From Agepedia

Jump to: navigation , search

EARTH ; the name of the planet which we inhabit. We may view it in regard to its physical, mathematical and political condition. (See Geography.) First, as to the form of the earth: to an observer whose view is not obstructed, it presents itself as a circular plain, on the circun> ference of which the heavens appear to rest. Accordingly, in remote antiquity, the earth was regarded as a flat, circular body, floating on the water. But the great distances which men were able to travel soon refuted this limited idea as an optical illusion; and, even in antiquity, the spherical form of the earth began to be suspected. On this supposition alone can all the phenomena relating to it be explained. A sphere of so great a magnitude as our EARTH, surrounded by a stra^ turn of air, or the visible firmament, must present to the eye of an observer, on a plain, the appearance just described. But how could the earth appear, from every possible position, as a surface bounded by the firmament, if it were not a sphere encircled by it ? How else could the horizon grow wider and wider, the higher the position we choose ? How else "%au the fact be explained, that we see the tops of towers and of mountains, at a distance, before the bases become visible ? But besides these proofs of the sphericity of the earth, there are many otjiers, such as its circular shadow on the moon during an eclipse, the gradual appearance ana disappearance of the sun, the inequality of day and night, the changes in the pos'w tion and course of the stars, and the gradual disappearance%of some and appearance of others, as we go from the equator to the poles. Finally, if the earth were not spherical, it would be impossible to sail round it, which is frequently done. The cause of the earth's sphericity is very evident, if we consider it as having been, at first, a yielding mass, capable of assuming any form: then, by the force of gravity, every particle contained in it tending towards the common centre, the globular form is the necessary consequence. As to the objection to the sphericity of the earth, drawn by weak and ignorant people, from the imagination that oui antipodes (q. v.) would tall from its surface, and many similar ones, they will appear to have no force whatever, when we consider that, in a globe of the magnitude of the earth, eveiy thing on the surface tends to the centre, and that, if we speak of what is above and below, the whole surface of the earth is below, and the surrounding atmosphere above. The earth is not, however, an exact sphere, but is flattened at the poles. Philosophers were first led to observe this by the variation in the vibrations of the pendulum under the equator and near the poles. It was found that the pendulum performed its vibrations slower the nearer it approached the equator, and hence was inferred the variableness of the force of gravity. This was easily explained on the theory just mentioned, because, the circle of daily revolution being greatest at the equator, all bodies revolve proportionally faster there than at the poles, so that the centrifugal force is greater, and the force of gravity less, than at other parts of the earth's surface ; and because,, at the equator, the centrifugal force is exactly opposed to that of gravity, but towards the poles, being oblique to it, produces less effect. From these observations it was justly inferred, that the earth is a sphere flattened at the poles, or a spheroid ; and this form was satisfactorily accounted for by the fact that the particles of a yielding mass, which revolves on its own axis, depart from the poles and tend to the centre, by which the poles are, of course, flattened, and the middle elevated. Various measurements liave put this beyond all doubt. (See Maupertuis, and Condamine, and Degree, Measurement of.) Another important desideratum for a more intimate acquaintance with the earth was, to fix its magni o tude. The labors of the ancients, in this respect were all fruitless, owing tc their want of suitable instruments. Accurate results were first obtained in the year 1615. Willibrord Snellius, a Dutchman, first struck into the only true way, and measured an arc of a meridian from Alcmaar to Leyden and Bergen op Zoom, by means of triangles. After him,/the measurements of Picard, and the later ones of Maupertuis, approximated nearer the truth. These made the circumference of a great circle of the earth 25,000 miles. But it is to be remarked that, in this calculation, the earth is regarded as a perfect sphere. Further measurements of all parts of the surface of the earth will be necessary to find, rigidly and accuratety, the true magnitude of it. (See Account of Experiments, to determine the Figure of the Earth, by Means of the Pendulwn, fyc, by Captain Ed. Sabine (London, 1825, 4to.), under the direction of the board of longitude.) if we take a view of our earth in its relation to the solar system, astronomy teaches us that, contrary to appearances, which make the sun revolve about the earth, the earth and ten other planets revolve about the sun, and, being themselves opaque bodies, receive from the sun light and heat. The earth completes its revolution round the sun in about 365 days and 6 hours, which forms our common year. The orbit of the earth is an ellipse, with the sun in one of its foci. Hence the earth is not equally distant from the sun in all parts of the year: its least distance is estimated at 93,336,000 miles, and its