Earth

Third planet in order of distance from the sun; period of revolution about the sun 365.2564 days, about its axis 23h., 56m., 4.09s.; equatorial diameter 7,926.592 miles; polar diameter 7,901.476 miles; density 5.58 times that of water. Our direct knowledge of the earth is confined almost exclusively to the surface, it being po ible to penetrate only an insignificant distance into the interior. From such data as we po e the temperature is found to increase as we descend at the rate of 1o F. for every 50 or 60 ft. If this rate continues, of which we have no direct knowledge either way, at a comparatively small depth the temperature will be sufficiently great to melt all known substances; hence the conclusion at one time universally held that the center is a molten ma , the outside crust being at most only a few hundred miles in thickne . On the other hand Lord Kelvin and G. H. Darwin, from investigations of the phenomena of tidal action and of prece ion. have reached the conclusion that the earth as a whole must be a solid body, more rigid than if made of solid gla . In form it is an oblate spheroid, eccentricity being 1: 294.98, as deduced by Clarke. The cubic contents are 260,000,000,000 cubic miles. Its weight is 5,852,000,000,000,000 tons. The total superficial area is 192,200,000 sq. m., of which 55,000,000 are land and 137,200,000 are covered by the sea. The mean height of the land is estimated by Murray at 2,250 ft., and the mean IMG:content-1238.jpg:[blocks in formation] IMG:content-1239.png:[blocks in formation] IMG:content-1240.png:[blocks in formation] IMG:content-1241.jpg:[graphic] Comparative Sizes of Sun and Planets. proved that it is an oblate spheroid, very closely, having an ellipticity of; that is, the amount of flattening at the two poles isth of the equatorial diameter. Erastothenes ab, 230 B.C. found that at Syene in Upper Egypt on the day of the summer solstice the sun pa ed exactly through the zenith, while at Alexandria its meridian zenith distance was 7° 12′. The distance between these places being measured, the length of 7° 12′ became known, and by a simple proce that of 360°, or the circumference of the earth. This method was perfectly correct in principle. A similar measurement was made by Posidonius, and by Caliph Almamoum 814. We are unacquainted with the results of these operations, but they could not have been more than rough approximations. The first determination entitled to much confidence from a scientific point of view was that of Picard in France 1669. From this source Newton obtained the data nece ary to his famous researches relating to gravity. About this time doubts began to arise whether the earth were an exact sphere. Theory had advanced sufficiently to indicate an oblate spheroid as the probable form, while some measurements made in France by J. and D. Ca ini unexpectedly and erroneously indicated a prolate spheroid. To settle the question, which had now attracted great attention, the French Academy in 1755 determined to obtain measurements at two points differing as widely as po ible in latitude. Expeditions, which are famous in the history of science, were sent to Lapland and to Peru; the results then arrived at have not been greatly changed by subsequent investigation. The latest researches indicate the general form of the earth to be that of an ellipsoid, the three axes having the following lengths: polar axis, 41,708,954 feet; equatorial diam. ( a ) 41,853,258 feet, equatorial diam. ( b ) 41,850,210 feet. The greatest equatorial axis being in longitude 8° 15′ west of Green wich. The continental elevations and ocean depre ions constitute, how ever, very appreciable departures from a true mathematical figure.