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



obtained in the neighbourhood of the more important islands visited by the Challenger 

 may be enumerated. 



Madeira and Tenerife were the first islands visited in the Atlantic, and the differ- 

 ences between the observations on their shores and the normal results on board were of a 

 nature to invite closer inquiry in other islands as time permitted. Thus at Madeira 

 observations of the inclination were made at a distance of one foot and 3 \ feet from 

 the ground, with a difference of 7-g° in the result ; whilst at Santa Cruz, Tenerife, the 

 inclination was 2^° in excess of the normal on board the ship. 



Bermuda. — Here the local disturbance was such as to invite particular examination, 

 especially as during the two visits of the Challenger time permitted many observa- 

 tions to be made. Previously to these visits observers in positions at short distances 

 apart differed considerably in their results. Our men-of-war, too, in the process of 

 swinging for the deviation of their compasses at the different anchorages, noticed 

 constant errors for all directions of the ship's head, which were confined to Bermuda 

 alone, and evidently proceeded from some local magnetic disturbance, the character of 

 which required to be definitely examined by means of instruments with which they 

 were unprovided. This was therefore an opportunity for doing immediate practical 

 service whilst instituting scientific inquiry by means of the excellent equipment of 

 instruments furnished to the Challenger both for absolute and relative determinations. 



With these objects in view, the declination was observed at seventeen stations on 

 land, the inclination at ten, and the intensity at seven. The ship was swung at sea 

 1 5' south of the green outside the dockyard, and normal values of the three magnetic 

 elements for the green deduced therefrom. Comparing the observed values with their 

 respective normals, it was found that the greatest differences in the declination were 

 + 2° 39' at Clarence Cove, and —3° 5' at Barge Island ; in the inclination, + 1° 47' at 

 Spanish Point and Mount Langton, and in vertical force at Spanish Point, +0"314 

 (British Units). Combining the observations taken in the western portion of the group 

 with eleven others of declination taken at different stations in previous years, plotting 

 the differences from the normal on a chart and drawing curves of equal value, as shown 

 on Plate I., it was found that between the Governor's House at Mount Langton and the 

 lighthouse on Gibb's Hill, there is a disturbing magnetic focus attracting the north- 

 seeking end of the needle with a force considerably in excess of that due to the position 

 of Bermuda on the earth considered as a mas-net. 



O 



Magnetic disturbance was also found at three other stations in the eastern parts 

 of this group of islands, but the observations made were too few in number to determine 

 any distinct source for it. It is satisfactory, however, to be able to point out to vessels 

 visiting the usual anchorage in Grassy Bay that there is little or no disturbance, whilst 

 at two positions half a mile on either side of it there may be as much as 2° difference in 

 the observed magnetic bearing of an object the true bearing of which is common to 



