The third planet in order of distance from the sun, Mercury and Venus being nearer to it. It is in shape a sphere slightly flattened at the poles and bulged at the equator, hence it is called an oblate spheroid. The equatorial diameter or axis measures 7,926 miles and 1.041 yds., and the polar diameter is 7,899 miles and 1.023 yds. The earth revolves upon its axis, completing its diurnal or daily revolution in a sidereal day, which is 3 minutes and 55.9 seconds shorter than a mean solar day. It revolves around the sun in one sidereal year, which is 365 days, 6 hours, 9 minutes, and 9 seconds. Its orbit or path around the sun is an ellipse, having the sun in one of the foci. The earth’s mean distance from the sun is 93,000,000 miles. Its axis is inclined to the plane of its orbit at an angle of 23° 27′ 12.68″. The circumference at the equator measures 24,899 miles. The total surface is 196,900,278 sq. miles, and the solid contents is 260,000,000,000 cubic miles. As we descend into the earth the temperature rises at the rate of 1° Fahr. for every 50 ft. At the depth of 10 or 12 miles the earth is red-hot, and at a depth of 100 miles the temperature is such that at the surface of the earth it would liquefy all solid matter in the earth. What Causes Hail? Hail is the name given to the small masses of ice which fall in showers, and which are called hailstones. When a hailstone is examined it is found usually to consist of a central nucleus of compact snow, surrounded by successive layers of ice and snow. Hail falls chiefly in Spring and Summer, and often accompanies a thunderstorm. Hailstones are formed by the gradual rise and fall, through different degrees of temperature (by the action of windstorms), and they then take on a covering of ice or frozen snow, according as they are carried through a region of rain or snow. With regard to rain, it may be said, in popular language, that under the influence of solar heat, water is constantly rising into the air by evaporation from the surface of the sea, lakes, rivers, and the moist surface of the ground. Of the vapors thus formed the greater part is returned to the earth as rain. The moisture, originally invisible, first makes its appearance as cloud, mist or fog; and under certain atmospheric conditions the condensation proceeds still further until the moisture falls to the earth as rain. Simply and briefly, then, rain is caused by the cooling of the air charged with moisture. Why Does a Human Being Have To Learn to Swim? It is strange, isn’t it, that almost every animal, excepting man and possibly the monkey, knows how to swim naturally; others such as birds, horses, dogs, cows, elephants, can swim as soon as they can move about alone.