bodies seem to move daily from east to west, was at no great distance from the Earth; and that all the celestial bodies were created solely for its use and ornament. Heraclitus imagined the Earth to have the shape of a canoe; Anaximander supposed it to be cylindrical; and Aristotle, the great oracle of antiquity, gave it the form of a timbrel. Such of the ancients, however, as understood any thing of astronomy, and especially the doctrine of eclipses, must have been acquainted with the round figure of the Earth; as the ancient Babylonian astronomers, who had calculated eclipses long before the time of Alexander, and Thales, the Grecian, who predicted an eclipse of the Sun.

A very little reflection, and a very little travelling either by sea or land, must soon convince any one that the Earth is of a spherical form. For let a person occupy any station in a level country, and mark carefully the objects within the range of his horizon, let him then advance in any direction, and as he moves the objects behind him gradually disappear, and new objects in front come in view. Before he has travelled twenty miles in the same direction, he will find that every object that was at first visible to him is lost to his view, and that he is now in the centre of a new horizon. As a similar change takes place at every part of the globe where the same experiment has been tried, it follows that the Earth is a spherical body. The same inference may be deduced from observing the appearance of a ship at a distance at sea, or from observing the gradual rising of the coast as the ship approaches the shore. In the former case, the top of the mast is first seen, and as the vessel approaches the land, the whole of her gradually becomes visible. In the latter, the hills, or the higher parts of the buildings, are first discovered, but by degrees every part of the building and even the beach itself is seen. These are appearances which can only be reconciled with the spherical figure of the Earth. The same conclusion may be drawn from observing the altitude of the pole star, after travelling north or south a considerable number of miles. In travelling northward its altitude will be increased; but in travelling south it will be diminished.

The globular figure of the Earth is also inferred from the operation of levelling, in which it is found necessary to make, an allowance for the difference between the true and apparent level; and the allowance which is made, and found to answer, is on the principle that the Earth is spherical.

Another proof of the Earth being of a spherical form, is obtained from its shadow in an eclipse of the Moon; for when the shadow of the Earth falls on the Moon she is eclipsed, and the shadow always appears circular upon the face of the Moon, when she is not totally eclipsed, although the Earth is constantly turning on its axis. Hence it follows, that the body which projects the shadow, must be spherical. But the most convincing proof of the spherical figure of the Earth, is, that many navigators have sailed round it; not on an exact circle, it is true, because the winding of the shores would not admit of it, but by going in and out as the shores happened to lie, and still keeping the same course, they have at last arrived at the port from which they departed. The first who succeeded in this daring enterprise was Ferdinand Magellan, a Portuguese, in the year 1519, and who completed his voyage in 1124 days; in the year 1557, Francis Drake performed the same in 1056 days; in the year 1586, Sir Thomas Cavendish made the same voyage in 777 days; in the year 1598, Oliver Noort, a Hollander, in 1077 days; Van Schouten, in the year 1615, in 749 days; Jac. Heremites and Joh. Huygens, in the year 1623, in 802 days and many others have performed the same navigation, particularly Anson, Bougainville, and Cook.

Some of these navigators sailed eastward, some westward. till they again arrived in Europe, whence they set out; and in the course of their voyage observed, that all the phenomena, both of the heavens and Earth, confirmed the doctrine of the spherical figure of the Earth. The unevenness or irregularity of the Earth's surface, such as mountains and valleys, afford no objection to its being considered as a giobular body; for the loftiest mountains bear no greater proportion to the vast magnitude of the Earth, than grains of sand to the size of an artificial globe of thirteen inches in diameter. This is the reason that no deviation from the spherical figure of its shadow is perceptible in an eclipse of the Moon.

2. From the most accurate measurement, lately made by mathematicians, it is found that the terrestrial meridian is nearly an ellipse; that the figure of the Earth is not exactly a sphere, but nearly an oblate spheroid, its equatorial diameter being about 25 miles longer than its axis or polar diameter, and its mean diameter 7914 miles.

3. By the application of a new theory of most probable results to the determination of the magnitude and figure of the Earth, Dr. Adrain has found the ratio of the axis to the equatorial diameter to be at 320 to 321, the true mean diameter of the Earth, considered as a globe, to be 7918.7 miles, and consequently its circumference 24877.4 miles, and a degree of a great circle equal to 69.1039 miles.

According to La Place the polar diameter is to the equitorial as 331 to 332; he makes the equatorial diameter 7924 miles: hence the polar diameter is 7900 miles, and the mean diameter 7912. In the preceding part of this work, the mean diameter of the Earth has been taken equal to 7920 miles, its circumference 24,880 miles, and the length of a degree 69 miles; the same numbers shall therefore be used in the subsequent part: they are nearly those given by Dr. Adrain, and which are considered to be the most exact measures of the magnitude of the Earth.

