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. 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 per- When Sir Isaac Newton had declined the office haps too abstruse and complicated for human assigned him, it was given to Mr. Molineux, one comprehension. of the commissioners of the Admiralty, who en This difficulty however was to be encoun-gaged in it with no great inclination to favour tered; and by close and steady perseverance of me; but however thought that one of the instru attention I at last subdued, or thought myself ments, which, to confirm my own opinion, and 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 endeavored 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. 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 for his own use. This treatment naturally produced remon strances and altercations, which indeed did not continue long, for Mr. Molineux died soon afterwards; 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 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 1 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, 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 Dr. Mortimer. I received the thanks of the society; and was solicited to reposit my theory properly sealed and attested among their ar chives, 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 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 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 titude 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 Čape 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 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 the ory. 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 philo sopher; 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. CONSIDERATIONS ON THE PLANS OFFERED FOR THE CONSTRUCTION OF BLACKFRIARS BRIDGE. IN THREE LETTERS, TO THE PRINTER OF THE GAZETTEER. SIR, LETTER I. may be demonstrated to excel in strength the Dec. 1st, 1759. elliptical arch, which approaching nearer to a straight line, must be constructed with stones THE Plans which have been offered by differ- whose diminution downwards is very little, and ent architects, of different reputation and abili- of which the pressure is almost perpendicular. ties, for the construction of the Bridge intended It has yet been sometimes asserted by hardy to be built at Blackfriars, are, by the rejection ignorance, that the elliptical arch is stronger than of the greater part, now reduced to a small num-the semicircular; or in other terms, that any ber; 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? 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 none of its stability, it will follow, that all arcuation is useless, and that the bridge may at last without any inconvenience, consist of stone laid in straight lines from pillar to pillar. But if a straight line will bear no weight, which is evident at the first view, it is plain likewise, that un ellipsis will bear very little; and that as the arch is more curved, its strength is increased. 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 Having thus evinced the superior strength of preferred, if with greater strength it has greater the semicircular arch, we have sufficiently proved, beauty. that it ought to be preferred; but to leave no obThose who are acquainted with the mathe-jection unprevented, we think it proper likewise matical principles of architecture, are not many; to observe, that the elliptical arch must always and yet fewer are they who will, upon any single appear to want elevation and dignity; and that occasion, endure any laborious stretch of thought, | if beauty be to be determined by suffrages, the or harass their minds with unaccustomed inves- elliptical arch will have little to boast, since the tigations. We shall therefore attempt to show only bridge of that kind has now stood two hunthe weakness of the ellipticul arch, by arguments dred years without imitation. which appear simply to common reason, and If in opposition to these arguments, and in dewhich will yet stand the test of geometrical ex-fiance at once of right reason and general auamination. thority, the elliptical arch should at last be chr sen, what will the world believe, than that some other motive than reason influenced the determination? And some degree of partiality cannot but be sus pected 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 eliptical 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 wide; but the force which laid upon a flat wontd 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, 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 reverence for the opinion of posterity, than to less easily be forced downwards, and as their disgrace themselves, and the metropolis cf 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 with laterally towards the piers, and so much less per-out any claim to such superiority, either by greatpendicularly towards the vacuity. ness of birth, dignity of employment, extent of He that in the list of the committee chosen for Upon this plain principle the semicircular arch | knowledge, or largeness of fortune. 582 SIR LETTER II. Dec. 8th, 1759. In questions of general concern, there is no law of government or rule of decency, that forbids 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 carts, 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. 