XII. Magnetic survey of the Eastern Archipelago

Abstract

In accordance with instructions received from the Honourable the Court of Directors of the East India Company, and at the recommendation of the Royal Society, I commenced, in the month of January 1846, the Magnetic Survey of the Eastern Archipelago. As, in the prosecution of this work, I was left entirely to my own discretion, I may be permitted to state, that the principal object of the Survey appeared to me to consist in determining the position of certain magnetic lines which were included within the space I had to traverse; such, for instance, as the line of no dip, and of the maximum horizontal component of the force; from these, to determine the line of minimum in­tensity; and finally, the line of no variation and its direction. The determination of these lines I considered to be the principal object; but in addition, I was anxious to take hourly observations of the elements of the earth’s magnetism, in order to ascertain whether the changes of declination and of magnetic intensity were uniformly similar over so large an area. The fixed stations for this latter purpose were sixteen in number, and the time employed at each station varied from a few days to several months. The fixed stations were spread over an area of 28° of latitude and of 45° of longitude, viz. from 16° latitude north to 12° latitude south, and from 80° to 125° longitude east. With reference to the line of no dip, which in this part of the globe coincides very nearly with the line of minimum intensity, I may state, that of the sixteen stations, nine were to the south, three to the north, and four in its immediate vicinity. With reference to geographical position, four were in the islands adjacent to Singapore; one in Borneo; one in the island of Java; two in Sumatra; one in the island of Mindanao; one in Celebes; one at the Cocos or Keeling Islands, which was the most southern station to which I could venture; one at Penang, and one in its immediate vicinity; one at Nicobar, in the bay of Bengal; one at Moulmein, which was my most northern; and, finally, one at Madras, which was my most western station. The Survey is however incomplete, as it would have been desirable to extend it considerably more to the eastward, in order to lay down with greater certainty the continuation, of the line of no dip and of the line of minimum intensity; and likewise I should have wished to proceed more to the northward, to ascertain with greater exactitude, at what distance north of the line of minimum intensity, the magnetic declination changes those periods of extreme easterly and extreme westerly variation, by which it is characterized in the southern magnetic hemisphere.