I remember that I learned these data when I was living rooted to or anchored upon the earth; but the explanation which my masters gave me was not so clear as I could have wished. Their considerations on the declensions of the sun, on the obliquity of the ecliptic, on the necessity of exactly reducing the earth to a simple point in relation to the distance of the stars, to the end that the phenomena occurring (_rapportés_) on parallel planes would be nearly identical with those observed from the centre of the globe, or from a point situated on its surface; all these fine things, which demanded a certain faculty of geometrical intuition, left a curious vagueness of idea upon my mind. I accepted them, under the influence of authority, as beyond discussion, but I was by no means satisfied whenever I wished to ascertain their foundation. * * * * * But now--in the regions of space--everything grows simple before the mind, in which, apparently, all my power of thought is concentrated. Behold the illuminated atmosphere; it reminds me vividly of the disc of the full moon. Ah, what exquisite iris colours! They mark the meeting-points of the bright with the obscured hemisphere: a circular line carried through all these points would exactly separate day and night. On the one side, motion and life and glow; on the other, silence and shade and calm. This line _moves_, carrying with it in its movement day and night, the illuminated and the darkened hemisphere; it moves from east to west, so that the bright hemisphere and the shadowy one, whose union forms what may be called the _photo-adumbrated_ sphere, revolves, in four-and-twenty hours, round an axis which coincides at this moment--the 21st day of March, according to the "terrestrial worms,"--with the axis of the earth rotating on itself in an inverse direction, that is to say, from west to east. Let us note this coincidence: it is remarkable; inasmuch as, at the equinoxes, the terrestrial equator divides exactly the illuminated and the obscured hemisphere into two equal parts, one of which is situated to the north, the other to the south, and their line of separation coincides with a meridian circle. * * * * *