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, 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 iron rail, whether gilt or varnished, ap- The first excellence of a bridge is now allowed pears to me unworthy of debate. I suppose to be strength; and it has been asserted, that a every judicious eye will discern it to be minute semi-ellipsis has less strength than a semicircle. and trifling, equally unfit to make a part of a To this he first answers, that granting this posigreat design, whatever be its colour. I shall only tion for a moment, the semi-ellipsis may yet have observe how little the writer understands his own strength sufficient for the purpose of commerce. positions, when he recommends it to be cast in This grant, which was made but for a moment, whole pieces from pier to pier. That iron forged needed not to have been made at all; for before is stronger than iron cast, every smith can in- he concludes his Letter, he undertakes to prove form him; and if it be cast in large pieces, the that the elliptical arch must in all respects be supefracture of a single bar must be repaired by arior in strength to the semicircle. For this daring 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. assertion he made way by the intermediate paragraphs; in which he observes, that the converily of a semi-ellipsis may be increased at will to any degree that strength may require: which is, that aa 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 The architrave of Perault, which has been ellipsis? It seems that the difference, whether pompously produced, bears nothing but its en-small or great, is to the advantage of the semitablature; 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. circle; 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 semi ellipsis, 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 anti quity have so little weight: yet in defiance of all this contemptuous superiority, I must again venture to declare, that a straight line will bear no weight; being convinced, that not even the science of Vasari can make that form strong which the laws of nature have condemned to weakness. By the position, that a straight line | will bear nothing, is meant, that it receives no strength from straightness; for that many bodies, laid in straight lines, will support weight by the cohesion of their parts, every one has found, who has seen dishes on a shelf, or a thief upon the gallows. It is not denied, that stones may be so crushed together by enormous pressure on each side, that a heavy mass may safely be laid upon them; but the strength must be derived merely from the lateral resistance; and the line so loaded will be itself part of the load. The semi-elliptical arch has one recommendation yet unexamined; we are told that it is dif ficult of execution. Why difficulty should be The friend of Mr. M, however he may SOME THOUGHTS ON AGRICULTURE, WITH AN ACCOUNT OF THE HONOUR DUE TO AN ENGLISH FARMER cease to be so, and that the most necessary and AGRICULTURE, in the primeval ages, was the | common parent of traffic: for the opulence of mankind then consisted in cattle, and the product of tillage; which are now very essential for the Agriculture was in no part of the world in promotion of trade in general, but more particu- higher consideration than Egypt, where it was larly so to such nations as are most abundant in the particular object of government and policy: cattle, corn, and fruits. The labour of the farmer nor was any country ever better peopled, richer, gives employment to the manufacturer, and yields or more powerful. The Satrapa, among the Asa support for the other parts of the community: syrians and Persians, were rewarded, if the lands it is now the spring which sets the whole grand in their governments were well cultivated; but machine of commerce in motion; and the sail were punished, if that part of their duty was ne could not be spread without the assistance of theglected. Africa abounded in corn, but the most plough. But though the farmers are of such famous countries were Thrace, Sardinia, and utility in a state, we find them in general too Sicily. much disregarded among the politer kind of people in the present age; while we cannot help observing the honour that antiquity has always paid to the profession of the husbandman; which naturally leads us into some reflections upon that occasion. Cato, the censor, has justly called Sicily the magazine and nursing mother of the Roman peo ple, who were supplied from thence with almost all their corn, both for the use of the city, and the subsistence of her armies: though we also find in Livy, that the Romans received no incon Though mines of gold and silver should be ex-siderable quantities of corn from Sardinia. But, hausted, and the species made of them lost; when Rome had made herself mistress of Carthough diamonds and pearls should remain con- thage and Alexandria, Africa and Egypt became cealed in the bowels of the earth, and the womb her storehouses: for those cities sent such nuof the sea; though commerce with strangers be merous fleets every year, freighted with corn, to prohibited; though all arts which have no other Rome, that Alexandria alone annually supplied object than splendour and embellishment, should twenty millions of bushels: and, when the harbe abolished; yet the fertility of the earth alone | vest happened to fail in one of these provinces, would afford an abundant supply for the occa- the other came in to its aid, and supported the sions of an industrious people, by furnishing sub-metropolis of the world; which, without this sistence for them, and such armies as should be mustered in their defence. We, therefore, ought not to be surprised, that agriculture was in so much honour among the ancients: for it ought rather to seem wonderful that it should ever supply, would have been in danger of perishing 1 |