greatest, at 95,484,572, making a difference of more than 2,000,000 of" miles. In winter, we are nearest the sun, and in summer, farthest from it; for the difference in the seasons is not occasioned by the greater or less distance of the earth from the sun, but by the more or less oblique direction of the sun's rays. The length of the path travelled over by the earth is estimated at 567,019,740 miles, and, as this immense distance is passed over in a year, the earth must move 17 miles a seconda rapidity so far exceeding our conceptions, that it gave very just occasion to the pleasant remark of Lichtenberg, that, while one man salutes another in the street, he goes many miles bareheaded without catching cold. Besides this annual motion about the sun, the earth has also a daily motion about its own axis (according to mean time, in 23 hours, 56 minutes and 4 seconds). This diurnal revolution is the occasion of the alternation of dny and night. But as the axis on which the earth performs its diurnal rotation forms, with its path about the sun, an angle of 23£ degrees, the sun ascends, from March 21 to June 21, about 23£ degrees above the equator towards the north pole, and descends again towards the equator from June 21 to September 23; it then sinks till December 21, about 23£ degrees below the equator, towards the south pole, and returns again to the equator by March 21. This arrangement is the cause of the seasons, and the inequality of day and night attending them, which, for all countries lying beyond the equator, are equal only twice in the year, when the ecliptic coincides with the equator. The moon, again, revolves about the earth, in a similar elliptical path, in 28 days and 14 hours. Copernicus first laid down this as the system of the universe.To the physical knowledge of the earth belongs, especially, the consideration of its surface and its interior. The earth's surface contains over 196,000,000 square miles, of which scarcely a third part is dry land; the remaining two thirds are water. Of the surface of the earth, Europe comprises about one 54th part; Asia, one 14th ; Africa, a 17th ; and America, a 16th. The islands of the Pacific, taken together, are somewhat larger than Europe. The population of the whole earth is estimated at from 800 to 1000 millions. The interior of the earth is entirely unknown to us, as the depth to which we have been able to penetrate is nothing in comparison with its diameter. Many modern speculators are of opinion that the interior is composed of a metallic mass. Respecting the origin and gradual formation of the earth, there are various hypotheses. (See Geology; see also Day,Cycle, Degree, &c.; and Mountain, Volcano, Earthquake, Current, &c.) Eajih, Motion of the. The earth has two motions, the daily motion round its axis, and the yearly motion in its orbit round the sun. The theory of the motion of the earth has become memorable in the history of the human mind, showing, as it does, a marked ability in man to resist the impressions produced by appearances, and to believe the contrary of that which had been believed and taught for many centuries. The theory of Copernicus not only founded the modern system of astronomy, but made men eager to examine other articles of their creed, after they were thus convinced that they had erroneously taught and believed the earth to be stationary for 6000 years. All the opinions of the ancients respecting the motion of the earth were speculative hypotheses, arising from the Pythagorean school, which, as we know, considered fire the centre of the world, round which all was moving. Thus we ought to explain the passage of Aristarchus of Samos, mentioned by Aristotle *\n his Arenario. Aristarchus, as a Pythagorean, held the idea, that the earth revolves round its axis, and, at the same time, in an oblique circle round the sun; and that the distance of the stars is so great, that this circle is but a point in comparison with their orbits, and therefore the motion of the earth produces no apparent motion in them. Every Pythagorean might have entertained this idea, who considered the sun or fire as the centre of the world, and who was, at the same time, so correct a thinker, and so good an astronomer, as " Aristarchus of Samos. But this was not the Copernican system of the world. It was the motions of the planets, their stations and their retrogradations, which astronomers could not explain, and which led them to the complicated motions of the epicycles, in which the planets moved in cycloids round the earth. Aristarchus lived 280 B. C, Hipparchus, the great astronomer of antiquity, 150 B. C, therefore 130 years later. At tliis time, all the writings of Aristarchus were extant, and, had the Copernican system been set forth in them, Hipparchus would not have despaired of explaining the motions of the planets. The same is true of Ptolemy, in whose Almagest, the most complete work of antiquity on astronomy, this system is not mentioned in the account of Aristarchus. Every Copernican speaks of the motion of the earth, but not every one who speaks of the motion of the earth is a Copernican. Copernicus was led to the discovery of his system by a consideration of the complicated motion of the planets, and, in the dedication of his immortal work, De Revolutionibus Orbium, to pope Paul III, he says, that the truth of his system is proved by the motion of the planets, since their successive stations and retrogradations are the simple and necessary consequence of the motion