[1] Mechanical Philosophy, vol. iii. p. 54.
On this we would remark, that the human body, and that of terrestrial animals in general, is not adapted for the pressure of water at great depths; even could man by any contrivance breathe, such a pressure would destroy life; and, indeed, few aquatic animals are constituted for oceanic existence in the depths of the sea. While the surface is alive with its myriads, the depths are still and untenanted; while bays, shores, reefs, and sandbanks, covered by many fathoms of water, are teeming with shelled mollusks, fishes, and thousands of wondrous creeping things, the profundity of ocean is a comparative desert; whatever lives there must be so constituted as to sustain a tremendous amount of aqueous pressure. Indeed, whales, which often plunge to a considerable depth, and remain submerged for twenty minutes, during which time respiration is suspended, are provided by their coating of blubber, and by the peculiar arrangement of their arterial and venous systems, for the pressure they then undergo; but this pressure often repeated, as it is when the animal is wounded and hard driven, soon produces great exhaustion. Captain Scoresby, for example, harpooned a whale, which, on receiving the weapon, descended four hundred fathoms, at the rate of eight miles an hour; but these animals, when suffering from the torture of the harpoon, often descend to a much greater depth, and sometimes strike so violently against a hard bed of the ocean as to fracture their jaws. At the depth of eight hundred fathoms, captain Scoresby calculates the pressure at 211,200 tons. On the other hand, the organization of man (and other animals) is as ill calculated for a much lighter pressure than that of our atmosphere at sea-level, as it is for great pressure in the depths of ocean.
In proportion as we ascend the alpine elevations of our globe, or mount upwards in a balloon, we find the air more and more rarefied. These elevations are, however, but trifling; nevertheless, trifling as they are, what an effect the decrease of pressure produces on the human frame! The heart beats with violence, the lungs gasp for more air, they have not pressure enough; the blood begins to ooze out of the minute vessels ramifying through the tissue of their delicate cells; blood issues from the nose, the eyes, the ears; the slightest exertion becomes oppressive--a mile or two higher, and death is inevitable. The difficulties attendant upon the ascent of Mont Blanc, the vast Himalaya chain, and the heights of the Cordilleras, are quite as much connected with the state of the air as with the terrible ravines and precipices which obstruct the way. Indeed, as is well known, on the elevated plateaus of South America or Thibet, men and animals accustomed to low plains, or even to gently undulating grounds, are for a long time distressed for breath, and incapable of bodily exertion. Time alone habituates them to the rarer and lighter atmosphere. But what is an elevation of 13,000 or 14,000 feet, nay of 15,668 feet, (Mont Blanc,) of 25,747 feet, or 28,077 feet, (Jewahir and Dhawalagiri, peaks of the Himalaya,) to that of twenty or thirty miles? At an elevation of twenty miles, the heart of a human being would burst, his lungs become gorged with blood, from every pore of his body a sanguine stream would gush forth--he would immediately die. Is not, then, the pressure of the atmosphere necessary for the existence of man, constituted as he is for the planet which he inhabits? But the atmosphere, with regard to its relationship to the solid globe it environs, demands a few words.