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 greatestattention 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 and 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 be 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 a of it, may, it is true, be considered as being perfectly level

and horizontál, 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 re. garded 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 disappear; 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 eastward, 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 stadia, 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 his 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

parts of its surface, the results were so far from agreeing one with another, that philosophers began to suspect the earth not to be a perfect sphere, all whose axes were equal; and the difficulty was to ascertain in what direction the longest and shortest axes were situated. From considerations arising from the nature of the earth itself, and the motions to which, it is subjected, Sir Isaac Newton proved, that the shortest axis must be that which passes through the centre from north to south, and the longest that which passes through from east to west: this opinion was, however, combated by other learned men on the continent; but the repeated measurements of degrees on various and distant parts of the earth's surface, performed with the greatest care, and by means of the highly improved instruments of modern times, have fully established the general truth of Sir Isaac's theory. It has even been supposed that the axis passing through the centre of the earth, from west to east, is about 34 English miles longer than that which passes from north to south. Upon taking a medium of all the different dimensions of the earth, it has been found that if it were a perfect sphere, the axis would be about 7,930 English miles, and consequently the circumference about 24,913 miles. Hence the superficial area of the globe would be about 197,560,000 square English miles, and the length of a degree on a great circle of such a sphere would be equal to 69,02 English miles.

When we observe the sun, the moon, and the stars begin regularly to appear or rise in the east, advance to their greatest elevation, and then disappear or set in the west, it is natural for us to imagine that these bodies are actually in motion from east to west, whilst we on the earth are perfectly fixed and motionless: but when our views of the magnitudes and distances of these bodies come to be more enlarged, we will be convinced that the same appearances may be produced, with respect to us, if we suppose that the whole

heavenly