Page:Popular Science Monthly Volume 76.djvu/574

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HE atmosphere is commonly considered as a body of gases surrounding the globe, but hardly as a part of our sphere. We must, however, look upon it as being of the very substance of our earth, an integral part of the planet as truly as the waters or the solid crust. The geologist and the geographer, indeed, habitually speak of three envelopes of the globe, the atmosphere, the hydrosphere and the lithosphere. A certain assemblage of gases, all of which may be found in the waters and the rocks, remains in a more attenuated condition as the outer part of the earth. The degree of attenuation increases as we go from the surface of the solid part. Whether the atmosphere actually ceases a few hundred miles, or some hundreds of thousands of miles, from the lithosphere is not important to our present purpose, for its effective work is done within a few miles of altitude.

Looking at the atmosphere as a whole, its calms are exceptional and its movements are the rule. We may find the gentle breeze, the cyclonic wind or the resistless tornado, but always activity. These movements do not tamely confine themselves to horizontal paths, but the gases rise and plunge, pursue broad curves and narrow spirals, and would present, to an eye that could see them from above, a tumult, like the sea in storm. If we add to these mechanical operations the efficient chemical functions of the atmosphere, we shall be ready to agree that it is one of the most powerful agencies that help to mold the form and fashion the quality of the outer parts of our planet.

We well understand that all organic life is dependent on the atmosphere for its existence, and that interchange of materials is constant. The forms of the land are nearly as dependent upon this medium as are those of life. Manhattan Island was once a mountainous tract. The first making of the rocks that composed it was conditioned by an atmosphere. The forces that filed it down to its present forms and heights could not have worked without the gaseous envelope. The channels that invite ships to its water line are an indirect product of atmospheric activity. Indeed, the Palisades Ridge, and the submergence of the coast line are the only features of your inorganic environment that have chiefly been due to underground forces.

The atmosphere is interwoven with all forces operating on or near the surface. Other, or subterranean, energy could produce but a few types of form. We might have great and swelling ridges or domes, or cliffs due to faulting, involving fracture and dislocation, or volcanic cones, streams or sheet outflows. For such initial forms, apart from an