and another at Souterwode. A froft happened juft after the country round Leyden had been overflowed; by which means he was enabled to take two ftations upon the ice, the distance between which he carefully measured 3 times over; and then from these stations he observed the angles which the vifual rays pointing at thofe towers made with the ftraight line upon the ice. From thefe confiderations profeffor MUSCHENBROEK was induced to make new calculations and form triangles upon the fundamental base of Snellius, which he did in 1700; and from thefe he afligns 57,033 toifes to a degree, which is only 27 lefs than had been done by the academicians.

In confequence of various opinions being entertained concerning the true figure of the earth, and the magnitude of a degree upon its furface, Meff. MAUPERTUIS, CLAIRAULT, CAMUS, and OUTHEIR, of France, were fent by Lewis XV. to measure an arch of the meridian in the northern regions of the earth.

They began their operations, affifted by M. CELSUS, an eminent aftronomer of Sweden, in Swedish Lapland, in July 1736; and finished them by the end of May following. They obtained the measure of that degree whofe middle point was in lat. 66° 20′ N. and found it to be $7,439 toifes when reduced to the level of the fea. About the fame time another company of Philofophers were sent to South America, viz. Meff. GODIN, BOUGUER, and CONDAMINE, of France; To whom were joined Don JORGE JUAN, and Don ANTONIO DE ULLOA, of Spain. They left Europe in 1735, and began their operations in the province of Quito in Peru, in October 1736, and finished them after many interruptions in about 8 years. The Spanish gentlemen published a feparate account, and aligned for a measure of a degree of the meridian, at the equator, 56,768 toifes. M. Bouger makes it 56,753 toifes, when reduced to the level of the fea; and M. Coudamine ftates it at 49,749 toifes.

M. LA CAILLE, being at the Cape of Good Hope in 1752, found the length of a degree of the meridian in lat. 33° 18' 30 S. to be 57,037 toifes. In 1755, Father BoscovICH found the length of a degree in lat. 43° to be 56,972 toises, as measured between Rome and Rimini in Italy.

In 1740, Meffrs Caffini again examined the former measures in France; and, after making all the neceffary corrections, found the measure of a degree whofe middle point is in lat. 49° 22' N. to be 57,074 toifes; and in the lat. of 48° it was $7,050 toifes. In 1764, F. BECCARTA, having measured a portion of the meridian near Turin, found the length of a degree whofe middle point was 44° 44' N. to be 57,024 toifes. Vienna 3 degrees of the meridian were measured, from which it may be concluded that a degree in lat. 47° 40′ N. may be reckoned to be 57,091 Paris toifes. In 1766, Meffers MASON and DIXON measured a part of the meridian in Maryland and Pennsylvania, and found that the length of a degree whofe middle point is 39° 12' N. was 36,763 English feet, or 56,904 toiles.

that of ERATOSTHENES, thought the beft of when antiquity can boaft, was nevertheless exceedingly imperfect and inaccurate. It contained little more than the states of Greece, and the dominions of the fucceffors of Alexander, digested according to the furveys above mentioned. He had feen, indeed, and has quoted the voyages of PYTHEAS into the great Atlantic ocean, which gave him fome faint idea of the western parts of Europe; but fo imperfect, that they could not be realifed into the ones of a chart. Strabo fays, he was extremely gnorant of Gaul, Spain, Germany, Britain, Italy, the coa's or the Adriatic, Pontus, and all the countries towards the north. He made the diftance between Epidamnus or Dyrrhachium on the Adriatic, and the bay of Therme on the Ægean fea, to be only 900 ftadia, when in reality it was above coo; and enlarged the distance from Carthage to Alexandria to 15,000 ftadia, when in reality it was only 9000.

Such was the ftate of geography and the nature of the maps prior to the time of HIPPARCHUS who made a clofer connection between geogra phy and aftronomy, by determining the longi tudes and latitudes from celeftial obfervations. But the leading steps to this new projection of the sphere had been in a great measure made eafy by ARCHIMEDES, upwards of go years before the time of Hipparchus, when he invented his noble theorems for measuring the furface of a sphere and its different fegments.

