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turnings of the road, he concluded that 68 Italian miles were equal to a degree of the earth.

According to these methods many other measurements of the earth's circumference have since that time been made, with much greater accuracy.

Though the maps of Eratosthenes were the best of his time, they were yet very imperfect aud inaccurate. They contained little more than the states of Greece, and the dominions of the successors of Alexander, digested according to the surveys above-mentioned. He had indeed seen, and has quoted, the voyages of Pythias into the great Atlantic ocean, which gave him some faint ideas of the western parts of Europe; but so imperfect, that they could not be realized into the outlines of a chart. Strabo says he was very ignorant of Gaul, Spain, Germany, and Britain; and he was equally ignorant of Italy, the coast of the Adriatic, Pontus, and all the countries towards the north.

Such was the state of geography, and the nature of the maps, before the time of Hipparchus. He made a closer connection between geography and astronomy, by determining the latitudes and longitudes from celestial observations.

War has usually been the occasion of making or improving the maps of countries; and accordingly geography made great advances from the progress of the Roman arms. In all the provinces occupied by that people, camps were every where constructed at proper intervals, and good roads made for communication between them; and thus civilization and surveying were carried on according to system through the whole extent of that large empire. Every new war produced a new survey and itinerary of the countries where the scenes of action passed; so that the materials of geography were accumulated by every additional conquest. Polybius says, that at the beginning of the second Punic war, when Hannibal was preparing his expedition against Rome, the countries through which he was to pass were carefully measured by the Romans. And Julius Cæsar caused a general survey of the Roman empire to be made, by a decree of the senate. Three surveyors had this task assigned them, which they completed in twenty-five years. The Roman itineraries that are still extant, also shew what care and pains they had been at in making surveys in all the different provinces of their empire, and Pliny has filled the 3d, 4th, and 5th books of his Natural History

with the geographical distances that were thus measured. Other maps are also still preserved, known by the name of the Pentigerian Tables, published by Welser and Bertius, which give a good specimen of what Vegetius calls the itinera picta, for the better direction of their armies in their march.

The Roman empire had been enlarged to its greatest extent, and all its provinces well known and surveyed, when Ptolemy, about 150 years after Christ, composed his system of geography. The chief materials he employed in composing this work, were the proportions of the gnomon to its shadow, taken by different astronomers at the times of the equinoxes and solstices; calculations founded on the length of the longest days; the measured or computed distances of the principal roads contained in their surveys and itineraries; and the various reports of travellers and navigators. All these were compared together, and digested into one uniform body or system; and afterwards were translated by him into a new mathematical language, expressing the different degrees of latitude and longitude, after the invention of Hipparchus, which had been neglected for 250 years.

Ptolemy's system of geography, notwithstanding it was still very imperfect, continued in vogue till the last three or four centuries, within which time the great improvements in astronomy, the many discoveries of new countries by voyagers, and the progress of war and arms, have contributed to bring it to a very considerable degree of perfection.

SECT. II.-Principles of Geography.

The fundamental principles of geography are, the spherical figure of the earth, its rotation on its axis, its revolution round the sun, and the position of the axis or line round which it revolves with regard to the celestial luminaries. That the earth and sea taken together constitute one vast sphere, is demonstrable by the following arguments: 1. To people at sea the land disappears, though near enough to be visible were it not for the intervening convexity of the water. 2. The higher the eye is placed, the more extensive is the prospect ; whence it is common for sailors to climb up to the tops of the masts to discover land or ships at a distance. But this would give them no advantage, were it not for the convexity of the earth; for upon an

infinitely extended plane objects would be visible at the same distance whether the eye was high or low, nor would any of them vanish till the angle under which they appeared became too small to be perceived. 3. To people on shore the mast of a ship at sea ap pears before the hull; but were the earth an infinite plane, not the highest objects, but the largest, would be longest visible; and the mast of a ship would disappear, by the smallness of its angle, long before the hull did so. 4. The convexity of any piece of still water of a mile or two in extent may be perceived by the eye. A little boat, for instance, may be perceived by a man who is any height above the water; but if he stoops down or lays his eye near the surface, he will find that the fluid appears to rise and intercept the view of the boat entirely. 5. The earth has been often sailed round, as by Magellan, Drake, Dampier, Anson, Cook, and many other navigators, which demonstrates that the surface of the ocean is spherical; and that the land is very little different, may easily be proved from the small elevation of any part of it above the surface of the water. The mouths of rivers which run 1000 miles are not more than one mile below their sources, and the highest mountains are not quite four miles of perpendicular height; so that, though some parts of the land are elevated into hills, and others depressed into valleys, the whole may still be accounted spherical. 6. An unde niable, and indeed ocular, demonstration of the spherical figure of the earth is taken from the round figure of its shadow which falls upon the moon in time of eclipses. As various sides of the earth are turned towards the sun during the time of different phænomena of this kind, and the shadow in all cases appears circular, it is impos. sible to suppose the figure of the earth to be any other than spherical. The inequalities of its surface have no effect upon the earth's shadow on the moon; for as the diameter of the terraqueous globe is very little less than 8000 miles, and the height of the highest mountains on earth not quite four, we cannot account the latter any more than the 2000th part of the former, so that the mountains bear no more proportion to the bulk of the earth, than grains of dust bear to that of a common globe.

