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IV. Contributions to Terrestrial 31agnetism. By Major Edward Sabine, R.A. 
V.P.R.S. 
Received March 12, — Read March 19, 1840. 
§ 1. Lines of Inclination and Intensity in the Atlantic Ocean. § 2. Lines of 
Intensity between the Cape of Good Hope and Australia. 
^ 1. Lines of Inclination and Intensity in the Atlantic Ocean. 
The value of observations of the magnetic inclination and intensity made at sea, in 
determining the position and direction of the magnetic lines over those large portions 
of the globe which are covered by the ocean, has been justly pointed out in the scien- 
tific memoranda prepared by the Royal Society for the Antarctic Expedition ; and 
the same publication notices the precautions which are necessary in order to elimi- 
nate, in results so obtained, the disturbing influence of the ship’s iron. 
The observations of Lieut. Bartholomew James Sulivan, R.N., which, by his wish 
and with permission of the Admiralty, will form a part of this communication, afford 
a practical illustration of the valuable and useful information, which a series of such 
observations, conducted with a proper measure of skill and patience, will produce, 
and of the degree of certainty which may be looked for when they are thus conducted, 
and when the requisite precautions are taken. 
At a time when increased activity has been given to magnetic observations, in the 
view of obtaining correct maps of the magnetic phenomena over the whole surface of 
the globe corresponding to the present epoch, it has appeared desirable to place in 
the hands of those engaged in these researches the theoretical maps of M. Gauss, 
accompanied by others in which the results of the most recent and authentic obser- 
vations are inserted in their respective geographical positions ; thus bringing under 
notice, as well as into comparison, the actual state both of theory and of experiment, 
and directing attention to those localities where observations have been most spar- 
ingly distributed, and where, consequently, they are most to be desired. This has 
been done, with regard to the maps of declination and intensity, in the publication of 
the Royal Society already referred to ; and the theoretical map of the inclination has 
been republished from the German original in the Seventh Number of Taylor’s Sci- 
entific Memoirs : but a map which should exhibit the actual observations of the latter 
element is yet wanting. The endeavour is made in the present communication to 
supply this deficiency, as far as regards that portion of the globe which is occupied 
by the Atlantic Ocean and its adjoining coasts. The sea observations which have 
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