agnosco, eademque, summa cum voluptate, a me perlecta sunt. Filius meus te plurimum salutat. Vale, doctissime vir, meisque verbis D. Caveum saluta, atque amare perge, Tuum, Dabam Leidis, A. D. xiv. KAL. Maias, A. D. MDCCXLVII. ABRAHAMUM GRONOVIUM. POSTSCRIPT. AND now my character is placed above all suspicion of fraud by authentick documents, I will make bold, at last, to pull off the mask, and declare sincerely the true motive that induced me to interpolate a few lines into some of the authors quoted by me in my Essay on Milton, which was this: Knowing the prepossession in favour of Milton, how deeply it was rooted in many, I was willing to make trial, if the partial admirers of that author would admit a translation of his own words to pass for his sense, or exhibit his meaning; which I thought they would not: nor was I mistaken in my conjecture, forasmuch as several gentlemen, seemingly persons of judgment and learning, assured me, they humbly conceived I had not proved my point, and that Milton might have written as he has done, supposing he had never seen these authors, or they had never existed. Such is the force of prejudice! This exactly confirms the judicious observation of the excellent moralist and poet : Pravo favore labi mortales solent; For, had I designed, as the vindicator of Milton supposes, Grotius, transcribed by his son, Mr. James. The truth of this particular consists perfectly well with the knowledge of the Doctor's brother, John Lawson, esq. counsellor at law; who also had the same thing lately confirmed to him by Mr. Gronovius himself in Holland. to impose a trick on the publick, and procure credit to my assertions by an imposture, I would never have drawn lines from Hog's translation of Milton, a book common at every sale, I had almost said, at every stall, nor ascribed them to authors so easily attained: I would have gone another way to work, by translating forty or fifty lines, and assigning them to an author, whose works possibly might not be found till the world expire at the general conflagration. My imposing, therefore, on the publick in general, instead of a few obstinate persons, for whose sake alone the stratagem was designed, is the only thing culpable in my conduct, for which again I most humbly ask pardon and that this, and this only, was, as no other could be, my design, no one, I think, can doubt, from the account I have just now given; and whether that was so criminal, as it has been represented, I shall leave every impartial mind to determine. AN ACCOUNT OF AN ATTEMPT TO ASCERTAIN THE LONGITUDE. FIRST PRINTED IN THE YEAR 1755. IT is well known to seamen and philosophers, that, after the numerous improvements produced by the extensive commerce of the later ages, the great defect 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 recompenses to those, who, 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. By the splendour 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 ¶ 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. 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 inquiry 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 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 magnetick needle, from the year 1300, when it is generally supposed to have been first applied by Flavio Gioia, of Amalfi, 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 extrinsick 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 Portuguese, who, at the expense of the king of England, 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 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 esta |