REPORT ON THE MAGNETICAL RESULTS. 15 



circles are mounted on board ship, especially as it is necessary that the ship should not 

 cover much distance during the time of observation, and consequently the engines 

 be moving slowly. 



Having thus followed in detail the various steps which have been taken to produce 

 the representation of the elements of terrestrial magnetism contained in the accompanying 

 charts, a few remarks on the degree of dependence to be placed upon them seem 

 desirable. The most reliable portion will be found in the zone contained between 

 the parallels of 70° N. and 50° S.— the weakest portions of that zone being the interior 

 of Africa and South America, and even on the coasts of the former there is a large space 

 not yet examined for magnetical purposes. In portions of North America, other 

 than the United States where an extensive magnetic survey is in progress, observations 

 are much wanted, especially in the higher latitudes of British America, In the 

 southern hemisphere the regions south of the parallel of 50° S. are largely dependent 

 upon Eoss's survey, corrected only by the results obtained in the Challenger, and 

 more recently by those of the International Polar Expedition of 1882-83. 



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. 



Referring to the familiar hypothesis of Halley, announced in the early part of the 

 last century, it will be found that its main features were that of a solid globe or terella, 

 with two poles or foci of intensity rotating within and independently of the outer shell 

 of the earth, which also possessed two poles or foci of intensity, the axes of the two 

 globes being inclined one to the other, but having a common centre, the variable rela- 

 tions of these poles causing the secular change. 



Again, Hansteen in the early part of the present century, with better materials at 

 hand, came to a conclusion similar to that of Halley, as to there being four poles of 

 attraction. Hansteen " 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. From computation he found that the North American point or pole 

 required 1740 years to complete its grand circle round the terrestrial pole, the Siberian 

 860 years, the -pole in the Antarctic regions south of Australia 4609 years, and a 

 secondary pole near Cape Horn 1304 years." 



In later years Sabine added his opinion, that the secular change is caused by the 

 progressive translation of the point of attraction at present in Northern Siberia, such 

 point of attraction resulting from magnetism induced in the earth by cosmical action. 

 The hypothesis, therefore, of the translation of one or more of the points of greatest 

 attraction or foci of intensity was clearly held by these magneticians. 



A later contributor 1 to terrestrial magnetism writes thus: "Sabine and Walker 



1 The late Balfour Stewart. 



