and thus we have another turning back, and a constant mingling. Tho curved arrows, h j k and h' f' k', are intended, on the "diagram of the winds" (Plate I.), to represent this rising up from the counter trades and turning back with the upper current.
229. Supposing the air visible, the spectacle that would he presented between the upper and lower currents.—Let us imagine the air to be visible, that we could see these different strata of winds, and the air as it is sloughed off from one stratum to join the other. We can only liken the spectacle that would be presented between the upper and the lower stratum of these winds to the combing of a succession of long waves as they come rolling in from the sea, and breaking one after another, upon the beach. They curl over and are caught up, leaving foam from their white caps behind, but nevertheless stirring up the sea and mixing up its waters so as to keep them all alike.
230. The importance of atmospherical circulation.—If the ordinances of nature require a constant circulation and continual mixing up of the water in the sea, that it become not stagnant, and that it may be kept in a wholesome state for its inhabitants, and subserve properly the various offices required of it in the terrestrial economy, how much more imperative must they not be with the air? It is more liable to corruption than water; stagnation is ruinous to it. It is both the sewer and the laboratory for the whole animal and vegetable kingdoms. Ceaseless motion has been given to it; perpetual circulation and intermingling of its ingredients are required of it. Personal experience teaches us this, as is manifest in the recognized necessity of ventilation in our buildings—the wholesome influences of fresh air, and the noxious qualities of "an atmosphere that has in circulation." Hence, continual mixing up of particles in the atmosphere being required of the winds in their circuits, is it possible for the human mind to conceive of the appointment of "circuits" for them (§ 216) which are so admirably designed and exquisitely adapted to the purpose as are those which this view suggests?
231. Its vertical movements—how produced.—As a physical necessity, the vertical circulation of the air seems to be no less important than its horizontal movements, which we call wind. One begets the other. The wind, when it blows across parallels of latitude—as it always must, except at the equator, for it blows