A great many of the terrestrial phænomena depend upon the glo bular figure of the earth, and the position of its axis with regard to the sun, particularly the rising and setting of the celestial luminaries, the length of the days and nights, &c.

Though the sun rises and sets all over the world, the circumstances of his doing so are very different in different countries. The most remarkable of these circumstances is the duration of the light, not only of the sun himself, but of the twilight before he rises, and after he sets. In the equatorial regions, for instance, darkness comes ou very soon after sunset; because the convexity of the earth comes quickly in between the eye of the observer and the luminary, the motion of the earth being much more rapid there than any where else. In our climate the twilight always continues two hours, or thereabouts, and during the summer season it continues in a considerable degree during the whole night. In countries farther to the northward or southward, the twilight becomes brighter and brighter as we approach the poles, until at last the sun does not appear to touch the horizon, but goes in a circle at some distance above it for many days successively. In like manner, during the winter, the same luminary sinks lower and lower, untill at last he does not appear at all; and there is only a dim twinkling of twilight for an hour or two in the middle of the day. By reason of the refraction of the atmosphere, however, the time of darkness, even in the most inhospitable climates, is always less than that of light; and so remarkable is the effect of this property, that in the year 1682, when some Dutch navigators wintered in Nova Zembla, the sun was visible to them sixteen days before he could have been seen above the lorizon, had there been no atmosphere. The reason of all this is, that in the northern and southern regions only a small part of the convexity of the globe is interposed betwixt us and the sun for many days, and in the high latitudes none at all. In the warmer climates the sun has often a beautiful appearance at rising and setting, from the refraction of his light through the vapours which are copiously raised in those parts. In the colder regions, halos, parhelia, aurora borealis, and other meteors are frequent; the two former owing to the great quantity of vapour continually flying from the warm regions of the equator to the colder ones of the poles. In the high northern latitudes, thunder and lightning are unknown, or but seldom heard of; but the more terrible phænomena of earthquakes, volca. noes, &c. are by no means unfrequent. These, however, seem only to affect islands and the maritime parts of the continent..

Notwithstanding the seeming inequality in the distribution of light and darkness, however, it is certain, that throughout the whole

world there is nearly an equal proportion of light diffused on every part, abstracting from what is absorbed by clouds, vapours, and the atmosphere itself. The equatorial regions have indeed the most intense light during the day, but the nights are long and dark; while, on the other hand, in the northerly and southerly parts, though the the sun shines less powerfully, yet the length of time that he appears above the horizon, with the greater duration of the twilight, compensates for the seeming deficiency.

Were the earth a perfect plane, the sun would appear to be vertical in every part of it; for, in comparison with the immense magnitude of that luminary, the diameter of this globe itself is but very small; and as the sun, were he near to us, would do much more than cover the whole earth, so, though he were removed to any distance, the whole diameter of the latter would make no difference in the apparent angle of altitude. By means of the globular figure of the earth also, along with the great disparity between the diameters of the two bodies, some advantage is given to the day over the night; for thus the sun, being immensely the larger of the two, shines upon more than one half of the earth; whence the unenlightened part has a shorter way to go before it again receives the benefit of his rays. This difference is greater in the inferior planets Venus and Mercury than in the earth.

To the globular form of the earth likewise is owing the long moonlight which the inhabitants of the polar regions enjoy. The same figure likewise occasions the appearance and disappearance of certain stars at some seasons of the year in some countries; for, were the earth flat, they would all be visible in every part of the world at the same time. Hence most probably has arisen the opinion of the influence of certain stars upon the weather and other sublunary matters. In short, on the globular figure of the earth depends the whole present appearance of nature around us; and were the shape of the planet we inhabit to be altered to any other, besides the real dif ferences which would of consequence take place, the apparent ones would be so great, that we cannot form any idea of the face which nature would then present to us.

In geography the circles which the sun apparently describes in the heavens are supposed to be extended as far as the earth, aud marked on its surface; and in like manner we may imagine as many circles as we please to be described on the earth, and their planes.

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