War has been often the occafion of making or improving the maps of different countries; and therefore geography made great advances from the progrefs of the Roman arms. In all the provinces occupied by the people, camps were every where conftructed at proper intervals; and roads were raised with substantial materials, for making an eafier communication between them; and thus civilization and surveying were carried on accord ing to fyftem throughout the extent of that large empire. Every new war produced a new furvey and itinerary of the countries where the scenes of action paffed; fo that the materials of geography were accumulated by every additional conqueft. Polybius tells us, that at the begining of the fecond Punic war, when HANNIBAL was preparing his expedition against Rome, the countries through which he was topals were carefully meafured by the Romans.

JULIUS CAESAR caufed a general urvey of the Roman empire to be made, by a decree of the fenate. Three ferveyors, ZENODOXUS, THEODOTUS, and POLYCLITUS, had this talk affigned them, and are faid to have completed it in 25 years. The Roman itineraries, that are ftill extant, At alfo fhow what care and pains they had been at,, in making furveys in all the different provinces of their empire; and Pliny has filled the 3d, 4th, and 5th, books of his Natural Hiftory with the geographical distances that were thus measured. Another fet of maps are ftil preferved, known bự the name of the Peutingerian Tables, published by Weifer and Bertius, which give a fufficient speci men of what Vegetius calls the litera Piča, for the clearer direction of their armies in their march.

The inveftigation of this problem of the circumference of the earth was effentially neceflary for determining the radical principics et ali map VOL. X. PART I.

The Roman empire had been enlarged to its S9 greates

as far as the fea, which appears to furround the extremity, have been more fully disclosed. On the whole the geography of the utmost extremities of the globe is now in a fair way of being much better known to the moderns, than that of the moft adjacent countries was to the ancients.

greateft extent, and all its provinces well known northern boundaries of that vaft continent, even and furveyed, when PTOLEMY, in the reign of Antoninus Pius, about A. D. 150, composed his fyftem of geography. The principal materials he made ufe of for compofing this work, were the proportions of the gnomon to its shadow, taken by different aftronomers at the time of the equinoxes and folftices: calculation founded upon the length of the longest days: the measure or computed diftances of the principal roads contained in their furveys and itineraries; and the various reports of travellers and navigators, who often determined the distances of places by hearfay and conjecture. All these were compared together, and digested into one uniform body or fyftem; and afterwards were tranflated by him into a new mathematical language, expreffing the different degrees of longitude and latitude, according to the invention of Hipparchus; but which Ptolemy had the merit of carrying into full practice and execution, after it had been neglected for upwards of 250 years. With fuch imperfect and inaccuratè materials, it is no wonder to find many errors in Ptolemy's fyftem. Neither were thefe errors fuch as had been introduced in the more diftant extremities of his maps, but even in the very centre of that part of the world which was beft known to the ancient Greeks and Romans, and where all the famed ancient aftronomers had made their obfervations. Yet this fyftem, with all its imperfections, continued in vogue till the end of the 15th century..

The improvements in geography, which, fince that period, have taken place, were owing to the great progrefs made in aftronomy by Copernicus. Galileo, Newton, and other eminent men who lived within thefe 3 laft centuries. More correct methods and instruments for obferving the latitude were found out; and the difcovery of Jupiter's fatellites afforded a much eafter method of find ing the longitudes than was formerly known. The voyages alfo made by celebrated navigators of different nations, which were now become much more frequent than formerly, brought to the konwledge of the Europeans a vaft number of countries totally unknown to them before.

Geographical knowledge, however, appears to have received greater additions, within the laft 40 years, than it had done in the same space at any former period. In the three voyages of Cook, and thofe of fucceeding navigators, particularly La Peyroufe, Marchand, Vancouver, and Pages, important difcoveries have been made in the great South Sea and in other parts of the world; (See NEW HOLLAND, &c.) and the accounts of thefe navigators afford us more exact furveys of the coatts of the countries they vifited, than could have been found from any that preceded them, or than even the moft fanguine could before have expected. In this refpect alfo, the accounts of the late embaflies to China, Tibet, and Ava, are very valuable, and the refearches of the Afiatic Society have added greatly to our knowledge of Hindooftan, while the exertions of the African fociety have been no lefs remarkable for dispelling our ignorance of the interior of that country, which has been fo far explored by Park, Brown, and Barrow. Our knowedge of America is likewife daily increafing; and ia the journeys of Hearne and Mackenzie, the

Still however, it must be owned, that geography is a science even yet far from perfection. Mr Pinkerton in his Geography, (an excellent work pub. lithed within thefe few years,) fpeaking of the im perfections of this branch of science, obferves that, "a production of real value in univerfal geography requires a wider extent of various knowledge than any other literary department, as embracing topics of multifarious defcription. There is, however, one name, that of d'Anville, peculiarly and justly eminent in this fcience; but his reputation is chiefly derived from his maps, and from his illuf. trations of various parts of ancient geography. In special departments Gosselin and other foreigners, have alfo been recently diftinguished; nor is it ne ceffary to remind the reader of Rennell and Vincent."

