7o6 



NATURE 



[Dec. 5, 1889 



was applied to the solitary islands of the ocean visited 

 by the Challenger, and the following are some of the 

 ^principal results. 



At Madeira there was a difference of 7^' in the ob- 

 served inclination between observations made at i foot 

 and i\ above the ground ; and at Santa Cruz, Tenerife, 

 the inclination was lY in excess of the normal observed 

 in the ship. 



It was at Bermuda, however, that the most remarkable 

 results were obtained. For some years previously, ob- 

 servers in different parts of the group had obtained very 

 different values of the declination, and our men-of-war 

 when swinging for deviations of the compass had found 

 constant errors for every direction of the ship's head 

 which were peculiar to Bermuda. It could only, therefore, 

 be by a properly equipped expedition like that of the 

 Challenger, and systematic observation, that the imme- 

 diate cause of all this local magnetic disturbance could 

 be traced. 



For this purpose the declination was observed at 

 seventeen stations, the inclination at ten, and the intensity 

 at seven. Combining these observations with others made 

 by previous observers, 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 magnet. The 

 normal values of the magnetic elements were obtained 

 by swinging the ship at sea 15' south of the green outside 

 the dockyard. The difference between the observed 

 declination at Clarence Cove and Barge Island was 5° 4+'. 

 The greatest difference in the inclination was i^ 47', and 

 in the vertical force -|-o'3i4 (Brit, units). 



Local magnetic disturbances were also noted at St. 

 Vincent, Cape de Verde Islands, Tristan d'Acunha, Ker- 

 guelen Island, Sandwich Islands, Juan Fernandez, and 

 Ascension, but not at St. Paul Rocks. 



By applying the same method of obtaining normal 

 values at sea, and observing on other adjacent solitary 

 islands such as St. Helena, similar effects result, and the 

 following general conclusions seem to be supported by 

 fact with regard to local magnetic disturbance : — 



(i) That in islands north of the magnetic equator, the 

 north-seeking end of the needle is generally attracted 

 vertically downwards, and horizontally towards the higher 

 parts of the land ; (2) south of the magnetic equator the 

 opposite effects are observed, the north-seeking end of 

 the needle being repelled : in both cases by an amount 

 above that due to the position of the island on the earth 

 considered as a magnet. 



Interesting as these conclusions may possibly be from 

 a scientific point of view, they are of real importance in 

 practical navigation. Navigators have asserted that their 

 compasses were disturbed when passing the land in 

 certain parts of the world. We learn from the Challenger 

 observations that within 5 feet from the soil the greatest 

 magnetic disturbance did not exceed 3° in the declina- 

 tion and 2|° in the inclination. Remembering the law of 

 magnetic attraction and repulsion, it is impossible that 

 a compass in such case could be disturbed in a vessel 

 passing the land at the ordinary distance. In point of 

 fact, it has been shown that it is to submerged magnetic 

 land comparatively near the ship's bottom, that the dis- 

 turbance of the compass is due. The remarkable instance 

 at Cossack in North- West Australia may be cited in sup- 

 port of this conclusion. Thus in H. M.S. i]/^^a, sailing on a 

 line of transit of two objects on land for a quarter of an 

 hour in 8 fathoms of water, it was found that the compass 

 was steadily deflected 30°, no visible land being nearer 

 than 3 miles. 



Great as the gain must be to the navigator to be thus 

 warned of a formidable danger in certain places, it also 

 lays upon him the important duty of being on his guard 



against similar disturbances elsewhere, reporting any new 

 discoveries as he would a rock or shoal. 



Large as was the Challenger's contribution to the 

 magnetic charts for 1880, it will be readily understood 

 that it required considerable reinforcement from other 

 sources, as their construction was dependent on observa- 

 tion alone. Every available observation between the years 

 1 865-87 was utilized. Beyond the published sources of infor- 

 mation on this subject may be mentioned the observations 

 made on the east coast of Africa by the officers of H.M.S. 

 Nassau in 1874-76, and on the west coast of Australia in 

 1885-86 by H.M.S. Meda. Also the sea observations 

 between Australia and Cape Horn of the declination in 

 H.M.SS. Esk, Pearl, and Thalia, between 1867-87, not 

 forgetting those of the New Zealand Shipping Company's 

 vessels in 1885-86. 



To combine this twenty years' observation usefully, a 

 somewhat extended knowledge of the distribution and 

 amount of secular change became a necessity. Generally 

 speaking, it is only at fixed observatories that this 

 element of terrestrial magnetism is known with precision, 

 for, as already shown, observations a few feet apart often 

 give very different results. In the more frequented parts 

 of the earth this secular change is approximately known, 

 especially in the United States, where valuable work has 

 been accomplished. 



One great object of the voyage of the Challenger was 

 to visit certain unfrequented positions where previous ob- 

 servers had been, rather than the beaten tracks. Thus 

 Ross's position of 1840 on St. Paul Rocks was visited, 

 and the secular change during thirty-three years obtained. 

 Then Tristan d'Acunha, an important station situated in 

 mid-ocean, rarely visited for magnetic purposes. At 

 Kerguelen Island, another of Ross's positions, observa- 

 tions of all three principal magnetic elements were made, 

 and the secular change found approximately. 



In the Indian Ocean generally, north of 30^ S., the 

 secular change of the declination rarely exceeds i' an- 

 nually, but at Kerguelen Island the westerly declination 

 is increasing at least 5' annually. 



It was, however, from two positions on the homeward 

 voyage that the most novel and remarkable values of the 

 secular change were obtained — Sandy Point, Magellan 

 Straits, and the Island of Asce ision, with its adjacent 

 waters. 



At Sandy Point, with the horizontal force nearly 

 stationary, and the declination decreasing 3' annually, it 

 was hardly suspected until 1876, when the Challenger 

 visited the place, that the inclination was apparently 

 changing 11' annually. Comparing the Challenger's 

 results by swinging near the Island of Ascension with 

 Sabine of 1842 '5, the following values of the secular 

 change are obtained : declination increasing 8' annu- 

 ally ; south inclination increasing 14'. 



From these results the notable fact is made evident, that 

 the north-seeking end of the needle is found to be moving 

 in opposite directions, downwards at Sandy Point, and 

 more strongly upwards at Ascension. Extending the 

 inquiry into the surrounding seas and countries, it was 

 found that these opposite movements of the needle were 

 not confined to the spots where they were discovered. 



The author of this Report, after having discussed his 

 collection of a large number of observations of the 

 magnetic elements for all parts of the world — in many 

 cases extending over several years — obtained approximate 

 values of their secular change for the epoch 1840-80. 



These several values were weighted according to their 

 relative accuracy, and entered on maps against the places 

 of observation. Lines of equal value were then drawn 

 for each element, and the following general results ob- 

 tained with regard to the movements of the north-seek- 

 ing end of the needle. 



I. Decimation. — The principal lines of little or no 

 change were found to take the course from St. John's, 



