so far as regarded descriptions of the various positions of the earth. The art, however, of representing such portions of the earth, on a plane surface, cannot well be traced back beyond the days of the Greek philosopher Thales, and his successor Anaximander, who, about the year 580 before the Christian era, produced a geographical table, perhaps a map, exhibiting the situation of Greece and the neighbouring countries.

After Anaximander came a succession of geographers, of whose writings, in general, only imperfect fragments have come down to our times: but, at last, in the reign of Augustus Cæsar, about the year 19 of Christ, appeared Strabo, a native of Amasia, in Lesser Asia, who composed a general system of geography, which has, happily, been preserved to our day, and which, besides topographical and historical informations concerning all such parts of the earth as were then known, many of which he had visited, contains . sundry curious discussions on disputed points of geography, together with numerous extracts from the writings of prior travellers and geographers, which, but for Strabo's work, would have been utterly lost to the world.

Still, however, was wanting a treatise which, to the informations contained in the writings of Strabo and his predecessors, should add the philosophical and geometrical principles on which geography rests, as forming a part of the science of the universe. Such a treatise was produced, about a century and a half after Strabo, by Ptolemy the Alexandrian, containing instructions for the due construction of maps, for the representation of a sphere on a plane surface, and for determining the positions of countries, towns, &c. agreeably to their proper and relative situations on the surface of the earth, as ascertained by what is termed their latitude and longitude.

Of modern geographers it may be sufficient to mention the names of De Lisle and D'Anville in France, particularly

the.

the latter, who, for the learning and the sagacity displayed in his numerous dissertations on many portions of ancient and modern geography, as well as for his maps, deservedly enjoys the highest reputation: amongst ourselves, the labours of Rennell and Vincent are entitled to the greatest attention from every lover of genuine geographical dis

cussion.

The uninformed and inconsiderate part of mankind, in modern, as well as in ancient times, have been of opinion, that this earth is a vast extended plane, bounded on all sides by the sea and the heavens: more attentive observers were, however, long ago persuaded that the earth is a round ball, globe, or sphere, maintaining its appointed place amongst the innumerable bodies composing the universe, and far removed from contact with any other body of either the same or a different kind.

How the ancients came to be convinced of the spherical form of the earth, we have now no means of discovering: but, by attending to the following facts, we may easily be led to adopt the same opinion. When we stand on the margin of a lake or arm of the sea of considerable breadth, and carefully observe such objects on the opposite side as seem to touch the surface of the water; if we stoop the eye slowly down to the ground, we will gradually lose sight of the objects we had at first remarked on the contrary, if instead of lowering the eye we shall raise it, by ascending an eminence, climbing up a ship's mast, or the like, we will, as we ascend gradually, discover new objects lower than those at first noticed along the surface of the water, and their number will be increased in proportion to the height to which the eye is elevated.

These effects can he produced by no other cause than the rounded swelling surface of the water between the objects and the eye, which surface, if we take only a small portion of it, may, it is true, be considered as being perfectly level

and horizontal, because the circle, of which this small por tion is a part, is of so great a diameter, that the line joining the eye and the object, and the circumference, may be regarded as entirely coinciding: but when the portion of this circle is enlarged, the curvature of the surface will depart so sensibly from, and rise so much above the lines between the eye and the object, as entirely to intercept the view. (See this subject farther explained in Practical Geometry, page 437, vol. I.)

Again, when a ship leaves the land, the observers on shore first lose sight of her hull, and then of the lower sails, until, by increasing her distance, the tops of the masts themselves disa pear and, on the contrary, when a ship approaches the land, or another ship at sea, observers at a distance first perceive the top of her rigging, but as their distance lessens, more and more of the sails is discovered, and last of all the hull; which phenomenon can only be occasioned as before, by the round swelling surface of the water between the ship and the observers; and the effect is not confined to the sur face of the sea, for those who have traversed the vast plains of Flanders, Lombardy, &c. must have observed similar appearances in the gradual discovery of steeples, towers, &c. in proportion as the intervening space was diminished.

Another proof that the earth is not a plane surface, but spherical, is drawn from the voyages repeatedly performed by those who have sailed round the world, who have, in a general sense, proceeded on in the same direction, some going always westward, others always castward, until they. returned to the port where the voyage began.

An observer on board a vessel bound for India in the middle of winter, perceives that, as he quits the coast of England, and advances towards the Cape of Good Hope, standing in general in a southerly direction, the sun comes daily to approach more and more to be directly over him at noon, until at last this actually happens; and as he advances

still farther on his course, the sun which at first appeared to bear to the southward of him, now bears to the northward: after he has doubled the Cape, however, and steers a course tending in general northerly, he again brings the sun directly over him at noon, before his arrival at Calcutta, when that luminary again appears always to the southward, as was the case before the traveller left England.

This appearance is not, however, confined to the sun, for as the observer proceeds to the southward from England, he will gradually discover stars appearing in the southern parts of the horizon, which had not before been seen, whilst others in the northern parts of the horizon, and even the north pole star itself, will gradually cease to appear, and those situated about the south pole will constantly be visible. If no interruptions from either land or bodies of ice presented themselves to the navigator, in steering his course still forward in the direction from north to south, he would pass under the south pole, and soon after discover the north pole, which, as he advanced in his voyage, would by degrees seem to rise higher and higher in the heavens, until he passed under it, and arrived at the place from which he sailed, when that pole would have regained the position it originally occupied in the hemisphere.

These several appearances, it must be evident, can only be explained on the supposition that the earth is not a flat circular plane, but a spherical body, totally unconnected with any other part of the universe.

In an eclipse of the moon, which, as shall be explained when we come to treat of astronomy, is occasioned by the earth coming in between her and the sun, and so intercepting his light, the boundary of the shadow of the earth, as it appears on the moon's body, is invariably of a circular form, which could not be the case if the body producing this shadow were not itself circular, and circular in all directions, or, in other words, spherical; for the eclipses of

the

the moon happening in very various positions of the body of the earth, both with respect to her and to the sun, if that body were not a globe instead of a round flat surface like a table, the shadow cast on the moon would, at one time or other, assume the appearance of an ellipse, of a straight line, or of some other form different from that which it has constantly been found to present.

When the earth was understood to be a spherical body, attempts were naturally made to ascertain its dimensions. Eratosthenes, a celebrated geographer of Cyrene in Africa, and keeper of the Alexandrian Library, who was born 276 years before our era, by means of observations of the sun's meridian altitude at Alexandria and Syene, a town of Upper Egypt, nearly due south from Alexandria, calculated the circumference of the earth, supposing it to be a perfect sphere, to be 250,000 stadià, each stadium containing 547,4 English feet consequently one degree, or the 360th part of the circumference would be 694; stadia, equal to about 71,24 English miles, and the whole circumference equal to about 25646,4 English miles, which exceeds the truth, but which is a wonderful approach to it, considering the very imperfect state of the science of geography in his time, as well as the defectiveness of the instruments he must have employed in bis operations.

Ptolemy mentions other attempts made to ascertain the dimensions of the earth, and gives it as his own opinion, that the circumference was only 180,000 stadia; but when the various measures known by the ancients under the common name of stadium are considered, it appears that the dimensions here assigned were equal to about 24873 English miles, the 360th part of which, or 1 degree, would be 69,1 miles, which is but a little less than that now agreed upon since the latest observations.

When measurements for the purpose of determining the dimensions of the earth were made in later times on remote

parts