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OF THE EARTH.

More distant still our Earth comes rolling on,
And forms a wider circle round the sun;
With her the moon, companion ever dear;

Her course attending through the shining year.

BAKER.

The earth performs its revolution round the Sun in an orbit between that of Venus and Mars, at the distance of 95,173,000, in 365 days, 5 hours, 48 minutes, 49 seconds, which is called the solar or tropical year. In performing its annual circuit, the Earth travels at the rate of 68,000 miles every hour, which is 140 times faster than that of a cannon ball. The diameter of the Earth is 7,912 English miles, and its circumference 24,855.42 miles.*

The Earth is usually represented by the character, in works which treat of Astronomy.

In the early ages of the world, many fanciful and absurd notions respecting the figure of the Earth prevailed; some of which were adopted because they appeared to agree with the slight and inaccurate observations of the vulgar, whilst others represented this matter in the way which best accorded with their preconceived opinions in philosophy or religion. The most general opinion was, that the Earth was a great circular plane, extending on all sides to an infinite distance; that the firmament above, in which all the heavenly 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.+ Cosmas Indopleustes supposed that it was an immense plane, whose length was much greater than its breadth, and surrounded by an unpassable ocean.-Towards the north he placed a huge mountain, round which the sun and stars performed their diurnal revolutions; and from the conical shape which he ascribed to it, with the oblique motion of the sun, he accounted for the inequality of the days and the variety of the seasons. The vault of heaven he conceived rested upon the earth, which extended beyond the ocean, and supported by two vast columns. Beneath the arch, angels conducted the stars in their various motions; above it were the celestial waters, and over all he placed the supreme heavens.

Concerning the figure and infinitude of the Earth no accurate information can be derived from the ancients. But this is not to be wondered at when we consider that they were not only unacquainted with the laws of motion, and the use of a lloptical and mathematical instruments; but they were unacquainted with the very existence of extensive islands and countries at no great distance from their own.

*This is what the French mathematicians have lately deduced from a measurement of above 12° of the meridian.

+ 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.

A very little reflection, however, 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 which 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 experiment is 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 a 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 buildings, 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 that projects it 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. Among those who have succeeded in this daring enterprise, may be mentioned Magellan, a Portuguese, in the year 1519, who completed the voyage in 1124 days; Francis Drake performed the same in 1056 days; Sir Thomas Cavendish in 777 days; Van Schoulen in 749 days; and many others have since performed the same navigation, particularly Anson, Baugainville, and Cook. Some of these navigators sailed eastward, some westward, till they again arrived in Europe; and in the course of their voyage observed

that all the phenomena, both of the heavens and the 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 globular 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 twelve 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.

Although every one of the observations which have just been made 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 eclipse, 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 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 Mr. 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 towards 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 about 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 diameter would be 229 to 230 nearly.

As all conclusions, however, deduced from the length of pendulums at different places of 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 determining the magnitude of the earth. A solution of this important problem has, however, been attempted at various periods, by other

means, and has at last been accomplished in a most accurate and satisfactory 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 of 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 7° 12′, or a fiftieth part of the circumference of a great circle. Supposing, then, that Àlexandria 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 Passedonius, who lived in the time of Pompey; but it is impossible for us to judge how far these results correspond with the more accurate measurements 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.

Our countryman, Mr. Norwood, measured the distance between London and York, 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. For he says, "Sometimes I measured, sometimes I paced, and I believe I am within a Scantling of the truth."

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 lat. 51° 2′ 9′′ N. to Formentera, the southernmost of the Balearic isles in lat. 38° 38 56" N. comprising an arc of 12° 23 13". But this has lately been extended to the Shetland islands. The whole amplitude of the arc is therefore above 22°.

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 increase 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 polar diameter being to the equatorial as 311 to 312. And by taking the mean length of a degree, or that measured in France at latitude 45°,

and multiplying it by 360, the degrees in the circle, the circumference of the earth in the direction of the meridian is found to be 24,855-84 English 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 7910 nearly, and the length of one degree 69 English miles.

OF MARS.

In larger circuit rolls the orb of Mars,
Guiltless of stern debate, and wasteful wars,
As some have erring taught: he journies on,
Impell'd and nourish'd by the attractive sun;
Like us, his seasons and his days he owes
To the vast bounty which from Phoebus flows.

BROWN.

Mars is the next planet, after the earth, in the order of distance from the sun; and as his orbit includes that of the earth, he is called the first of the superior planets. He is usually represented by the character, which is said to be rudely formed from a man holding a spear protruded, representing the god of war, which is the title of Mars in the heathen mythology.

He may easily be distinguished from any other planet, or star, by the red colour of his light.

Mars is the smallest of all the ancient planets, except Mercury, his diameter being about 4200 miles. The time he takes to perform his sedereal revolution round the sun is 686 days, 23 hours, 30 minutes, 36 seconds, which is performed at the distance of about 145 millions of miles, at the rate of 55,000 miles per hour. The time he takes to revolve on his axis is 24 39' of our time. The quantity of light and heat which Mars receives from the sun is only about half what the earth receives from him; and the sun only appears half as large to Mars as to the earth. If any satellite revolves round Mars it must be very small, as it has not yet been discovered, notwithstanding the great number of observations which have been made on this planet with the most powerful telescopes.

To Mars, the earth and moon appear like two moons, a larger and a smaller, changing places with each other, and appearing sometimes horned, sometimes half and three quarters enlightened, but never full; and never above a quarter of a degree distant from one another, although they are 240,000 miles asunder.

The red colour of this planet is ascribed to the density of its atmosphere. For the atmosphere which surrounds Mars is not only of great density, but of great height, that is, extends a great way from his surface, as appears from the occultations to which the fixed stars are subject on approaching his disc.

Dr. Smith, in his Optics, mentions an observation made by Cassini, on a star in the constellation Aquarius, at the distance of six

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