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



increasing in amount until a focus of + 0'009 was found on the west coast of Portugal, 

 gradually diminishing again towards the Atlantic seaboard of North America on the 

 west, and towards the Aral Sea on the east. In China, also, there appears to be a 

 minor focus of increasing annual change. 



But the changes going on in the horizontal component of the earth's intensity 

 were far exceeded by those in the vertical component. Commencing at the Cape Horn 

 focus there was found an annual change in the vertical force of 0"055 (B.U.), drawing 

 the north-seeking end of the needle doivnwards, the change diminishing 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 da Cunha, was reached. To the northward and eastward of this zero line, 

 there were found increasing values in the annual change in the upward vertical force 

 acting on the north-seeking end of the needle, until the Guinea focus was reached, 

 where its full value was increasing - 025 annually. From the Guinea focus to Northern 

 Europe, Asia, and the Atlantic seaboard, the change gradually decreased in amount. 

 In China a minor focus of change in this element was found, the north-seeking end of 

 the needle being drawn downwards. Apparently there was no great change going on 

 in the Indian and Pacific Oceans, but there were signs of increase in the vertical force 

 on the west coast of Mexico and the United States as far as San Francisco. 



From these remarks upon the means adopted for obtaining the corrections for 

 observations taken at different epochs, it may be fairly accepted that the possibilities 

 of error in reducing them to the common epoch of 1880 have been brought within 

 satisfactory limits, especially as one of the chief factors in the compilation of the maps 

 of the three elements — the observations taken in the Challenger — were separated 

 from it by only a mean number of five years. 



Hitherto only the special points of interest in the Challenger's results have 

 been reviewed in their order of time, but the ship's track may now be usefully followed 

 as marked out by magnetic observations. These were begun late in 1872; when 

 starting from England the ship went to Lisbon, and on to Gibraltar, where the first 

 swinging abroad took place, and shore observations were made. These were valuable, as 

 little had been done for terrestrial magnetism at the latter place since the visit of the 

 Austrian frigate " Novara " in 1 857. Proceeding on the voyage by Madeira and Tenerife, 

 and westward near the parallel of 20° N., the island of St. Thomas was reached ; 

 thence northward to Bermuda and Halifax, N.S., back to Bermuda, and on to the 

 Azores, and a second time to Madeira. A large portion of this division of the voyage 

 was over entirely new ground. Sailing south by way of St. Vincent, Cape de Verde 

 Islands, and St. Paul's Rocks, Bahia, near the magnetic equator, was reached. Complete 

 sets of observations having "been made there, the voyage was continued by Tristan 

 da Cunha to Cape Town. It may be remarked that on account of the moderate time 



