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It is well known to seamen and philosophers, of the atmosphere, the effects of different effluvia 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.

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.

That navigation might at length be 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 But when the discovery of the new world not without hopes, that I might be considered turned the attention of mankind upon the naval as an assistant to some greater genius, and re-sciences, and long courses required greater ceive from the justice of my country the wages offered to an honest and not unsuccessful labourer in science.

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 ob servations; 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.

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.

Considering the various means by which this important inquiry has been pursued, I found that the observation of the eclipses, either of the As the next century was a time of naval adprimary or secondary planets, being possible but ventures, it might be expected that the variation at certain times, could be of no use to the sailor; once observed, should have been well studied: that the motions of the moon had been long yet it seems to have been little heeded; for it attended, however accurately, without any con- was supposed to be constant and always the sequence; that other astronomical observations same in the same place, till in 1625 Gellibrand were difficult and uncertain with every advan-noted its changes, and published his observatage of situation, instruments, and knowledge: tions.

and were therefore utterly impracticable to the From this time the philosophical world had a sailor, tost upon the water, ill provided with new subject of speculation, and the students of instruments, and not very skilful in their ap-magnetism employed their researches upon the plication.

gradual changes of the needle's direction, or the The hope of an accurate clock or time-keeper variations of the variation, which have hitherto is more specious. But when I began these stu- appeared so desultory and capricious, as to dies, no movements had yet been made that elude all the schemes which the most fanciful of were not evidently inaccurate and uncertain: the philosophical dreamers could devise for its and even of the mechanical labours which I now explication. Any system that could have united hear so loudly celebrated, when I consider the these tormenting diversities, they seem inclined obstruction of movements by friction, the waste to have received, and would have contentedly of their parts by attrition, the various pressure numbered the revolutions of a central magnet, with very little concern about its existence,

*An Account of an attempt to ascertain the Longitude could they have assigned it any motion, or vicisat Sea, by an exact Theory of the Variation of the Mag-situde of motions, which could have correspondnetical Needle; with a Table of Variations at the most remarkable cities in Europe, from the year 1660 to 1860.ed with the changes of the needle. By Zachariah Williams. 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.

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 improve-
ments 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
of the commissioners of the Admiralty, who en-
assigned him, it was given to Mr. Molineux, one
gaged in it with no great inclination to favour
me; but however thought that one of the instru-
ments, which, to confirm my own opinion, and
to confute Mr. Whiston's, I had exhibited to the
Admiralty, so curious or useful, that he surrepti
tiously copied it on paper, and clandestinely
endeavoured to have it imitated by a workman fo
his own use.

This treatment naturally produced remon continue long, for Mr. Molineux died soon afterstrances and altercations, which indeed did not wards; and my proposals were for a time for gotten.

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 exami


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 fa-nation. vourers 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 endeavored to evince what he has at last confessed in the narrative of his life, the uncertainty and inefficacy of his method.

It is one of the melancholy pleasures of an About the year 1729, my subscribers explained my pretensions to the Lords of the Admiralty, old man, to recollect the kindness of friends, and the Lord Torrington declared my claim just whose kindness he shall experience no more. to the reward assigned in the last clause of the I have now none left to favour my studies; and act to those who should make discoveries con- therefore naturally turn my thoughts on those ducive to the perfection of the art of sailing. by whom I was favoured in better days; and I This he pressed with so much warmth, that the hope the vanity of age may be forgiven, when commissioners agreed to lay my tables before I declare that I can boast among my friends, Sir Isaac Newton, who excused himself, by almost every name of my time that is now rereason of his age, from a regular examination: membered: and that in that great period of but when he was informed that I held the varia-mathematical competition scarce any man failed tion at London to be still increasing, which he to appear as my defender, who did not appear as and the other philosophers, his pupils, thought my antagonist. 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.

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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, exhibited no imperfect system of magnetical attraction. The experiment was shown by Mr. Hawkesbee, and the explanation with which it was accompanied, was read by society; and was solicited to reposit my theory Dr. Mortimer. I received the thanks of the 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 public notice,

and applied myself wholly to the continuation | titude 50 40'; but that this is its true situation, of my experiments, the confirmation of my I cannot be certain. The latitude of many places system, and the completion of my tables, with is unknown, and the longitude is known of very no other companion than Mr. Gray, who shared few; and even those who are unacquainted with all my studies and amusements, and used to re- science, will be convinced that it is not easily to pay my communications of magnetism, with his be found, when they are told how many degrees discoveries in electricity. Thus I proceeded Dr. Halley, and the French mathematicians, with incessant diligence; and perhaps in the place the Cape of Good Hope distant from each zeal of inquiry did not sufficiently reflect on other. 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 error 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 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. la

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 inconveniences 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.

