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



secular change of the elements deduced from the observations made after this interval 

 of thirty-three years may be considered as approximately correct. 



Then Tristan da Cunha. Since H.M.S. " Herald "■ touched at this island in 1852, 

 and observed the declination, no British observer seems to have made magnetic 

 observations until the Challenger called there in 1873, and obtained values of all 

 three elements. H.M.S. "Sapphire" called here in 1883, and obtained a good value 

 of the declination in the process of swinging near the same position as the 

 Challenger. The change in this element may therefore be considered fairly 

 established, whilst for that of the inclination and intensity one is obliged to rely on 

 Sabine's lines for 1842-45, based on observations made in the neighbourhood as a means 

 of comparison with the Challenger's results. Situated in mid-ocean and rarely 

 visited, results obtained at this island form an important link for the purposes of 

 terrestrial magnetism. 



Not long after leaving the well-known Cape of Good Hope, the ship anchored in 

 Christmas Harbour, Kerguelen Island, and here another of Ross's positions on land 

 was visited. Unfortunately for our immediate purpose, the stations occupied during 

 the transit of Venus by the Rev. Father Perry were situated in quite a different part 

 of the island, and his otherwise valuable magnetical observations cannot strictly be 

 compared with those of Eoss and the Challenger, so that the secular change now 

 adopted dejDends upon the two latter authorities. 



At Cape Town and in the Indian Ocean north of the parallel of 30°, as well as on 

 the coasts of Australia, the secular change of the declination for many years past has 

 been found to be very small, rarely exceeding 1' annually ; it was therefore desirable 

 to know how far to the southward this slight movement of the needle extended. The 

 results from Christmas Harbour show that the north-seeking end of the needle is 

 moving westward at the rate of 5' annually at least. 



It was, however, from two positions on the homeward voyage that the most novel 

 and remarkable values of the secular change were obtained. These were Sandy Point, 

 Magellan Strait, and the island of Ascension with its adjacent waters. 



The United States monitor " Monadnock " visited Sandy Point in the year 

 1866, and took what were probably the first observations with absolute instruments of 

 the three magnetic elements. Subsequently the declination was observed by different 

 vessels, and the absolute horizontal force by H.M.S. "Nassau" in 1868-69. But the 

 secular change of those elements at this station is so moderate, — the horizontal force 

 being nearly stationary and the declination decreasing 3' annually, — that but little was 

 suspected of the large change which was going on in the inclination until the visit of 

 the Challenger in 1876 disclosed the fact that the latter element was apparently 

 decreasing about 1 1' annually. The results obtained by the observers of the French 

 Expedition to Orange Bay in 1882-83, who visited Sandy Point, somewhat modify this 



