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The Threefold Covering of the Earth.

What may in the largest and most general sense be called the superficies of the earth, is threefold in character, and yet one in function; consisting of a highly elastic body, the atmosphere, water, and the solid ground. These three forms are variously proportioned; the more elastic is universal, the fluid form is more restricted, and the solid one prevails still less. The more dense the body, the nearer it is found to the center of the earth. The lightest of all floats over the entire periphery of the globe like a grace ful mantle of cloud. Man, and in fact all organisms, live by contact with all three of these forms. The investigation of the elements, and phenomena of the air, regarded in themselves, is the province of meteorology. The mercury-column is the true language of the atmosphere, and tells us in distinct tones of all the changes there. Mineralogy and geology make us acquainted with all the qualities and all the elements of the soil, not in their relation to man, but regarded in themselves. Geography deals with the conflict of all these bodies, their relations to each other, their mutual action and reaction. Meteorology gives occasion for the study of climate, and for the observation of the phenomena of the lower strata of the atmosphere, the fall of rain and snow, for example. Geology and mineralogy give rise to the study of plains and mountain formations, as well as of volcanic phenomena, affecting the surface of the earth as they do in earthquakes, upheavals of whole districts, and the opening of hot springs. Thus, geography has it own province clearly defined, and uses all this and studies it in relation to the organic world, and to man foremost of all.

The most highly elastic covering of the earth is un

broken, the other two are sundered, and each only occupies a part of the surface. Formerly, in most ancient times, the water seems to have covered the entire earth. The study of this is, however, within the domain of geology. We have to do only with the historic period which followed. We have to look at the earth in its present relations, and as the home of man. Now, the portions covered with water are, by far, the largest part of the surface; a little less than three-fourths are water, a little more than one-fourth land. The whole water-mass is composed largely of the oceans, which, in one sense, constitute a continent of their own: in looking at them as we do now, we are not to regard them as ceasing at the outlines of the great land-masses, but as penetrating these as far as to the springs which feed the rivers; for the world of waters, embracing springs, brooks, rivers, lakes, seas, and oceans, is one, and but one.

The water is, in some respects, a form between the other two; its peculiarities, weight, density, freedom of movement, and changeableness of form, are a mean between the opposing extremes of air and the ground. Water can pass to a more fluid or a more solid state; it can become vapor or ice. The measurement of the depth of the world of waters has lately been so clearly connected with the needs of civilization, that geographers have made many exceedingly accurate investigations. Formerly, this was much neglected; up to Captain Cook's time 1500 feet was the greatest depth ascertained; in the course of the Arctic discoveries 7000 feet limited the plummet's descent; Captain Ross sounded, near St. Helena, to a depth of 30,000 feet; and Captain Denhorn, in the South Atlantic, reached a point 4€,000 feet from the surface-about twice the height of the loftiest mountains. And not single points

alone, but entire ocean districts have been traversed; the temperature of these great depths has been studied, the currents, the density, in fact all the features which must be known preliminarily to the laying of great lines of submarine telegraphs, such, for example, as that proposed between North America and Europe.

The atmosphere, too, is by no means thoroughly known to us. It rises to a height between 85 and 95 miles from the earth, of which man has explored in balloons only about five miles, or the height of the loftiest mountains. At loftier heights than we can live, the bright light of midday even fades into a dim kind of twilight, and meteoric masses of iron are seen in full glow, there being oxygen enough even there to support their combustion, and very little resistance to overcome from the density of the atmosphere. Astronomers, Benzenberg in particular, have calculated the distance of the meteors to range from 23 to 100 miles from the earth, and have studied them* in respect to the time when they were visible, their locality, and their direction. The limits of the atmosphere must be at that point where the expansive power of air and the attractive influence of the globe neutralize each other. The form of the atmospheric body is therefore, like the earth, spheroidal, but far more oblate than the earth, in consequence of its much greater fluidity. At the poles, the distance is therefore much less from the surface of the earth to the confines of the atmosphere than at the equa

* Meteors, which are nightly visible, are different from the periodic phenomena, seen in August and November, in different localities over the earth, and called falling stars. These exist outside of our atmosphere, and belong not to the earth, but rather to the great solar system.

tor.

The effect of this upon the refraction of light must be very great.

The investigation of the interior of the earth is more difficult to us than that of the atmosphere even. We cannot say that we know thoroughly more than we can learn by penetrating ooo part of the distance to the center of the globe. Deeper than that our lowest shafts have not sunk. The coal mines of England penetrate perhaps the farthest below the sea level; for the deep mines in Germany, in the Hartz district, for instance, have their entrance hundreds of feet above it. One coal mine near Durham, England, descends to a depth of about 1584 feet, and reaches a point where the thermometer is 79° Fahrenheit. The deep coal mines of England have, however, one rival, in a shaft at Liege, which is sunk 1800 feet.

The modern Artesian well has gone to still greater depths, in the effort to procure brine for the manufacture of salt, or fresh water for the use of cities. At Rehme, in Porta Westphalica, a point 2160 feet from the surface has been reached, and water brought up at a temperature. of 90° Fahrenheit, containing four per cent. of salt. By an ingenious application of mechanics to the process of well boring, doubtless a depth of 5000 feet could be attained. At Mondorf, in Luxemburg, a bore has been made through sandstone and the mineral formations lying beneath it, for a distance of 2700 feet, and water reached at 82° Fahrenheit.

The great upheavals caused by earthquakes and volcanoes disclose still vaster depths. In the eruptions of the latter, immense masses of the inner contents of the earth are thrown out, sometimes enough to form a not insignificant mountain, and to desolate large regions with their debris. As a general thing, the original mineral forms are

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lost and indistinguishable in the molten mass. Yet not seldom perfect specimens are hurled out, imbedded in lava and cinders; not always minute fragments, sometimes huge blocks, testifying not with any degree of completeness, yet clearly, to a certain extent, of the composition of the region bordering on the great inner sea of molten matter. Yet most of our knowledge upon this subject is hypothetical, and what we know only indicates painfully the great extent of that of which we are entirely ignorant.

The uniform increase of temperature, as we descend into the earth, at the rate of about 24° Fahrenheit for every 100 feet, the heat of some mineral springs, leads to the conclusion that, could we advance to a place about twenty-three miles from the surface, we should attain the limits where all becomes a molten mass. The cold surface, on which we walk in such security, seems, by all analogies, to envelop a liquid caldron which has been seething from the morning of the world. This internal mass is, of course, the source of all volcanic eruptions, and of all the phenomena to which I have alluded above. A distinguished geologist has well said that "light and heat are the two extremes of being the farther man goes away from the earth's surface, he encounters light; the farther he recedes inwardly from its surface, he encounters heat." It is true we are not absolutely certain that the rate of increase is uniform, at the rate of one degree Fahrenheit for every 45 feet; but if it is, we should reach the boiling point of water at less than 10,000 feet from the surface, and the melting point of iron (22° Fahrenheit) a little over 120,000 feet. The relation of this thickness to the entire diameter of the earth is about as 1 to 344, about the ratio of the thickness of an egg-shell to the egg.

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