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Again, how could water be subject to tension without the aid of air? You entertain no doubt, I suppose, that the jet of water in the amphitheatre, which is thrown from the centre of the arena to the highest pinnacle, is accompanied by tension of the water? And yet neither hand42 nor any other engine can send or force water more effectively than air. It lends itself readily to the influence of the air, by the compelling force of which within the pipe it is raised. Its nature is to flow down, but under pressure it mounts and accomplishes great results contrary to its nature. Yes, and do not heavily laden vessels also prove that it is the resistance of air, not of water, that prevents their sinking? The water of itself would give way, and would be unable to bear up the burthens, were it not itself upborne. So, too, a quoit thrown from a height into a pond does not fall straight in, but recoils, and that merely because the air bears it back. In what way, again, could the sound of a voice be transmitted through the thick barrier of a wall unless the solid masonry contained some air to receive and transmit the sound from without? The tension of the air, of course, affects not only what is exposed, but what is concealed and enclosed as well. This is easy for it to do, since it is never divided, but maintains an unbroken continuity even through the centre of objects by which it appears to be parted. The interposition of walls and high mountains renders it impassable by us, but is no obstacle to itself. The air is there all the same,43 but a portion is enclosed and we cannot follow it through; that’s all.