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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. needle, which after long experience I recom mend 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, is many parts of the earth, bodies which obstruct or intercept the general influence of magnetism; but those interruptions do not inf.inge the the ory. It is allowed, that water will run down a declivity, though sometimes a strong wind may force 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.







SIR, Dec. 1st, 1759. 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?

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.

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 of which the pressure is almost perpendicular.

It has yet been sometimes asserted by hardy 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 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 an ellipsis will bear very little; and that as the arch is more curved, its strength is increased.

Having thus evinced the superior strength of the semicircular arch, we have sufficiently proved, 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; 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 the weakness of the elliptical arch, by arguments 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.

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-Il-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 wide; but the force which laid upon a flat would press directly downwards, is dispersed each way He that in the list of the committee chosen for 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 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 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

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 shull 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 thin perhaps in any other of the world. That there is an elliptical part 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.

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 cireular 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. M- obtained 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 com petition is only between boys, and the prize given to excite laudable industry, not to reward con summate 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 dege. nerated 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 de cay, has its limits; and of mentioning the Monų, ment 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 posi❤ tion 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 supe rior in strength to the semicircle. For this daring assertion he made way by the intermediate paragraphs; in which he observes, that the conterity of a semi-ellipsis may be increased at will to any gree 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 sem circle, 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 sem 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 leal, and large stones, and a very thick arch; assistances which the semicircle does not require, and which can be yet less required by a sema 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 caa be made of his contrariety to others; nor dol think myself entitled to complain of disregard from one, with whom the performances of anth

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 chosen for its own sake, I am not able to discover; but it must not be forgotten, that as the convexity is increased, the difficulty is lessened; and I know not well whether this writer, whe appears equally ambitious of difficulty and studious of strength, will wish to increase the convexity for the gain of strength, or to lessen it for the love of difficulty.

The friend of Mr. M, however he may be mistaken in some of his opinions, does not want the appearance of reason, when he prefers facts to theories; and that I may not dismiss the question without some appeal to facts, I will borrow an example, suggested by a great artist, and recommended to those who may still doubt which of the two arches is the stronger, to press an egg first on the ends, and then upon the sides. I am, Sir, yours, &c.



| cease to be so, and that the most necessary and most indispensable of all professions should have fallen into any contempt.

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: ca:tle, 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 peo- Cato, the censor, has justly called Sicily the ple in the present age; while we cannot help ob- magazine and nursing mother of the Roman peoserving the honour that antiquity has always ple, who were supplied from thence with almost paid to the profession of the husbandman; which all their corn, both for the use of the city, and naturally leads us into some reflections upon that the subsistence of her armies: though we also occasion. find in Livy, that the Romans received no inconThough 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 orca- 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 by famine. Rome actually saw herself reduced to this condition under Augustus; for there remained only three days' provision of corn in the city; and that prince was so full of tenderness

for the people, that he had resolved to poison himself, if the expected fleets did not arrive before the expiration of that time; but they came, and the preservation of the Romans was attributed to the good fortune of their emperor; but wise precautions were taken to avoid the like danger for the future.

state, which inclines him to justice, temperance, sobriety, sincerity, and every virtue that can dignify human nature. This gave room for the poets to feign, that Astræa, the goddess of justice, had her last residence among husbandmen, before she quitted the earth. Hesiod and Virgil have brought the assistance of the muses in praise of agriculture. Kings, generals, and philosophers, have not thought it unworthy their birth, rank, and genius, to leave precepts to pos

When the seat of empire was transplanted to Constantinople, that city was supplied in the same manner; and when the emperor Septimius Severus died, there was corn in the public maga-terity upon the utility of the husbandman's prozines for seven years, expending daily 75,000 bushels in bread, for 600,000 men.

fession. Hiero, Attalus, and Archelaus, kings of Syracuse, Pergamus, and Cappadocia, have composed books for supporting and augmenting the fertility of their different countries. The Carthaginian general Mago wrote twenty-eight volumes upon this subject; and Cato, the censor, followed his example. Nor have Plato, Xe nophon, and Aristotle, omitted this article, which makes an essential part of their politics. And Cicero, speaking of the writings of Xenophon, says, "How fully and excellently does he, in that book called his 'Economics,' set out the advantages of husbandry, and a country life " When Britain was subject to the Romans, she

corn; and the Isle of Anglesea was then looked upon as the granary for the western provinces; but the Britons, both under the Romans and Saxons, were employed like slaves at the plough. On the intermixture of the Danes and Normans, possessions were better regulated, and the state of vassalage gradually declined, till it was entirely worn off under the reigns of Henry VII. and Edward VI.; for they hurt the old nobility by favouring the commons, who grew rich by trade, and purchased estates.

