By means of this loadstone, the island is made to rise and fall, and move from one place to another. For with respect to that part of the earth over which the monarch presides, the stone is endued at one of its sides with an attractive power, and at the other with a repulsive. Upon placing the magnet erect with its attracting end towards the earth, the island descends; but when the repelling extremity points downwards, the island mounts directly upwards. When the position of the stone is oblique, the motion of the island is so too. For in this magnet the forces always act in lines parallel to its direction.
By this oblique motion the island is conveyed to different parts of the monarch's dominions. To explain the manner of its progress, let AB represent a line drawn cross the dominions of Balnibarbi, let the line cd represent the loadstone, of which let d be the repelling end, and c the attracting end, the island being over C; let the stone be placed in the position cd, with its repelling end downwards; then the island will be driven upwards obliquely towards D. When it is arrived at D, let the stone be turned upon its axle, till its attracting end points towards E, and then the island will be carried obliquely towards E; where if the stone be again turned upon its axle till it stands in the position EF, with its repelling point downwards, the island will rise obliquely towards F, where by directing the attracting end towards G, the island may be carried to G, and from G to H, by turning the stone, so as to make its repelling extremity point directly downwards. And thus by changing the situation of the stone as often as there is occasion, the island is made to rise and fall by turns in an oblique direction, and by those alternate risings and fallings (the obliquity being not considerable) is conveyed from one part of the dominions to the other.
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 in that case the extremities of it being at equal distance from the earth, act with equal force, the one in drawing downwards, the other in pushing upwards, and consequently no motion can ensue.