FIGURE.

3. The form of the Earth is that of an oblate spheroid. The equatorial diameter is twenty-six and a half miles longer than the polar.

The equatorial diameter=7925.6 miles,

This gives a compression of thirteen and a quarter miles to each hemisphere. If the earth should cease to rotate on its axis, the waters of the ocean about the equator would flow towards the poles, seeking the lowest level, i. e. the position nearest the center of the earth. The direction of rivers running towards the equator would be reversed.

This form of the globe seems to indicate that its particles have been free to obey the centrifugal force. The other planets exhibit spheroidal forms. The equatorial diameter of the planet Jupiter, exceeds the polar diameter by more than six thousand miles.

DENSITY.

4. The density of the Earth is five and a half times that 5.67 of water. It weighs five and a half times as much as a Bailly globe of water of the same size. This is more than twice lat, the density of the most prevalent rocks at the surface. Hence it appears that the density increases towards the center. By the pressure exerted beneath the surface the bulk of bodies is compressed, and their density consequently increased. At the depth of thirty-four miles, air would become as dense as water; at three hundred and sixty-two miles, water as heavy as quicksilver; and at the center the average minerals of the surface would be compressed into

less than one-tenth their present bulk. This would make the mean density of the globe much greater than it is. Consequently the materials within must be different from those at the surface, or this compressing force must be counteracted by some expansive power. The density of the Earth is ascertained by astronomical processes; observing the deflection of the plumb-line caused by a mountain of known dimensions, the attraction of other planets, &c.

TEMPERATURE.

5. The temperature of the globe is determined by a variety of influences. The temperature of the surface is influenced by its latitude, being warmer near the equator, and diminishing toward the poles; by its power of absorbing and retaining, or reflecting the heat of the sun's rays; by its elevation above the general level, the temperature falling as we ascend; and by the distribution of land and water, the climate of the ocean and of islands being milder and more uniform than that of continents in the same latitude. Lines joining places having the same mean annual temperature, are called isothermal lines. These, on the ocean, are very nearly parallel to the equator and to each other, but over continents are modified by the extent and elevation of the land. The following table gives the latitude of isothermal lines on the American and European

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The mean temperature of the whole surface of the Earth is estimated to be 58°. The surface temperature determines the distribution and growth of plants and animals.

The heat of the sun penetrates the crust of the earth to a limited extent, rarely exceeding seventy feet. At this limit the temperature remains constant through the year. On descending below this point the temperature uniformly rises about 1° of Fahrenheit for every fifty-four feet of depth. The rate of increase varies with the nature of the rocks passed through, from 1° for thirty feet to 1° for seventy feet. This has been established by numerous experiments in mines, Artesian wells,* and mineral springs.

In the mine at New Salzwerk, near Minden, in Prussia, two thousand feet deep, the increase of temperature was at the rate of 1° for fifty-four feet. The mean result of a large number of observations in the Saxony mines, gives an increase of 1° for seventy-six feet at depths of two

* Artesian wells are borings through which the water rises nearly or quite to the surface, where no indication of springs existed. They are so called from the French province Artois-the ancient Artesium-where they were used as early as the 12th century of the Christian era.

Fig. 3.

Artesian Well.

a. porous water-bearing rock between the rocks e and d, which are impervious

to water. Through the well at b, the water rises as in a syphon.

thousand feet; while in a coal mine in Durham, at about the same depth, it is 1° for fifty-nine feet. The rate of increase observed in sinking the well at the Barriere de Grenelle, Paris, was 1° for fifty-eight feet. In a very deep Artesian well recently sunk at Mondorf, on the frontier of France and Luxembourg, to a depth of nearly two thousand three hundred feet, the water at two thousand two hundred feet had a temperature of 93° Fahrenheit, showing an increase at the rate of 1° Fahrenheit for fifty-four fcet.* With this rate of increase, at the depth of fifty miles, the heat would be sufficient to melt all known rocks.

SURFACE CONFIGURATION.

6. The surface outline of the Earth is very irregular, intersected by mountains and vallies, seas and rivers. The highest mountains exceed five miles.

Dhawalagiri, in the Himalayas, 28,072 feet.

The mean height of all the solid parts of the Earth's surface above the ocean, is estimated by Humboldt at about one thousand feet; that of South America, one thousand one hundred and fifty one; of North America, seven hundred and forty-eight; of Europe, six hundred and seventyone; and of Asia, one thousand one hundred and thirtytwo feet. The length of the chain of mountains between Siberia and India is ten thousand miles, and its breadth one thousand five hundred miles. The mountains of Amer*Ansted.