These 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 magnetic poles, which I have long ago begun, and, with just encouragement, should have long ago completed.







Dec. 1st, 1759. SIR, THE Plans which have been offered by different architects, of different reputation and abilities, for the construction of the Bridge intended to be built at Blackfriars, are, by the rejection of the greater part, now reduced to a small number; in which small number, three are supposed to be much superior to the rest; so that only three architects are now properly competitors for the honour of this great employment; by two of whom are proposed semicircular, and by the other elliptical arches.

The question is, therefore, whether an elliptical or semicircular arch is to be preferred?

may be demonstrated to excel in strength the elliptical arch, which approaching nearer to a straight line, must be constructed with stones whose diminution downwards is very little, and It has yet been sometimes asserted by hardy of which the pressure is almost perpendicular. ignorance, that the elliptical arch is stronger than the semicircular; or in other terms, that any mass is more strongly supported the less it rests upon the supporters. If the elliptical arch be equally strong with the semicircular, that is, if an arch, by approaching to a straight line, loses ation is useless, and that the bridge may at last none of its stability, it will follow, that all arcuin straight lines from pillar to pillar. But if a without any inconvenience, consist of stone laid straight line will bear no weight, which is evident at the first view, it is plain likewise, that an ellipsis will bear very little; and that as the arch Having thus evinced the superior strength of is more curved, its strength is increased. that it ought to be preferred; but to leave no obthe semicircular arch, we have sufficiently proved, Those who are acquainted with the mathe-jection unprevented, we think it proper likewise matical principles of architecture, are not many; and yet fewer are they who will, upon any single occasion, endure any laborious stretch of thought, or harass their minds with unaccustomed investigations. We shall therefore attempt to show If in opposition to these arguments, and in dethe weakness of the elliptical arch, by arguments which appear simply to common reason, and which will yet stand the test of geometrical ex-fiance at once of right reason and general auamination.

The first excellence of a bridge built for commerce over a large river, is strength; for a bridge which cannot stand, however beautiful, will boast its beauty but a little while; the stronger arch is therefore to be preferred, and much more to be preferred, if with greater strength it has greater beauty.

to observe, that the elliptical arch must always appear to want elevation and dignity; and that if beauty be to be determined by suffrages, the elliptical arch will have little to boast, since the only bridge of that kind has now stood two hundred years without imitation.

thority, the elliptical arch should at last be chosen, what will the world believe, than that some other motive than reason influenced the determination? And some degree of partiality cannot but be suspected by him, who has been told that one of the judges appointed to decide this question, is Mr. M-1-r, who having by ignorance, or thoughtlessness, already preferred the elliptical arch, will probably think himself obliged to maintain his own judgment, though his opinion will avail but little with the public, when it is known that Mr. S-ps-n declares it to be false.

All arches have a certain degree of weakness. No hollow building can be equally strong with a solid mass, of which every upper part presses perpendicularly upon the lower. Any weight laid upon the top of an arch, has a tendency to force that top into the vacuity below; and the arch thus loaded on the top, stands only because the stones that form it, being wider in the upper than in the lower parts, that part that fills a wider space cannot fall through a space less He that in the list of the committee chosen for wide; but the force which laid upon a flat would press directly downwards, is dispersed each way in a lateral direction, as the parts of a beam are the superintendency of the bridge, reads many pushed out to the right and left by a wedge driven of the most illustrious names of this great city, reverence for the opinion of posterity, than to between them. In proportion as the stones are will hope that the greater number will have more wider at the top than at the bottom, they can less easily be forced downwards, and as their disgrace themselves, and the metropolis of the lateral surfaces tend more from the centre to each kingdom, in compliance with any man, who, inside, to so much more is the pressure directed stead of voting, aspires to dictate, perhaps withness of birth, dignity of employment, extent of laterally towards the piers, and so much less per-out any claim to such superiority, either by greatpendicularly towards the vacuity.

Upon this plain principle the semicircular arch | knowledge, or largeness of fortune.




Dec. 8th, 1759.