The conftruction and accuracy of maps and charts have been very confiderably improved, from the works of recent navigators and travellers; but few, if any of them can yet be implicitly confid ed in; and we are affured by Major Rennell that even the maps of our own country are far from correct. In general, the maps which at prefent are most to be depended upon, are thofe of D'Anville, Arrowfmith, Caffini, and Faden; though there are maps of particular countries by others, which, in fome inftances may be preferable: fuch as, Evan's of Wales, Ainflie's of Scotland, Ferrari's of the Netherlands, Chauchard's of Germany, Sortzmann's of Pruffia, Lopez's of Spain, Geoff. roy's of Portugal, improved by Rainsford, Weiss's of Switzerland, Witfen's of Tartary, Robert's of Japan, Rennell's of Hindooftan, Rochette's of Perfia, Neibuhr's of Arabia, Bruce's of Abyffinia, Morfe's of the individual provinces of America, and Jeffreys's of the Weft India Islands. SECT. III. Of the FIGURE and MAGNITUDE of the EARTH.

ch

THE EARTH is one of the great bodies whi compofe the planetary fyftem. It moves roun the fun in an orbit nearly circular, and compleats its revolution in the course of a year, while at the fame time it revolves continually upon its own axis, which is inclined to the plane of its orbit at an angle of 66 degrees; the time of a revolution being 23 hours and 56 minutes. The revolution of the earth round the fun is called its ANNUAL MOTION, and the rotation it performs on its own axis is called its DIURNAL MOTION.

While the earth revolves round the fun in the courfe of its annual motion, its axis, round which the diurnal motion is conftantly performed, moves always parallel to itfeif. It is by the parallelifm of the axis, and the annual motion of the earth, that the changes of the feafons are produce, as has been already explained at large; (See ASTRONOMY, Part 3, Sel. 3,) while by the diurnal motion all places on the earth's surface are alternate ly turned towards the fun, and by these means the

changes

changes of day and night are produced. See As- dle is fituated in lat. 45° was by this means found TRONOMY, 412 and 413. to be 57,029 toifes.

The ACADEMY OF SCIENCES, in which this question had been warmly agitated, concluded with reafon, that the difference of magnitude in the degrees of the meridian, if real, would be moft fenfibly perceived by the comparison of degrees meafured at the equator and towards the poles. Accordingly a company of Academicians was fent to the equator, where, having measured a degree of the meridian, they found it to contain 56,753 toifes; which was fhorter, by 274 toifes than a degree in lat. 45° N. Other Academicians were fent to the north, and having measured a degree of the meridian in Lapland, about the lat. of 66° 20' they found it to be 57,458 toifes, which was greater than the degree at the equator by 685 toifes; and by these measurements, it was completely proved, that the earth was not exactly spheri cal; and other measurements of degrees made fince that period have all tended to fhew, that the degrees of the meridian gradually increase from the equator to the poles.

