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The atmosphere is in unbroken contact with the earth, in such close juxtaposition that it must always occupy the space that she has just quitted. It is a part, as I have said, of the universe. At the same time it receives all that the earth sends forth for the nourishment of the heavenly bodies; so that, of course, it should be understood in this connection as material rather than part. It is these earthy elements that cause its fickleness and constant turmoil. Some authorities believe the atmosphere to be composed of separate bodies as dust is, but they are sadly in error. For there can never be internal effort in a body held together in any other way than by unity,38 since the elements must be in agreement in order to contribute their united strength toward the tension. Now, the atmosphere, if assumed to be cut up into atoms, must be dispersed. Scattered elements cannot hold together as one body. But, as a matter of fact, the tension of the atmosphere is proved by inflated objects that will not yield to a blow. It is proved, too, by weights carried up to a great height merely by the support of the wind. It is proved by the sound of voices sinking or swelling, according to the stirring (= vibration) of the air. For what is voice save tension of the air moulded by a stroke of the tongue so as to become audible? What is all running and motion? Are they not the effects of tense air? This it is that imparts strength to the sinews, and endows the runner with his speed. When, being violently stirred, it has twisted itself into an eddy, it uproots trees and woods, carries aloft and shatters whole buildings. When the sea lies all peaceful, the air raises it in waves. Or, to descend to less violent manifestations, what song can be sung without tension of breath? Or, take horns and trumpets, or those organs that by means of hydraulic pressure can produce a greater volume of sound than the mouth is capable of doing: is it not through atmospheric tension that they display their functions? Or, let us note what an enormous force is exerted in secret by quite tiny seeds, whose smallness has allowed them to find a lodgment in the clefts of stones. Their slender diminutive roots gather strength enough to dislodge huge boulders, split statues, and cleave crags and rocks. And to what is this due but air tension, without which there is no strength, over which no strength can prevail? The unity of the atmosphere may, in fact, be inferred from the mere coherence of our bodies. What else is it that holds them together save air? What else is it by which the soul is stirred (literally, moved)?39 What constitutes that motion if it be not tension? What tension can there be except from unity? What unity could there be unless it were in the air? What else, too, brings forth from the earth its fruits and slender grain, and sets erect the verdant trees, and stretches out their branches, or sets them on high, but the tension and unity of air?