Besides the errors which the attraction of the iron produced in the compasses at the binnacle of the Investigator, differences are frequently mentioned in the course of this voyage as having been found in the magnetic needle on shore, and on board the ship in the vicinity of land. That there are few masses of stone totally devoid of iron, and that all iron which has long remained in the same position will acquire magnetism, or a power of attracting one end of the magnetic needle towards one part of it, and the opposite end towards another, is, I believe, generally admitted. The kinds of stone which I have observed to exert the greatest influence on the needle, are iron ore, porphyry, granite, and basaltes; and the least, are sand or free stone, and calcareous rock, and the argillaceous earths very little.
The iron in the ship attracted the south end of the needle in the southern hemisphere, and in the same part of the world it was the same end of the needle upon which the land had an attraction. The following are some instances:
In King George's Sound, the west variation was 6° greater on the western head of Michaelmas Island, than it was on the east side of a flat rock in the sound. The stone here is granite.
On approaching the granite islands of the Archipelago of the Recherche, from the west, the corrected variation on board the ship was increased from 5° 25′ to 6° 22′ west, contrary to the regular order; but when Termination Island bore nearly West, and the principal cluster N.N.W., the corrected variation was no more than 0° 51′; and after clearing the Archipelago some distance, it again increased to 4½° west.[1]- ↑ M. Beautemps-Beaupré (in Vol. I. p. 605, of the work before cited) gives the following instance of attraction in the stone of this Archipelago. The compass was placed upon one of the capes of the main land, to set the bearing of a point. "When the beating had been taken, the compass was removed six feet from its place, beyond a large stone; where the vane being by chance directed to the same point, a difference of four degrees was found in the bearing, although the object were so far distant that the change of place should scarcely have produced a difference of one minute. Fully persuaded that we had made an error, either in reading off the bearing or in writing it down, the first observation was verified; but we had the same result within a few minutes as had been marked on the paper, and it was certain that the stone near which the observation had been made, had solely caused this great error."