That the earth is nearly of a spherical figure. Although the spherical figure be the most fimmay be proved by many arguments: the chief of pie, and it is natural for man to fuppofe objects thefe have been given under ASTRONOMY, 387 to be of that form which he most readily conand 389. See alfo EARTH, IV, ii. And fince ceives, yet the implicity of nature is not always this conclufion has been drawn from phænomena measured by that of our conceptions. Infinitely vawhich were not greatly complicated in their na- ried in her effects, Nature is only fimple in her cauture, and which were intimately connected with fes; and her economy confifts in producing a the common affairs of life, it is reasonable to con- great number of phænomena, often the most comclude that the attention which was neceffary to plicated, by means of a few general laws. The determine the returns of the proper feasons, for figure of the earth is a refult of these laws, which performing the labours of husbandry, and for the modified by a great variety of circumftances, may regulation of civil affairs, would lead men at an cause it to deviate fenfibly from a fpherical figure; early period of society to form pretty juft notions and certain fmall variations, obferved in the length of the figure of the earth. When the earth was of degrees of the meridian in France, fufficiently once known to be fpherical, the curiofity of man indicated that fuch a deviation did exift; but the would naturally lead him to endeavour to mea errors, which were unavoidable in fuch obfervafure its dimenfions; and we accordingly learn from tions, left this important phenomenon in a state of hiftory, that fuch attempts were made; as has uncertainty. been already noticed in latt fection. But the first accurate measure that was made of the earth, of which we have any certain knowledge, was that executed by M. PICARD, in France, towards the end of the last century, and which has been verified feveral times fince that period. It is not difficult to understand in what way the earth may be measured; the direction of gravity is always perpendicular to the earth's furface; hence it follows that the zenith of any place, or point of the heavens directly over our head, and alfo the horizon which is a plane touching the earth's surface at that place, will be continually changing accord. ing as we change our pofition on the earth's furface. Hence it follows, that as we travel from S. to N., the pole of the heavens, (or that point in the heavens, in which the earth's axis when produced meets the fphere of the fixed ftars) will be more and more elevated above the horizon; the meridian altitude alfo of the ftars in the northern regions of the heavens will appear to increafe; while that of the stars in the southern quarter will be diminished. By the elevation or depreffion of the ftars, we shall know the angle formed at the point of concourfe of perpendiculars drawn to the earth's furface at each extremity of the terreftrial arc; for this angle is equal to the difference of the meridian altitude of the fame ftar as feen from the extremities of the arc, diminished by the angle which the arc itself fubtends as feen from the ftar; which laft angle is altogether infenfible. The number of degrees in the arc being found, it is only neceffary to determine its length in fome known meafure, as a fathom, &c. but, as it would be a work of great labour to apply a meafure to an arc of great extent, it will be fufficient if its extremities be connected by a series of triangles to thofe of a bafe line of 3, or 4000 feet in length; and confidering the accuracy with which the angles of these triangles can be obferved, the length of the arc may be found with great precision. It was in this way that degrees of the meridian have been repeatedly measured. In France, for example, about 1793, an arc was measured extending from Dunkirk to Barcelona; (for the purpofe of fettling an univerfal ftandard of weights and meafures: See MEASURE ;) and the degree whofe mid

The ELLIPSE is the next curve in point of fimplicity to the circle, and the earth has been confiflered as a spheroid formed by the revolution of an ellipfe about its leffer axis: its oblatenefs or compreflion, in the direction of its poles, is a neceffary confequence of the obferved increase of the degrees of the meridian from the equator to the poles. For the radii of these degrees being in the direction of gravity, they are by the law of the equilibrium of fluids perpendicular to the furface of the ocean, with which the earth is in a great measure covered. They do not therefore, as in the fphere, tend to the centre of the fpheroid; neither are they in the fame direction, nor of the fame magnitude, as the radii drawn from the centre to its furface; which cut it obliquely every where, except at the equator and poles. The point at which two adjoining perpendiculars, fituated under the fame meridian, meet each other, is the centre of the small terreftrial arc which they comprehend between them. If this are were a ftraight line, thefe perpendiculars would be parallel; or they could only be confidered as meeting at an infinite diftance; but in proportion as this arc became curved, they would meet at a diftance fo much the lefs, as the curvature of the are S$2

was the greater.' Hence it follows, that feeing the extremity of the lefer axis is the point where the curvature of the ellipfic the leaft, the radius of a degree at the pole, and confequently that degree itfelf, muft be the greatest of any degree on the earth's furface. On the contrary, at the equator, or at the extremity of the greater axis, the curvature is the leaft, and therefore the degrce in the direction of the meridian is there the fmalleft. And in going from the equator to the pole, the degrees increase in fuch a manner, that if the ellipfe be not very eccentric, the increafe is nearly proportional to the fquare of the fine of the latitude. If the earth were exactly an oblate fpheroid, its magnityde, as well as the proportion of its axes, might be determined by the menfuration of two degrees in the direction of the meridian, as has been already explained. See EARTH, § IV, ii. It fhould alfo follow, that by a comparison of all the degrees hitherto measured, taken two and two, we should obtain the fame proportion between the axes. This, however, has not been the cafe The refuits have indeed fhewn, that the earth is flattened at the poles; but they have left an un certainty as to the quantity of the compreffion, ex tending from between the 170th to the 330th part of the radius of the equator. Between these two quantities, the former of which is nearly double of the latter, most of the refults are placed; but in fuch a manner, that thofe moft entitled to credit are much nearer to the leaft extremethan to the greater. In confequence of this difagreement in the re. fult of comparifons of degrees of the meridian, measured in different latitudes, it has been concluded by mathematicians, that the figure of the earth it not that of a fpheroid; nor does it even appear, that the parts of it on cach fide of the equator are exactly similar.

