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It is well kumn to seamaa and poschers after the numera imprirements produced the extensive ommerce of the later ages, the great deText in the art of sailing is ignorance of longitude, or of the distance to which the ship has passed eastward or westward, from any given meridian.

That navigation might be at length set free from this uncertainty, the legislative power of this kingdom incited the industry of searchers into nature, by a large reward proposed to him who should show a practicable method of finding the longitude at sea; and proportionable recompences to those, wko, though they should not fully attain this great end, might yet make such advances and discoveries as should facilitate the work to those that might succeed them.

• An Account of an attempt to ascertain the Longitude at Sea, by an exact Theory of the Variation of the Magnetical Needle ; with a Table of Variations at the most remarkable cities in Europe, from the year 1660 to 1860. By Zachariah Williams.

By the splendor of this golden encouragement many eyes were dazzled, which nature never intended to pry into her secrets. By the hope of sudden riches many understandings were set on work very little proportioned to their strength, among whom whether mine shall be numbered, must be left to the candour of posterity: for I, among others, laid aside the business of my profession, to apply myself to the study of the longitude, not indeed in expectation of the reward due to a complete discovery; yet not without hopes, that I might be considered as an assistant to some greater genius, and receive from the justice of my country the wages offered to an honest and not unsuccessful labourer in science.

Considering the various means by which this important enquiry has been pursued, I found that the observation of the eclipses, either. of the primary or secondary planets, being possible but at certain times, could be of no use to the sailor; that the motions of the moon had been long attended, however accurately, without any consequence; that other astronomical observations were difficult and uncertain with every advantage of situation, instruments and knowledge ; and were therefore utterly impracticable to the sailor, tost upon the water, ill provided with instruments, and not very skilful in their application.

The hope of an accurate clock or time-keeper is more specious. But when I began these studies, no movements had yet been made that were not evidently unaccurate and uncertain : and even of the mechanical labours which I now hear so loudly celebrated, when I consider the obstruction of movements VOL. X.

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by friction, the waste of their parts by attrition, the various pressure of the atmosphere, the effects of different effluvia upon metals, the power of heat and cold upon all matter, the changes of gravitation and the hazard of concussion, I cannot but fear that they will supply the world with another instance of fruitless ingenuity, though I hope they will not leave upon this country the reproach of unrewarded diligence. · I saw therefore nothing on which I could fix with probability of success, but the magnetical needle, an instrument easily portable, and little subject to accidental injuries, with which the sailor has had a long acquaintance, which he will willingly study, and can easily consult.

The magnetic needle from the year 1300, when it is generally supposed to have been first applied by John Goia, of Amalphi, to the seaman's use, seems to have been long thought to point exactly to the north and south by the navigators of those times who sailing commonly on the calm Mediterranean, or making only short voyages, had no need of very accurate observations; and who, if they ever transiently observed any deviations from the meridian, either ascribed them to some extrinsic and accidental cause, or willingly neglected what it was not necessary to understand.

But when the discovery of the new world turned the attention of mankind upon the naval sciences, and long courses required greater niceties of practice, the variation of the needle soon became observable, and was recorded in 1500 by Sebastian Cabot, a Por

tuguese, who, at the expence of the king of Eng. land, discovered the northern coasts of America.

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 tornienting 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 lati. tude : 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

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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 phænomena 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. 1 frequently shewed 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 narra

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