As the next century was a time of naval adventures, it might be expected that the variation once observed, should have been well studied:  yet it seems to have been little heeded; for it was supposed to be constant, and always the same in the same place, till, in 1625, Gellibrand noted its changes, and published his observations.

From this time the philosophical world had a new subject of speculation, and the students of magnetism employed their researches upon the gradual changes of the needle’s direction, or the variations of the variation, which have hitherto appeared so desultory and capricious, as to elude all the schemes which the most fanciful of the philosophical dreamers could devise for its explication.  Any system that could have united these tormenting diversities, they seem inclined to have received, and would have contentedly numbered the revolutions of a central magnet, with very little concern about its existence, could they have assigned it any motion, or vicissitude of motions, which would have corresponded with the changes of the needle.

Yet upon this secret property of magnetism I ventured to build my hopes of ascertaining the longitude at sea.  I found it undeniably certain that the needle varies its direction in a course eastward or westward between any assignable parallels of latitude:  and, supposing nature to be in this, as in all other operations, uniform and consistent, I doubted not but the variation proceeded in some established method, though, perhaps, too abstruse and complicated for human comprehension.

This difficulty, however, was to be encountered; and by close and steady perseverance of attention I at last subdued, or thought myself to have subdued it:  having formed a regular system in which all the phenomena seemed to be reconciled; and, being able, from the variation in places where it is known, to trace it to those where it is unknown; or from the past to predict the future; and, consequently, knowing the latitude and variation, to assign the true longitude of any place.

With this system I came to London, where, having laid my proposals before a number of ingenious gentlemen, it was agreed that during the time required to the completion of my experiments, I should be supported by a joint subscription to be repaid out of the reward, to which they concluded me entitled.  Among the subscribers, was Mr. Rowley, the memorable constructor of the orrery; and among my favourers was the lord Piesley, a title not unknown among magnetical philosophers.  I frequently showed, upon a globe of brass, experiments by which my system was confirmed, at the house of Mr. Rowley, where the learned and curious of that time generally assembled.

At this time great expectations were raised by Mr. Whiston, of ascertaining the longitude by the inclination of the needle, which he supposed to increase or diminish regularly.  With this learned man I had many conferences, in which I endeavoured to evince what he has at last confessed in the narrative of his life, the uncertainty and inefficacy of his method.