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 eause 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 our antipodes (q. v.) would fall 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, every 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 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 have 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 magnitude. The labors of the ancients, in this respect, were all fruitless, owing to 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 Alemaar 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 accurately, the true magnitude of it. (See Account of Experiments, to determine the Figure of the Earth, by Means of the Pendulum, & C 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 inore 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 second-a 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 day and night. But as the axis on which the earth performs its diurnal rotation forms, EARTH-MOTION OF THE EARTH. with its path about the sun, an angle of 234 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 234 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.) Earth, 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 367 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 in 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 this 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 these means, the zeal for astronomy had been highly excited. Every city in Italy was then a little Athens, in which the arts and sciences 368 MOTION OF THE EARTH. 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, does 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 Transactions, and were 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, 13 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 northward. 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.) 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. EARTHQUAKE-EARTHS. EARTHQUAKE; a shaking of certain a shaking of certain parts of the earth's surface, produced by causes not perceivable by our senses. This motion occurs in very different ways, and in various degrees of violence. Sometimes it is perpendicular, throwing portions of the ground into the air, and making others sink. Sometimes it is a horizontal, undulating motion, and sometimes it appears to be of a whirling nature. Sometimes it is quickly over; sometimes continues long, or recurs at intervals of weeks, days or months. At one time, it is confined within a small circle; at another, it extends for many miles. At one time, it is hardly perceptible; at another, it is so so violent, that it not only demolishes the works of human art, but changes the appearance of the ground itself. Sometimes the surface of the ground remains unbroken; sometimes it bursts open into clefts and chasms; and then occasionally appears the phenomenon of the eruption of gases, and also of flames, with the ejection of water, mud and stones, as in volcanic eruptions. The eruptions of proj and permanent volcanoes are preceded by, and proportionate to, the agitations of the earth in their neighborhood. These observations furnish grounds for the conclusion, that earthquakes cannot proceed from external causes, but arise from certain powers operating within the circumference or crust of the earth. Moreover, all the phenomena of earthquakes bear so much affinity to those of volcanoes, that there can hardly be a doubt, that both proceed from the same causes, acting differently, according to the difference of situation, or different nature of the surface on which they operate. A volcano differs from an earthquake, principally, by having a permanent crater, and by the reappearance of the eruptions in the same place, or in its immediate vicinity. All the other phenomena of a volcano, such as the subterranean thunder-like noises, the shaking, raising and bursting asunder of the earth, and the emission of elastic fluids, the fire and flames, the ejection, too, of mineral substances, all occur, now and then, more or less, in earthquakes as well as in volcanic eruptions, even when at a distance from active volcanoes; and the genuine volcanic eruptions are, as has been remarked, accompanied or announced by shakings of the earth. All our observations go to prove, that volcanic eruptions, earthquakes, the heaving of the ground from within, and the disruption of it in the same way, are produced by one and the same cause, by one and the 369 same chemical process, which must have its seat at a great depth beneath the present surface of the earth. The most remarkable earthquakes of modern times are those which destroyed Lina, in 1746, and Lisbon, in 1755; in the latter, 20,000 persons were killed. It extended from Greenland to Africa and America. A similar fate befell Calabria, in 1783, the province of Caracas, in South America, in 1812, and Aleppo, in Syria, in 1822. Several earthquakes have taken place quite lately, in South America, one particularly dreadful at Lima. The city of Guatemala, also, was nearly destroyed in the spring of 1830, by earthquakes, which continued five days successively. EARTHS. The term earth is applied, in common life, to denote a tasteless, inodorous, dry, uninflammable, sparingly-soluble substance, which is difficultly fusible, and of a moderate specific gravity. Several of the earths are found in a state of purity in nature; but their general mode of occurrence is in intimate union with each other, and with various acids and metallic oxides. Under these circumstances, they constitute by far the greatest part of the strata, gravel and soil, which go to make up the mountains, valleys and plains of our globe. Their number is ten, and their names are siler, alumina, magnesia, lime, barytes, strontites, zircon, glucine, yttria and thorina. The four first have long been known to mankind; the remainder have been discovered in our own times. Silex exists nearly pure, in large masses, forming entire rocks, as quartz rock, and