Although every one of the observations which have just been made, (in the preceding article) respecting the figure. of the Earth, affords sufficient evidence that the surface of the Earth is curved, yet none of them, except, perhaps, the form of the shadow on the disc of the Moon in a lunar éclipse, entitles us to infer that the figure of the Earth is that of a globe, or perfect sphere. It was natural, however, for those who first discovered that the Earth had a round shape, to suppose that it was truly spherical. This, however, is now known not to be the case; its true figure being that of an oblate spheroid, or sphere flattened a little at the poles, and raised about the equator: so that the axis or polar diameter is less than the equatorial. What first led to this discovery was the observations of some French and English philosophers in the East Indies and other parts, who found that pendulums required longer time to perform their vibrations the nearer they were to the equator; for Richer in a voyage to Cayenne, near the equator, found that it was absolutely necessary to shorten the pendulum of his clock about one eleventh part of a Paris inch, in order to make it vibrate in the same time as it did in the latitude of Paris. From this it appeared that the force of gravity was less at places near the equator than at Paris; and consequently that those parts

are at a greater distance from the Earth's centre. This circumstance put Newton and Huygens upon attempting to discover the cause, which they attributed to the revolution of the Earth on its axis. If the Earth were in a fluid state, its rotation on its axis would necessarily make it assume such a figure, because the centrifugal force being greatest at the equator, the fluid would there rise and swell most; and that its figure really should be so now, seems necessary to keep the sea in the equatorial regions from overflowing the land in those parts.

Newton in his Principia demonstrates, that by the operation of the power, called gravity, the figure of the Earth must be that of an oblate spheroid, if all parts of the Earth be of a uniform density throughout, and that the proportion of the polar to the equatorial would be 229 to 230 nearly.

As all conclusions, however, deduced from the length of pendulums at different places on the Earth's surface, proceed upon the supposition that the Earth is a homogeneous body, which is very improbable, the true figure of the Earth can scarcely be expected to be discovered by the pendulum; and at any rate it can be of no use in determing the magnitude of the Earth. A solution of this important problem has, how ever, been attempted at various periods, by other means, and has at last been accomplished in a most accurate and satis factory manner, by the actual measurement of a very large arc of a meridian circle on the Earth's surface. The earliest attempt of this kind of which we have any account, is that or Eratosthenes of Alexandria, in Egypt. By measuring the Sun's distance from the zenith of Alexandria, on the solstitial day, and by knowing, as he thought he did, that the Sun was in the zenith of Syené, on the same day, he found the distance in the heavens between the parallels of these places to be 70 12', or part of the circumference of a great circle. Supposing then that Alexandria and Syené were on the same meridian, nothing more was required than to find the distance between them, which multiplied by 50, would give the circumference of the globe. But it does not appear that Eratosthenes took any trouble either to ascertain the bearing or the distance of the two places; for Syené is considerably east of Alexandria, and it appears that the distance was not measured till long afterwards, when it was done by the command of the Emperor Nero. A similar attempt was made by Possedonius, who lived in the time of Pompey; but it is imossible for us to judge how far these results correspond with the most accurate measurement of the moderns, as we

are unacquainted with the stadium, the measure in which the results were expressed.

The first arc of the meridian measured in modern times, with any degree of accuracy, was by Snellius, a Dutch mathematician. The arc was between Bergen-op-zoom and Alkmaar, and the length of the degree that resulted was 55,021 toises; but upon repeating the operations afterwards with greater accuracy, the degree came out 57,033 toises, which is not far from the truth.

The next who undertook this measurement was Norwood, who, in the year 1635, measured the distance between London and York with a chain, from whence he deduced the length of a degree to be 57,800 toises, which has been found to be a near approximation, considering the method he took to determine it.

Picard was the first person who employed the trigonometrical method with any degree of accuracy; but since his time very large arcs of the meridian have been measured in various parts of the world, particularly in Lapland, Peru, India, France, and England. The arc which has been measured in France extends from Dunkirk, in latitude 510 29" N. to Formentera, the southernmost of the Balearic isles, in latitude 38°38′56′′ N.comprising an arc of 12°23′13′′. But this has lately been extended to the Shetland islands. 'The whole extent of the arc is therefore above 22 degrees. From comparing the lengths of the degrees of the meridian which have thus been measured at different parts of the Earth with each other, it is found that they gradually in crease in length from the equator to the poles; which proves, beyond the possibility of doubt, that the true figure of the Earth is that of an oblate spheroid, its axis or pola diameter, according to some mathematicians, being to the equatorial as 311 to 312. And by taking the mean length of a degree, or that measured in France in latitude 45°, and multiplying it by 360, the degrees in the circle, the circumference of the Earth in direction of the meridian is found to be 24,855.84 miles. The circumference of the equator is 24,896.16 miles, which is about 40 miles greater than the preceding. The mean diameter of the Earth is therefore 9710 nearly, and length of one degree is 69 American miles.

4. As the axis of the Earth is perpendicular to the plane of the equinoctial, and its orbit makes an angle of 23° 28' with the plane of that circle, the