sky.—Upon the proper adjustments of the dynamical forces which keep up these ceaseless movements the life of organic nature depends. If the air that is breathed were not taken away and renewed, warm-blooded life would cease; if carbon, and oxygen, and hydrogen, and water were not in due quantities dispensed by the restless air to the flora of the earth, all vegetation would perish for lack of food. That our planet may be liable to no such calamity, power has been given to the wayward wind, as it "bloweth where it listeth," to bring down from the pure blue sky fresh supplies of life-giving air wherever it is wanted, and to catch up from the earth wherever it may be found, that which has become stale—to force it up, there to be deflagrated among the clouds, purified and renovated by processes known only to Him whose ministers they are. The slightest change in the purity of the atmosphere, though it may be too slight for recognition by chemical analysis in the laboratory, is sure to be detected by its effects upon the nicer chemistry of the human system, for it is known to be productive of disease and death. No chemical tests are sensitive enough to tell us what those changes are, but experience has taught us the necessity of ventilation in our buildings, of circulation through our groves. The cry in cities for fresh air from the mountains or the sea, reminds us continually of the life-giving virtues of circulation. Experience teaches that all air when pent up and deprived of circulation becomes impure and poisonous.
239. Beautiful and benign arrangements.—How minute, then, pervading, and general, benignant, sure, and perfect must be that system of circulation which invests the atmosphere and makes the whole world kin?" In the system of vertical circulation which I have been endeavouring to describe, we see, as in a figure, the lither sky filled with crystal vessels full of life-giving air continually ascending and descending between the bottom and the top of the atmospherical ocean; these buckets are let down by invisible hands from above, and, as they are taken up again, they carry off from the surface, to be purified in the laboratory of the skies, phials of mephitic vapours and noxious gases, with the dank and deadly air of marshes, ponds, and rivers.
240. Their influences upon the mind.—Whenever, by study and research, we succeed in gaining an insight, though never so dim, into any one of the offices for which any particular part of the physical machinery of our planet was designed by the Great