constituting the chief ingredient in all granitic rocks and sandstones, so that it may safely be asserted to form more than one half of the crust of the earth. Alumine is found pure in two or three exceedingly rare minerals, but, in a mixed state, is well known as forming clays and a large family of rocks, usually called argillaceous. Lime, an earth well known from its important uses in society, occurs combined with carbonic acid, in which state it fonns limestone, marble, chalk, and the shells of snails. It exists also, upon a large scale, in combination with sulphuric acid, when it bears the name of gypsum. Magnesia is rare in a state of purity, but enters largely into the composition of some of the primary rocks, especially of the limestones. The remaining eight (if we except barytes, which, in combination with sulphuric acid, is often met with in metallic veins) are only known to the chemist as occurring in the composition of certain minerals, which, for the most part, are exceed ingly rare. The earths are very similar to the alkalies (q. v.), forming, with the acids, peculiar salts, and resembling the alkalies likewise in their composition. They consist of peculiar metals in combination with oxygen, and compose the greatest part of the solid contents of the globe. They differ from the alkalies principally in the following peculiarities: they are incombustible, and cannot, in their simple state, be volatilized by heat; with different acids, especially the carbonic, they form salts, insoluble, or soluble only with much difficulty, and with fat oils, soaps insoluble in water. They are divided into two classes, the alkaline and proper earths. The former have a greater similarity to the alkalies. In their active state, they are soluble in water, and these solutions may be crystallized. They change the vegetable colors almost in the same way as alkalies, and their affinity for acids is sometimes weaker and sometimes stronger than that of the alkalies. They combine with sulphur, and form compounds perfectly similar to the sulphureted alkalies. With carbonic acid, they form insoluble salts, which, however, become soluble in water by an excess of carbonic acid. The alkaline earths are as follows: 1. barytes, or heavy earth, so called from its great weight; 2. strontites (q. v.); both these earths are counted among the alkalies, by many chemists, on account of their easy solubility in water; 3. calcareous earth, or lime, forms one of the most abundant ingredients of our globe; 4. magnesia is a constituent of several minerals. The proper earths are wholly insoluble in water, infusible at the greatest heat of our furnaces, and, by being exposed to heat, in a greater or less degree, they lose their property of easy solubility in acids. Some of them are incapable of combining with carbonic acid, and the remainder form with it insoluble compounds. They are the following: 1. alumine; 2. glucine, which is found only in the beryl and emerald, and a few other minerals; 3. yttria is found in the gadolinite, in the yttrious oxide of columbium, &c.; 4. zirconia is found less frequently than the preceding, in the zircon and hyacinth; 5. silex. The earths were regarded as simple bodies until the brilliant researches of sir H. Davy proved them to be compounds of oxygen with peculiar bases, somewhat similar to those of the alkalies, potassium and sodium. Some of the heavier of the earths had often been imagined to be analogous to the metallic oxides; but every attempt to tors. effect their decomposition or reduction had proved unsuccessful. After ascertaining the compound nature of the alkalies, Davy submitted the earths to the same mode of analysis by which he had effected that fine discovery. The results obtained in his first experiments were less complete than those afforded with the alkalies, owing to the superior affinity between the principles of the earths, as well as to their being less perfect electrical conducBy submitting them to galvanic action, in mixture with potash, or with metallic oxides, more successful results were obtained; and a method employed by Berzelius and Pontin, of placing them in the galvanic circuit with quicksilver, terminated very perfectly in affording the bases of barytes and lime, in combination with this metal. By the same method, sir H. Davy decomposed strontites and magnesia; and, by submitting silex, alumine, zircon and glucine to the action of the galvanic battery, in fusion with potash or soda, or in contact with iron, or by fusing them with potassium and iron, appearances were obtained sufficiently indicative of their decomposition, and of the production of bases of a metallic nature. Thorina, the last discovered earth, was decomposed by heating the chloride of thorium with potassium. The metallic bases of the earths approach more nearly than those of the alkalies to the common metals, and the earths themselves have a stricter resemblance than the alkalies to metallic oxides. metallic oxides. Viewing them as forming part of a natural arrangement, they furnish the link which unites the alkalies to the metals. Accordingly, many of the more recent systems of chemistry treat of all these bodies as forming a single group under the name of the metallic class. Still (as doctor Ure justly remarks), whatever may be the revolutions of chemical nomenclature, mankind. will never cease to consider as earths those solid bodies composing the mineral strata, which are incombustible, colorless, not convertible into metals by all the ordinary methods of reduction, or, when reduced by scientific refinements, possessing but an evanescent metallic existence. (For a more particular account of the properties of the earths, and of their bases, consult the articles relating to them, respectively, in this work.) EARWIG; an insect whose name is derived from its supposed habit of insinuating itself into the ears of persons who incautiously sleep among grass where it is found. It is extremely doubtful whether |