AN ACCOUNT OF AN ATTEMPT TO ASCERTAIN THE LONGITUDE[1].
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 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 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.
About the year 1729, my subscribers explained my pretensions to the lords of the Admiralty, and the lord Torrington declared my claim just to the reward assigned, in the last clause of the act, to those who should make discoveries conducive to the perfection of the art of sailing. This he pressed with so much warmth, that the commissioners agreed to lay my tables before Sir Isaac Newton, who excused himself, by reason of his age, from a regular examination: but when he was informed that I held the variation at London to be still increasing; which he and the other philosophers, his pupils, thought to be then stationary, and on the point of regression, he declared that he believed my system visionary. I did not much murmur to be for a time overborne by that mighty name, even when I believed that the name only was against me: and I have lived till I am able to produce, in my favour, the testimony of time, the inflexible enemy of false hypotheses; the only testimony which it becomes human understanding to oppose to the authority of Newton.
My notions have, indeed, been since treated with equal superciliousness by those who have not the same title to confidence of decision; men who, though, perhaps, very learned in their own studies, have had little acquaintance with mine. Yet even this may be borne far better than the petulance of boys, whom I have seen shoot up into philosophers by experiments which I have long since made and neglected, and by improvements which I have so long transferred into my ordinary practice, that I cannot remember when I was without them.
When Sir Isaac Newton had declined the office assigned him, it was given to Mr. Molineux, one of the commissioners of the Admiralty, who engaged in it with no great inclination to favour me; but, however, thought one of the instruments, which, to confirm my own opinion, and to confute Mr. Whiston's, I had exhibited to the Admiralty, so curious or useful, that he surreptitiously copied it on paper, and clandestinely endeavoured to have it imitated by a workman for his own use.
This treatment naturally produced remonstrances and altercations, which, indeed, did not continue long, for Mr. Molineux died soon afterwards; and my proposals were for a time forgotten.
I will not, however, accuse him of designing to condemn me, without a trial; for he demanded a portion of my tables to be tried in a voyage to America, which I then thought I had reason to refuse him, not yet knowing how difficult it was to obtain, on any terms, an actual examination.
About this time the theory of Dr. Halley was the chief subject of mathematical conversation; and though I could not but consider him as too much a rival to be appealed to as a judge, yet his reputation determined me to solicit his acquaintance and hazard his opinion. I was introduced to him by Mr. Lowthorp and Dr. Desaguliers, and put my tables into his hands; which, after having had them about twenty days under consideration, he returned in the presence of the learned Mr. Machin, and many other skilful men, with an entreaty that I would publish them speedily; for I should do infinite service to mankind.
It is one of the melancholy pleasures of an old man, to recollect the kindness of friends, whose kindness he shall experience no more. I have now none left to favour my studies; and, therefore, naturally turn my thoughts on those by whom I was favoured in better days: and I hope the vanity of age may be forgiven, when I declare that I can boast among my friends, almost every name of my time that is now remembered: and that, in that great period of mathematical competition, scarce any man failed to appear as my defender, who did not appear as my antagonist.
By these friends I was encouraged to exhibit to the Royal Society, an ocular proof of the reasonableness of my theory by a sphere of iron, on which a small compass moved in various directions, exhibiting no imperfect system of magnetical attraction. The experiment was shown by Mr. Hawkesbee, and the explanation, with which it was accompanied, was read by Dr. Mortimer. I received the thanks of the society; and was solicited to reposit my theory, properly sealed and attested, among their archives, for the information of posterity. I am informed, that this whole transaction is recorded in their minutes.
After this I withdrew from publick notice, and applied myself wholly to the continuation of my experiments, the confirmation of my system, and the completion of my tables, with no other companion than Mr. Gray, who shared all my studies and amusements, and used to repay my communications of magnetism, with his discoveries in electricity. Thus I proceeded with incessant diligence; and, perhaps, in the zeal of inquiry, did not sufficiently reflect on the silent encroachments of time, or remember, that no man is in more danger of doing little, than he who flatters himself with abilities to do all. When I was forced out of my retirement, I came loaded with the infirmities of age, to struggle with the difficulties of a narrow fortune; cut off by the blindness of my daughter from the only assistance which I ever had; deprived by time of my patron and friends; a kind of stranger in a new world, where curiosity is now diverted to other objects, and where, having no means of ingratiating my labours, I stand the single votary of an obsolete science, the scoff of puny pupils of puny philosophers.
In this state of dereliction and depression, I have bequeathed to posterity the following table; which, if time shall verify my conjectures, will show that the variation was once known; and that mankind had once within their reach an easy method of discovering the longitude.
I will not, however, engage to maintain, that all my numbers are theoretically and minutely exact: I have not endeavoured at such degrees of accuracy as only distract inquiry without benefiting practice. The quantity of the variation has been settled partly by instruments, and partly by computation: instruments must always partake of the imperfection of the eyes and hands of those that make, and of those that use them: and computation, till it has been rectified by experiment, is always in danger of some omission in the premises, or some errour in the deduction.
It must be observed, in the use of this table, that though I name particular cities, for the sake of exciting attention, yet the tables are adjusted only to longitude and latitude. Thus when I predict that, at Prague, the variation will in the year 1800 be 24-1/4 W. I intend to say, that it will be such, if Prague be, as I-have placed it, after the best geographers in longitude, 14 30'. E. latitude 50 40'. but that this is its true situation I cannot be certain. The latitude of many places is unknown, and the longitude is known of very few; and even those who are unacquainted with science will be convinced that it is not easily to be found, when they are told how many degrees Dr. Halley, and the French mathematicians, place the cape of Good Hope distant from each other.
Those who would pursue this inquiry with philosophical nicety, must, likewise, procure better needles than those commonly in use. The needle, which, after long experience, I recommend to mariners, must be of pure steel, the spines and the cap of one piece, the whole length three inches, each spine containing four grains and a half of steel, and the cap thirteen grains and a half.
The common needles are so ill formed, or so unskilfully suspended, that they are affected by many causes besides magnetism; and, among other inconveniencies, have given occasion to the idle dream of a horary variation.
I doubt not but particular places may produce exceptions to my system. There may be, in many parts of the earth, bodies which obstruct or intercept the general influence of magnetism; but those interruptions do not infringe the theory. It is allowed, that water will run down a declivity, though sometimes a strong wind may force it upwards. It is granted, that the sun gives light at noon, though, in certain conjunctions, it may suffer an eclipse.
Those causes, whatever they are, that interrupt the course of the magnetical powers, are least likely to be found in the great ocean, when the earth, with all its minerals, is secluded from the compass by the vast body of uniform water. So that this method of finding the longitude, with a happy contrariety to all others, is most easy and practicable at sea.
This method, therefore, I recommend to the study and prosecution of the sailor and philosopher; and the appendant specimen I exhibit to the candid examination of the maritime nations, as a specimen of a general table, showing the variation at all times and places for the whole revolution of the magnetick poles, which I have long ago begun, and, with just encouragement, should have long ago completed.
[1] 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.
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Samuel Johnson's essay: An Account Of An Attempt To Ascertain The Longitude