Gullivers Travels by Part 3 Chapter 3 Page 7

But it must be observed, that this island cannot move beyond the extent of the dominions below, nor can it rise above the height of four miles. For which the astronomers (who have written large systems concerning the stone) assign the following reason: that the magnetic virtue does not extend beyond the distance of four miles, and that the mineral, which acts upon the stone in the bowels of the earth, and in the sea about six leagues distant from the shore, is not diffused through the whole globe, but terminated with the limits of the king’s dominions; and it was easy, from the great advantage of such a superior situation, for a prince to bring under his obedience whatever country lay within the attraction of that magnet.

When the stone is put parallel to the plane of the horizon, the island stands still; for