In questions of general concern, there is no law of government or rule of decency, that for bids open examination and public discussion. I shall therefore not betray, by a mean apology, that right which no man has power, and, I suppose, no wise man has desired to refuse me; but shall consider the Letter published by you last Friday, in defence of Mr. M-'s* design for a new bridge. Mr. M.proposes elliptical arches. It has been objected that elliptical arches are weak: and therefore improper for a bridge of commerce, in a country where greater weights are ordinarily carried by land than perhaps in any other part of the world. That there is an elliptical bridge at Florence is allowed, but the objectors maintain, that its stability is so much doubted, that carts are not permitted to pass over it.

To this no answer is made, but that it was built for coaches; and if it had been built for earts, it would have been made stronger: thus all the controvertists agree, that the bridge is too weak for carts; and it is of little importance, whether carts are prohibited because the bridge is weak, or whether the architect, knowing that carts were prohibited, voluntarily constructed a weak bridge. The instability of the elliptical arch has been sufficiently proved by argument, and Ammanuti's attempt has proved it by example.

The iron rail, whether gilt or varnished, appears to me unworthy of debate. I suppose every judicious eye will discern it to be minute and trifling, equally unfit to make a part of a great design, whatever be its colour. I shall only observe how little the writer understands his own positions, when he recommends it to be cast in whole pieces from pier to pier. That iron forged is stronger than iron cast, every smith can inform him; and if it be cast in large pieces, the fracture of a single bar must be repaired by a new piece.

The abrupt rise which is feared from firm circular arches, may be easily prevented, by a little extension of the abutment at each end, which will take away the objection, and add almost nothing to the expense.

The whole of the argument in favour of Mr. M, is only that there is an elliptical bridge at Florence, and an iron balustrade at Rome; the bridge is owned to be weak, and the iron balustrade we consider as mean; and are loth that our own country should unite two follies in a public work.

The architrave of Perault, which has been pompously produced, bears nothing but its entablature; and is so far from owing its support to the artful section of the stone, that it is held together by cramps of iron; to which I am afraid Mr. M must have recourse, if he persists in his ellipsis, or, to use the words of his vindicator, forms his arch of four segments of circles drawn from four different centres.

That Mr. Mobtained the prize of the architecture at Rome, a few months ago, is willingly confessed; nor do his opponents doubt that he obtained it by deserving it. May he

* Mr. Mylne.

continue to obtain whatever he deserves; but let it not be presumed that a prize granted at Rome, implies an irresistible degree of skill. The competition is only between boys, and the prize given to excite laudable industry, not to reward consummate excellence. Nor will the suffrage of the Romans much advance any name among those who know, what no man of science will deny, that architecture has for some time degenerated at Rome to the lowest state, and that the Pantheon is now deformed by petty decorations. I am, Sir, yours, &c,

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It is the common fate of erroneous positions, that they are betrayed by defence, and obscured by explanation; that their authors deviate from the main question into incidental disquisitions, and raise a mist where they should let in light.

Of all these concomitants of errors, the Letter of Dec. 10th, in favour of elliptical arches, has afforded examples. A great part of it is spent upon digressions. The writer allows, that the first excellence of a bridge is undoubtedly strength : but this concession affords him an opportunity of telling us, that strength, or provision against decay, has its limits; and of mentioning the Monument and Cupola, without any advance towards evidence or argument.

The first excellence of a bridge is now allowed to be strength; and it has been asserted, that a semi-ellipsis has less strength than a semicircle. To this he first answers, that granting this position for a moment, the semi-ellipsis may yet have strength sufficient for the purpose of commerce. This grant, which was made but for a moment, needed not to have been made at all; for before he concludes his Letter, he undertakes to prove that the elliptical arch must in all respects be superior in strength to the semicircle. For this daring assertion he made way by the intermediate paragraphs; in which he observes, that the convexity of a semi-ellipsis may be increased at will to any degree that strength may require: which is, that an elliptical arch may be made less elliptical, to be made less weak; or that an arch, which by its elliptical form is superior in strength to the semicircle, may become almost as strong as a semicircle, by being made almost semicircular.

That the longer diameter of an ellipsis may be shortened, till it shall differ little from a circle, is indisputably true; but why should the writer forget the semicircle differs as little from such an ellipsis? It seems that the difference, whether small or great, is to the advantage of the semicircle; for he does not promise that the elliptical arch, with all the convexity that his imagination can confer, will stand without cramps of iron, and melted lead, and large stones, and a very thick arch; assistances which the semicircle does not require, and which can be yet less required by a semiellipsis, which is in all respects superior in strength.

Of a man who loves opposition so well, as to be thus at variance with himself, little doubt can be made of his contrariety to others; nor do I think myself entitled to complain of disregard from one, with whom the performances of anu

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