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? I finally thought not. Their
      origin was to be looked for in the progressive removal of the
      customary atmospheric pressure upon the surface of the body, and
      consequent distention of the superficial blood-vessels—not in any
      positive disorganization of the animal system, as in the case of
      difficulty in breathing, where the atmospheric density is
      chemically insufficient for the due renovation of blood in a
      ventricle of the heart. Unless for default of this renovation, I
      could see no reason, therefore, why life could not be sustained
      even in a vacuum; for the expansion and compression of chest,
      commonly called breathing, is action purely muscular, and the
      cause, not the effect, of respiration.