The ancients were no less industrious in the cultivation of the vine than in that of corn, though they applied themselves to it later: for Noah planted it by order, and discovered the use that might be made of the fruit, by pressing out and preserving the juice. The vine was carried by the offspring of Noth into the several countries of the world: but Asia was the first to experience the sweets of this gift; from whence it was imparted to Europe and Africa. Greece and Italy, which were distinguished in so many other respects, were particularly so by the excellency of their wines. Greece was most cele-annually supplied them with great quantities of brated for the wines of Cyprus, Lesbos, and Chio; the forner of which is in great esteem at present: though the cultivation of the vine has been generally suppressed in the Turkish dominions. As the Romans were indebted to the Grecians for the arts and sciences, so were they likewise for the improvement of their wines; the best of which were produced in the country of Capua, and were called the Massick, Calenian, Formian, Cæc ban, and Falernian, so much celebrated by Horace. Domitian passed an edict for destroying all the vines, and that no more should be planted throughout the greatest part of the west; which continued almost two hundred years afterwards, when the emperor Probus employed his soldiers in planting vines in Europe, in the same manner as Hannibal had formerly employed his troops in planting olive-trees in Africa. Some of the ancients have endeavoured to prove, that the cultivation of vines is more beneficial than any other kind of husbandry: but, if this was thought so in the time of Columella, it is very different at present; nor were all the ancients of his opinion, for several gave the preference to pasture lands.

The wines of France, Portugal, and Spain, are now the best; while Italy can only boast of the wine made in Tuscany. The breeding of cattle, is now chiefly confined to Denmark and Ireland. The corn of Sicily is still is in great es teem, as well as what is produced in the northern countries: but England is the happiest spot in the universe for all the principal kinds of agriculture, and especially its great produc of com.

The improvement of our landed estates, is the enrichment of the kingdom; for, without this, how could we carry on our manufactures, or prosecute our commerce? We should look upon the English farmer as the most usefu. member of society. His arable grounds not only supply his fellow-subjects with all kinds of the best grain, but his industry enables him to

The breeding of cattle has always been considered as an important part of agriculture. The riches of Abrahim, Laban, and Job, consisted in their flocks and herds. We also find from La i-export great quantities to other kingdoms, which nus in Virgil, and Ulysses in Homer, that the might otherwise starve: particularly Spain and wealth of those princes consisted in cattle. It Portugal; for in one year, there have been exwas likewise the same among the Romans, till ported 51,520 quarters of barley, 219,781 of malt, the introduction of money, which put a value 1,920 of oatmeal, 1,329 of rye, and 153,343 of upon commodities, and established a new kind wheat; the bounty on which amounted to of barter. Varro has not disdained to give an 72,433 pounds. What a fund of treasure arises extensive account of all the beasts that are of from his pasture lands, which breed such innuany use to the country, either for tillage, breed, merable flocks of sheep, and afford such fine caringe, or other conveniences of man. And herds of cattle, to feed Britons, and clothe manCito, the censor, was of opinion, that the feed-kind! He rears flax and hemp for the making ing of ca tle was the most certain and speedy me hod of enriching a country.

of linen; while his plantations of apples and hops supply him with generous kinds of liquors. Luxury, avarice, injustice, violence, and am- The land-tax, when at four shillings in the bition, take up their ordinary residence in po-pound, produces 2,000,000 pounds a year. This pulous cities; while the hard and laborious life arises from the labour of the husbandman: it is of the husbandman will not admit of these vices. a great sum: but how greatly is it increased by The honest farmer lives in a wise and happy the means it furnishes for trade? Without the

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