It will, however, be fufficient for the purpofe of Geography, to fuppofe the earth a fpheroid. Upon this hypothefis, LA PLACE, by a comparifon of the arc of the meridian meafured at the equator, and another measured between DunKirk and Mountjoy, has found, that the polar diameter is lefs than the equatorial by one 334th part of the latter: and that a 4th part of the elliptic meridian is 5,130,740 toifes; the toife being that used in meafuring the earth in Peru, and reduced to a temperature of 16 degrees of a mercarial thermometer, divided into 100 degrees from the freezing point to that of water, boiling under a preffure equivalent to a column of mercury 6 centimetres in height, or about 30 inches English meafure. This determination alfo agrees nearly with the refults from the combination of a great number of experiments made at different places of the earth, upon the pendulum.

B. caufe the measure of a degree at the equator hs been affumed, in the preceding calculation at 56,753 toifes, it follows alfo from the method explained under the article EARTH, § IV, ii. that The equatorial diameter is 3,271,267, and the pojar diameter 3,263.471 toiles; the difference between them being 9 796 toifes. From thefe data and the rules of menftration, it will be eafy to find the furface, folidity, &c. of the earth, alfo the of miles in a degr, &c.

The following table of the dimenfions of the
earth is given by Dr HUTTON.
The diameter
The circumference
A degree contains
The fuperficies
The folidity

79,5794 miles
25,000 miles
694 English miles

198,944,206 fquare milea 263,930,000,000 cubic miles, SECT. IV. Of the CIRCLES fuppofed to be DESCRI

BED on the EARTH'S SURFACE.

In geography the circles, which the fun apps. rently defcribes in the heavens, are fuppofed to be extended as far as the earth, and marked on its furface. In like manner we may imagine as many circles as we please to be described on the earth, and their planes to be extended to the celeftial fphere, till they mark concentric ones on the hea. vens. The mok remarkable of thofe fuppofed by geographers to be described in this manner are the following:

The Axis of the earth is that imaginary line pafling through the earth's centre, round which it continually revolves, from weft to east.

The POLES of the earth are the points at which the axis meets the earth's furface. One of the is called the north pole, and the other the fouth pole. Thefe correfpond to the poles of the beavens, or the points where the earth's axis, when produced, meets the starry fphere.

The EQUATOR is a great circle on the earth's furface, equally diftant from both poles, and cor refponds to the equinoctial circle in the heavens. It divides the earth's furface into two equal po tions called the northern and Southern hemispheres. The equator is alfo fometimes called the LINE, OT EQUINOCTIAL LINE.

The diftance of any place, northward or fouthward from the equator, is called its LATITUDE, and is reckoned in degrees and minutes, &c. The diftance between the poles and equator, which is a quadrant of a great circle paffing thro the poles, has by all geographers litherto bera fuppofed to be divided into 90 degrees; and each of thefe again fubdivided into 60 minutes, &c. But fome French aftronomers, and in particular LA PLACE, in his Expofition du Syfleme du Monde, as well as in his Traite de Mecanique Celeste, by adopted the decimal divifion of the meridia. They have fuppofed the diftance between the e quator and the poles to be divided into rco de grees, and each degree to be fubdivided into tes minutes, each minute into 100 feconds, and fo on.

All places lying on the north side of the equator are faid to have north latitude: on the contrary, all places on the fouth fide of the equator are faid to have fouth latitude.

PARALLELS OF LATITUDE are leffer circles upon the earth's furface parallel to the equator. They may be confidered as indefinite in number; all places that lie directly eat or weft from each other, are faid to lie in the fame parallel of latitude.

The TROPICs are two leffer circles on the earth. parallel to the equator, and 231 degrees diftart from it. That which lies on the north fide of th; equator is called the TROPIC OF CANCER; and that which hes on the fouth fide is called the TOPIC OF CAPRICORN. Tu. Ces correl

pond