EARTH. 



771 



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 

 oeing greatest at the equator, all bodies revolve pro- 

 portionally taster 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 ex- 

 actly 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 sphe- 

 roid ; 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, Measure- 

 ment of. 



Another important desideratum for a more intimate 

 acquaintance with the earth was, to fix its magnitude. 

 The labours 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 Alcmaar to Leyden and Bergen-op-Zoom, by 

 means of triangles. After him, the measurements of 

 Picard, and the later ones of Maupertuis, approxi- 

 mated nearer the truth. These made the circumfer- 

 ence 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 lon- 

 gitude. 



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 six 

 hours, which forms our common year. The orbit of 

 the earth is an ellipse, with the sun hi 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, 

 tarthcst 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 seventeen 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 twenty-three hours, 

 fifty-six minutes, and four seconds). This diurnal 

 revolution is the occasion of the alternation of day 

 and night. But as the axis on which the earth per- 

 forms 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 Septem- 

 ber 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 in- 

 equality 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 twenty-eight 

 days and fourteen 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 in- 

 terior. 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 fifty- 

 fourth part ; Asia, one fourteenth ; Africa, a seven- 

 teenth ; and America, a sixteenth. The islands of 

 the Pacific, taken together, are somewhat larger than 

 Europe. The population of the whole earth is esti- 

 mated at froni 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 specu- 

 lators 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, P'olcano, Earthquake, Current, &C. 



Earth, Motion of the. 1'he 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 appear- 

 ances, 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 con- 

 vinced 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 

 Aristarchns 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 ex- 

 plain, and which led them to the complicated motions 

 of the epicycles, in which the planets moved in 

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