an angle of 23½°; and thus, when the northern end of the axis is directed away from the sun, the rays of light and heat do not reach it, but, on the other hand, the sun’s rays continually shine upon the southern end. This period is the winter of the northern hemisphere. When the earth has moved a quarter of the way around its orbit, the sun’s rays reach both the north and south poles. At the end of the second quarter of its revolution—or the summer solstice—the northern end of the axis is inclined toward the sun, and thus is continually lighted; while at the third quarter of the revolution once more the light reaches both poles. In the first position, the further north we go the shorter is the time during which the sun’s rays touch any one point, and thus the days are short and the nights long; and, as the days are short and the sun shines only for a short time, the weather is cold. This is the season of winter. Now, as the earth moves through the first quarter of its orbit, the days become longer on the northern hemisphere and shorter in the southern, till they become just equal about the 21st of March. This is called the spring-equinox. At the end of the second quarter the days are long in the north, the nights short and the season therefore warm; and at the third quarter once more the length of days and nights becomes equal, about September 23. At the summer-solstice of the northern hemisphere, when the north pole is inclined toward the sun, sunlight falls 23½° beyond the pole, and, as the earth rotates, all this region remains in daylight the whole 24 hours. At this time the south pole is turned away from the sun to the same extent. The circles bounding these regions of continuous daylight or darkness at the solstices are called the Arctic and Antarctic circles, and the spaces within them the north and south frigid zones. At the same time the sun is vertical at a distance of 23½° north of the equator. This is the highest northern latitude at which the vertical sun is experienced, and is called the tropic of Cancer, from the constellation in which the sun appears at that time. At the winter-solstice of the northern hemisphere the sun is vertical at a distance of 23½° south of the equator, or on the tropic of Capricorn. As the sun appears overhead in all places between these tropics twice in the year, and thus exerts its greatest heating power, this broad belt of the earth is called the torrid zone. The belts between the tropics and the polar circles are called the northern and southern temperate zones.
The sun’s heat is constantly at work breaking down the higher rocks and spreading the broken matter as soil over the lower ground. The circulating of water is the great instrument for this work: vapor raised from the oceans and carried by winds is condensed as rain on the highlands, and, returning to the sea in the forms of springs and streams, has a chief share in wearing down the surface of the land. This process would finally reduce the land to a common low level, were it not counteracted by the continual gentle elevations and depressions of the surface, due to internal changes. Animal and vegetable life is spread all over the globe and has had a large share in producing the condition and aspect of many parts of the earth, as is witnessed by the great coal-fields of the earth, the chalk, limestone and marble found in many regions and the coral reefs and islands of tropical seas. Man, too, has helped to change the appearance of the face of the earth.
The average density of the earth is about five and one half times as great as that of water. Since the density of the earth’s crust is very much less than this, it is not unlikely that the interior of the earth has a density as high as 7 or 8 and that it is composed largely of metals.
Some of the articles which will be useful in this connection are Africa, America, Australia, Earthquake, Europe, Geology, Glaciers, Gravitation, Pacific, Planets, Sun, Tides, Volcano.
Earthquake, a name given to any very considerable and very sudden disturbance of the earth’s crust. Since the late Lord Kelvin showed that the earth is more rigid than steel, it is not surprising that a disturbance at any point in the earth’s crust should be propagated rapidly to distant parts of the earth. Concerning the character of the original disturbance very little is known; the slipping of one layer of rock over another and the falling of great masses in the interior of a mountain or volcano have been suggested. But concerning the vibrations which are propagated from one part of the earth to another a great deal has been learned within the last few years by means of the seismograph, an instrument which records both the character and the time of the disturbance at any one point. The principle upon which these seismographs are constructed is simply that of an ordinary gate standing halfway open. If the post to which the gate is hinged be shaken to and fro, the center of gyration of the gate will remain fixed. If now a style be attached to the gate, it will trace out a line on the ground (which moves with the post); and this line will be a description of the displacement perpendicular to the direction of the gate. Now, a typical seismograph contains two open gates with heavy masses at the outer ends, each gate being at right angles to the other so that both components of any horizontal disturbance are obtained. A third heavy mass capable of motion only in a vertical
