Earth

[AS.] One of the planets of the solar system, coming next to Mercury and Venus in the order of distance from the sun. The princi¬ pal motions of the earth are (1) its daily revolu¬ tion 011 its axis; (2) its yearly revolution in its orbit round the sun. It was long believed that the earth stood still and the sun, planets, and stars revolved around it, but astronomers have proved that the earth moves round the sun, completing one revolution in about 365 days hours. The orbit is an ellipse, with the sun in one focus, so that the earth is not at the same distance from the sun at all periods of the year the mean distance is about 92,800,000miles. The motion of the earth in its orbit explains the apparent motion of the sun in the heavens during the course of the year. Many general considerations suggest to us the globular form of the earth. When a ship is sailing away from the laud, the hull will be seen to disappear while the masts are still visible, and by degrees the masts also sink below the horizon. If the sea were fiat, the body of the ve el would be visible as long as the masts. Mariners also have sailed round the earth, always steering in the same general direction. Measurements made at dif¬ ferent parts of the earth’s surface show that the length of a degree increases towards the poles, and that consequently the earth is not a perfect sphere, but is flattened at the poles. Its mean diameter is 7,918 miles, and circumference 24,875 miles, while it moves around the sun at a speed of 15 miles per second. The ma of the earth is rather more than five times as great as that of a globe of water of the same size would be. It has been found that the temperature increases about i° F. for every 64 feet of descent. If the temperature were to increase at this rate inwards, then at no great depth the heat would be suffi¬ cient to melt the ordinary materials of the crust known to us. Hot springs and volcanoes show that the interior of the earth is much warmer than the exterior. It is therefore supposed that, though the materials in the interior are at an exceedingly high temperature, yet owing to the great pre ure under which they exist they are most probably in the solid state. part of the surface of the earth, resulting from a shock inflicted on a solid portion of the earth at some point below the surface. Earthquakes occur with most frequency in volcanic regions. A few of the more remarkable earthquakes of modern times were those of Uisbon in 1755, Lima in 1746, Peru and Ecuador in 1868, and the Riviera in 1887. usually black, hard, and heavy, which can take on a fine polish. The finest is the heart wood of a tree found in Mauritius. Other trees in Ceylon and the East Indies also yield ebony. A legu¬ mino us tree in the West Indies yields green ebony. It is used for ornamental cabinet work, mosaic, flutes, knife handles, pianoforte keys, etc. Most of the furniture called ebony is made of cherry-wood dyed black. ing disk, whose axis of motion is not in its centre. By its use circular motion can be con¬ verted into intermittent motion. Eccentrics are used to work the valve-gear of steam engines and for many other purposes. It is caused by the sending back from a reflect¬ 6 the undula¬ Rhine those at the Whispering Galleries at St. Paul’s, London, the Observatory, Paris, and the Capitol, Washington, and those on some of the lochs in Scotland. (See Focus.) body which does not allow light to pa through it (an opaque body) is exposed to the light of