steadily, since he would only weigh a few pounds.

The densities, that of water being 1, as far as can be satisfactorily explained, are as follow:—Sun, 1²⁄₁₅; Mercury, 9¹⁄₆; Venus, 5¹¹⁄₁₅; Moon, 4½; Mars, 3²⁄₇; Ceres, 2; Pallas, 2; Jupiter, 1¹⁄₂₄; Saturn, ¹³⁄₃₂; Herschel, ⁹⁹⁄₁₀₀.

Similar observations apply to the influence of the atmosphere, in whatever point of view we consider it, that is, whether we regard its weight, its electrical condition, its illumination, its temperature, its dryness, or humidity.

1. Its Weight.—The weight of the atmosphere (an elastic, compressible, and expansible fluid,) is calculated to be from fourteen to fifteen pounds avoirdupois on every square inch, (pure water, taking bulk for bulk, being about 828 times as heavy.) Now, reckoning the surface of a middle-sized man to be fourteen feet, he sustains the pressure of eleven tons. Many of our readers have seen the philosophical experiment of placing two hollow metallic hemispheres rim to rim, the rims being nicely adjusted and smeared with lard; this being done, the air, by means of a stop-cock on the lower hemisphere, screwed into a powerful air-pump apparatus, is drawn out or exhausted—the stop-cock is turned—the globe is unscrewed from the air-pump, and placed in the hands of those who may consent to try their strength in pulling the two hemispheres asunder. Enormous is the force required; if the diameter of the circle be fourteen inches, the least force that will separate them will be equivalent to half a ton.

Such, then, being the pressure of the atmosphere, as is clearly proved by simple experiments, it may at first create some surprise that the human body is capable of sustaining it without being crushed; that the tiny insect, with its delicate wings—and in some these amount to several square inches of surface—is not reduced to atoms. But the wonder will cease, when we reflect that this pressure, or a pressure little short of it, is essential to the existence, not of man only, but of all terrestrial organic beings.

Dr. Robinson justly observes, that "the human body (he might have said every organic body) is a bundle of solids mixed with fluids, and there are few or no parts of it which are empty. All communicate either by vessels or by pores, and the entire surface is a sieve, through which the insensible perspiration is performed. The whole extended surface of the lungs is open to the pressure of the atmosphere; everything, therefore, is in equilibrio; and if free or speedy access be given to every part, the body will not be damaged by the pressure, however great, any more than a wet sponge would be deranged by pressing it to any depth in the water."[1]

[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.

This elastic fluid must be considered as a body of air revolving with the earth, whence it must be evident that the velocity of the strata of air, if we may use the word, increases as we recede from the earth's axis; for example, at the equator, that stratum of air, (if such there be,) which is twice as distant from the centre of the earth as the surface is, must revolve with twice the actual velocity of the air at the surface. Taking this fact into consideration, it results that, however attenuated, however rarefied, the atmosphere cannot extend beyond 20,000 miles from the surface of the earth; far above that elevation the centrifugal force would counteract the centripetal, or, in other words, the tendency of the particles to the earth would cease, and, consequently, unless air pervaded the universe, which is not the case, 20,000 miles are within the utmost range of possibility. The fact, however, appears to be