were also found : one near Kerguelen Island, the other 

 in the South Pacific. Another focus apparently exists 

 in Alaska. The general tendency was for the values of 

 the change to decrease gradually' from the foci to lines of 

 no change. 



2. lncl77iaiion.—S\m\\2ix\Y to that of the declination, 

 there are lines of no change, two principal foci of maximum 

 secular change, but only one minor focus. The lines of 

 no change are not so clearly defined as those for the 

 declination, data being still wanting. The principal foci 

 of maximum change in the inclination were found : (i) 

 near the Gulf of Guinea, between Ascension and St. 

 Thom^, which may be called the Guinea focus. Here 

 the north-seeking end of the needle was moving upwards 

 about 15' annually. (2) in the latitude of Cape Horn, 

 and about 80'' W. long. This may be called the Cape 

 Horn focus, and the annual change was n', needle 

 being drawn doiun^vards. It must be distinctly under- 

 stood that both the positions and values of the change 

 are only approximate, and only the general features in 

 the angular movement of the freely suspended needle 

 are to be accepted, as clearly shown by this investigation. 



3. Magnetic Intensity.— \r\ the horizontal force, the 

 annual change (B.U.) was about -0-002 near Cape Horn, 

 whilst between Valparaiso and Monte Video the focus of 

 greatest change was about - 0017. Again, on the west 

 coast of Portugal a focus of -f 0*009 (B.U.) occurred. 



Turning to the vertical component of the earth's 

 intensity, some remarkable results were observed. At 

 the Cape Horn focus an annual change of 0*055 (B.U.) 

 was found in the vertical force, the north-seeking end of 

 the needle being drawn downwards, the change diminish- 

 ing in value until the zero line extending from Callao 

 across the American continent to the west coast between 

 Bahia and Rio de Janeiro, and then taking a south- 

 easterly course north of Tristan d'Acunha, was reached. 

 Northward and eastward of this zero line there were 

 found increasing values in the annual change in the 

 7//7t/^zr^ vertical force acting on the north-seeking end of 

 the needle until the Guinea focus was reached, where its 

 full value was increasing 0*025 annually. From the 

 Guinea focus to Northern Europe, Asia, and the Atlantic 

 seaboard the change gradually decreased in amount. 

 There were signs of minor movements in the north-seek- 

 ing end of the needle in China, Mexico, and the United 

 States. 



One of the chief factors in the compilation of the pre- 

 viously mentioned maps of the three elements for the 

 epoch 1880 were the observations taken in the Challenger, 

 and these were reduced to the common epoch by means 

 of the investigation of annual change to which reference 

 has just been made. 



It may be truly said that the Challenger''s track was 

 studded with magnetic observations. After successfully 

 traversing the Atlantic Oceans in varying directions, the 

 three magnetic elements were obtained by swinging, in 

 probably the most southerly position since the days of 

 Ross in the hrebus and 1 error, in lat. 63° 30' S., and 

 long. 90° 47' E. But the most valuable part of the con- 

 tributions to terrestrial magnetism obtained in the Chal- 

 lenger were the observations made in the North and .. 



South Pacific. The route lay from Wellington, N.Z., to 

 Tongatabu, and Fiji, from the Admiralty Islands to Japan, 

 and thence in mid-ocean from nearly 40" N., through the 

 Sandwich Islands and Tahiti to 40° S., nearly at right 

 angles to the curves of equal magnetic inclination. 



During the voyage much experience was gained as to 

 the usefulness of the Fox circle as an instrument for use on 

 board ship at sea, the general result being that valuable 

 work may be done with it if frequently compared witii 

 the absolute instruments on land, and the instrument 

 mounted on a gimbal stand prepared to withstand the 

 vibrations caused by the engines of the vessel. 



Although on the general question of the secular change 

 of the magnetic elements much has been already written 

 in this Report, there yet remain some important points 

 which demand further discussion. 



As to the causes of the secular change various 

 hypotheses have been advanced. Thus in the early part 

 of the last century, Halley considered the change was 

 chiefly caused by a terella with two poles or foci of 

 intensity rotating within and independently of the outer 

 shell of the earth, which also possessed two foci of 

 intensity, the axes of the two globes being inclined one 

 to the other but having a common centre. 



Again, Hansteen at the beginning of the present 

 century concluded that there are four poles of attraction, 

 and computed both the geographical positions and the 

 probable period of the revolution of this dual system of 

 poles or points of attraction round the terrestrial pole. 



In later years Sabine considered the secular change to 

 be causedjby the progressive translation of the point 

 of attraction at present in Northern Siberia, this point 

 of attraction resulting from cosmical action. Walker 

 also agreed with Sabine as to the cosmical origin of the 

 change. 



Later still, Balfour Stewart gave reasons for attributing 

 the secular variation to the result of solar influence of a 

 cumulative nature. 



Keeping in view these hypotheses, and recalling the 

 chief results of observation during recent years, how do 

 they accord "i 



Observation generally points to the fixity of the mag- 

 netic poles — or two limited areas where the needle is 

 vertical — in respect to the geographical poles. Again, 

 in Siberia there is little or no apparent translation of the 

 greatest point of attraction in that region, and the North 

 American focus of intensity is probably at rest. 



Thus the results of observation in recent years are not 

 favourable to hypotheses founded on the translation of 

 the poles or foci of magnetic intensity. 



Let the terms blue and red magnetism be adopted, and 

 the movements of the red, or north-seeking, end of the 

 needle alone be considered. 



The question arises. What have recent observations 

 offered us instead ? They tell us that near a line drawn 

 from the North Cape of Norway across the Atlantic to 

 Cape Horn lie some of the foci of greatest known secular 

 change. It was also found that at the Cape Horn focus 

 of 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. At 

 the Guinea focus there was the equivalent to a red pole 

 of increasing power of repulsion, the freely suspended 

 needle being repelled over an extended region of un- 

 defined limits. The action of these two poles apparently 

 combine to produce a focus of considerable angular 

 movement in the horizontal needle near Brazil. 



In China there is a minor blue pole of increasing power 

 attracting the needle over a large area. 



With apparently small secular changes in Siberia, and 

 the horizontal needle moving somewhat rapidly to the 

 eastward at the focus of change in the declination in the 

 German Ocean, and similarly to the westward in Alaska, 