of the earth round the sun ; and we need not take refuge in the complicated epicycles. Copernicus did not live to see the persecutions which the Roman Catholic priests raised against his system. They began only 100 years later (about 1610), when the telescope was invented, when the moons of Jupiter and the phases of Venus were discovered, and, by tnese means, the zeal for astronomy had been highly excited. Every city in Italy was then a little Athens, in which the aits and science** o flourished. Galileo obtained high distinction, and defended the new system of the world. The Roman inquisition summoned him before its tribunal, and he was compelled to abjure this theory. (See Galileo.) The general sympathy for the fate of this astronomer increased the popularity of the system, and it was as violently defended on one side as it was attacked on the other. Among the arguments against the motion of the earth, it was alleged, that a stone, falling from a tower, did not fall westward of the tower, notwithstanding this had advanced eastward several hundred feet during the four or five seconds of the fall of the stone. Copernicus had answered justly : the cause of its remaining near the tower is, that it has the same motion eastward, and, in falling, does not lose this motion, but advances with the earth. Galileo said the same, and asserted that a stone, falling from the top of the mast of a vessel, at full sail, falls at the foot of the mast, notwithstanding the mast advances, perhaps, 10 or more feet during the fall. Gassendi tried these experiments in the harbor of Marseilles, and the stones fell at the foot of the mast, notwithstanding the vessel was under full sail. Galileo therefore maintained, that it is impossible to draw any conclusions concerning the motion of the earth from such experiments, since bodies would fall on the earth in motion precisely the same as on the earth at rest. In 1642, Galileo died. In the same year, Newton was born. He proved, in 1679, that the opinion of Galileo was erroneous, and that we certainly can try experiments on the motion of the earth ; that the balls would not deviate westward, but would fall a little eastward of the plumbline, .about a half inch at the height of 300 feet. The cause is this: since the top of the tower is at a greater distance from the axis of the earth than its base, the centrifugal force must be greater at the former point than at the latter; the ball, in falling, iloes not lose this impulse, and, therefore, advances before the plumbline, which strikes the foot of the tower, since it has a less impulse eastward. This hint, given by Newton, was followed by Hooke. He tried experiments on the motion of the earth, at a height of 160 feet, and asserts that he succeeded. The academy appointed a committee, Jan. 14,1680, in the presence of which he was to repeat his experiments. * Probably they were not satisfactory, since they have never been mentioned in the Philosophical Transaco lions, and w^re entirely forgotten. Only 112 years later, a young geometrician in Bologna, Guglielmini, attempted to repeat these experiments, which had been considered very difficult by astronomers, in the tower Degli Asinelli, in that city, at a height of 240 feet. After having surmounted all difficulties, he succeeded in causing the fall of 16 balls, which perceptibly deviated eastward. But Guglielmini committed an error in not suspending the lead every day when he tried his experiments, of which he often made three or four in one night. He did not drop the plummet until after he had finished all his experiments, and, as it did not come to a perpendicular position until six months, on account of stormy weather, the tower in the meantime was a little bent) the point at which the plummet should have fallen was altered, and his experiments were lost. This happened in 1792. Benzenberg, a German, performed similar experiments in 1804, in Michael's tower, in Hamburg. He let fall 30 balls, from the height of 235 feet: the balls deviated from the perpendicular four lines eastward. But they deviated, at the same time, 1£ line southward, probably owing to a gentle draft of air in the tower. He repeated these experiments in 1805, in a coalpit, at Schlebusch, in the county of Mark, at the height of 260 feet: there the balls deviated from the perpendicular five lines eastward, just as the theory of the motion of the earth requires for the latitude of 51°, but neither southward nor north* ward. From these experiments, Laplace calculated that the chances are 8000 to 1 that the earth turns round its axis. The invention of the telescope, by means of which the rotation of Jupiter was soon observed, but still more, Newton's discovery of universal gravity, and of the nature of the celestial motions, established the theory of the motion of the earth ; and, in modern times, no man of intelligence doubts it any longer. The French general Allix, however, endeavored to prove that the motion of the planets does not depend on the law of gravitation. The flattening of the earth (see Degree, Measurement of\ and the diminution of gravity in the vicinity of the equator, proved by the experiments of Richers and others on the motion of the pendulum in the equatorial regions (see Pendulum), also give as convincing proofs of the rotation of the earth, as the aberration of light (q. v.J affords of the revolution of the earth round the sun. Thus the human intellect has triumphed over the evidences of sense, and the opposition of authority.