Page:A Voyage to Terra Australis Volume 2.djvu/544

530 dipped about 18° only, I had some observations which countenance this supposition, though taken at too great a distance to afford a satisfactory proof.

The attraction of land has here been supposed to be so far uniform, as that, in high dips of the needle, the upper part has the same kind of attraction as the magnetic pole of its hemisphere; and that near the magnetic equator, the north end of the land attracts the south point of the compass, and vice versa; but it must be evident, that not only should there be a regular gradation from one to the other, as the dip increases or diminishes, but also that local discordances may take place in both cases, where, instead of one extended magnetic substance pervading the whole land, there are many detached masses, veins, or blocks. Each one of these will possess a north and a south polarity; and contrary attractions may therefore be found at different degrees of elevation and in short intervals of space, without the supposition of the general attraction in land being thereby overturned.

That even, small differences in elevation may produce a change in the magnetic needle appears from some instances in this voyage, where observations were taken on shore, either in the same spot or a few feet from it, with the ship's azimuth compasses and with a theodolite. The compasses stood on the ground, the theodolite upon legs about four and a half feet high, and the variations were as under: Some part of these differences might arise from erroneous construction of the instruments, but only a small part; for they were scarcely sensible in other cases where no hills existed in the neighbourhood, or where the theodolite was placed on the highest land. It is to be remarked, that the compasses come nearer to what appeared to be the true variation than did the theodolite; which I should attribute to the attraction being more equal all round, upon the instrument placed on the earth, and to the theodolite being influenced by the neighbouring hills.