The next point to be regarded was a matter of far greater importance. From indications afforded by the barometer, we find that, in ascensions from the surface of the earth we have, at the height of 1,000 feet, left below us about one-thirtieth of the entire mass of atmospheric air, that at 10,600 we have ascended through nearly one-third; and that at 18,000, which is not far from the elevation of Cotopaxi, we have surmounted one-half the material, or, at all events, one-half the ponderable, body of air incumbent upon our globe. It is also calculated that at an altitude not exceeding the hundredth part of the earth's diameter—that is, not exceeding eighty miles—the rarefaction would be so excessive that animal life could in no manner be sustained, and, moreover, that the most delicate means we possess of ascertaining the presence of the atmosphere would be inadequate to assure us of its existence. But I did not fail to perceive that these latter calculations are founded altogether on our experimental knowledge of the properties of air, and the mechanical laws regulating its dilation and compression, in what may be called, comparatively speaking, the immediate vicinity of the earth itself; and, at the same time, it is taken for granted that animal life is and must be essentially incapable of modification at any given unattainable distance from the surface. Now, all such reasoning and from such data must, of course, be simply analogical.

The greatest height ever reached by man was that of 25,000 feet, attained in the aeronautic expedition of Messieurs Gay-Lussac and Biot. This is a moderate altitude, even when compared with the eighty miles in question; and I could not help thinking that the subject admitted room for doubt and great latitude for speculation.

But, in point of fact, an ascension being made to any given altitude, the ponderable quantity of air surmounted in any farther ascension is by no means in proportion to the additional height ascended (as may be plainly seen from what has been stated before), but in a ratio constantly decreasing. It is therefore evident that, ascend as high as we may, we cannot, literally speaking, arrive at a limit beyond which no atmosphere is to be found. It must exist, I argued; although it may exist in a state of infinite rarefaction.

On the other hand, I was aware that arguments have not been wanting to prove the existence of a real and definite limit to the atmosphere, beyond which there is absolutely no air whatsoever. But a circumstance which has been left out of view by those who contend for such a limit seemed to me, although no positive refutation of their creed, still a point worthy very serious investigation. On comparing the intervals between the successive arrivals of Encke's comet at its perihelion, after giving credit, in the most exact manner, for all the disturbances due to the attractions of the planets, it appears that the periods are gradually diminishing; that is to say, the major axis of the comet's ellipse is growing shorter, in a slow but perfectly regular decrease. Now, this is precisely what ought to be the case, if we suppose a resistance experienced from the comet from an extremely rare ethereal medium pervading the regions of its orbit.

For it is evident that such a medium must, in retarding the comet's velocity, increase its centripetal, by weakening its centrifugal force. In other words, the sun's attraction would be constantly attaining greater power, and the comet would be drawn nearer at every revolution. Indeed, there is no other way of accounting for the variation in question. But again. The real diameter of the same comet's nebulosity is observed to contract rapidly as it approaches the sun, and dilate with equal rapidity in its departure towards its aphelion. Was I not justifiable in supposing with M. Valz, that this apparent condensation of volume has its origin in the compression of the same ethereal medium I have spoken of before, and which is only denser in proportion to its solar vicinity? The lenticular-shaped phenomenon, also called the zodiacal light, was a matter worthy of attention. This radiance, so apparent in the tropics, and which cannot be mistaken for any meteoric lustre, extends from the horizon obliquely upward, and follows generally the direction of the sun's equator. It appeared to me evidently in the nature of a rare atmosphere extending from the sun outward, beyond the orbit of Venus at least, and I believed indefinitely farther.[2] Indeed, this medium I could not suppose confined to the path of the comet's ellipse, or to the immediate neighborhood of the sun. It was easy, on the contrary, to imagine it pervading the entire regions of our planetary system, condensed into what we call atmosphere at the planets themselves, and perhaps at some of them modified by considerations, so to speak, purely geological.

Having adopted this view of the subject, I had little further hesitation. Granting that on my passage I should meet with atmosphere essentially the same as at the surface of the earth, I conceived that, by means of the very ingenious apparatus of M. Grimm, I should readily be enabled to condense it in sufficient quantity for the purposes of respiration. This would remove the chief obstacle in a journey to the moon. I had indeed spent some money and great labor in adapting the apparatus to the object intended, and confidently looked forward to its successful application, if I could manage to complete the voyage within any reasonable period. This brings me back to the rate at which it might be possible to travel.

It is true that balloons, in the first stage of their ascensions from the earth, are known to rise with a velocity comparatively moderate. Now, the power of elevation lies altogether in the superior lightness of the gas in the balloon compared with the atmospheric air; and, at first sight, it does not appear probable that, as the balloon acquires altitude, and consequently arrives successively in atmospheric strata of densities rapidly diminishing—I say, it does not appear at all reasonable that, in this its progress upwards, the original velocity should be accelerated. On the other hand, I was not aware that, in any recorded ascension, a diminution was apparent in the absolute rate of ascent; although such should have been the case, if on account of nothing else, on account of the escape of gas through balloons ill-constructed, and varnished with no better material than the ordinary varnish. It seemed, therefore, that the effect of such escape was only sufficient to counterbalance the effect of some accelerating power. I now considered that, provided in my passage I found the medium I had imagined, and provided that it should prove to be actually and essentially what we denominate atmospheric air, it could make comparatively little difference at what extreme state of rarefaction I should discover it—that is to say, in regard to my power of ascending—for the gas in the balloon would not only be itself subject to rarefaction partially similar (in proportion to the occurrence of which, I could suffer an escape of so much as would be requisite to prevent explosion), but, being what it was, would, at all events, continue specifically lighter than any compound whatever of mere nitrogen and oxygen. In the meantime, the force of gravitation would be constantly diminishing, in proportion to the squares of the distances, and thus, with a velocity prodigiously accelerating, I should at length arrive in those distant regions where the force of the earth's attraction would be superseded by that of the moon. In accordance with these ideas, I did not think it worth while to encumber myself with more provisions than would be sufficient for a period of forty days.

There was still, however, another difficulty, which occasioned me some little disquietude. It has been observed, that, in balloon ascensions to any considerable height, besides the pain attending respiration, great uneasiness is experienced about the head and body, often accompanied with bleeding at the nose, and other symptoms of an alarming kind, and growing more and more inconvenient in proportion to the altitude attained.[3] This was a reflection of a nature somewhat startling. Was it not probable that these symptoms would increase indefinitely, or at least until terminated by death itself?