Page:Proceedings of the Royal Society of London Vol 4.djvu/477

Rh menced, involving neither of them, but more nearly contiguous to k and X than to jj. From thence its course was towards tt Eridani, which star must have been covered by it, and was not seen ; this judgement of its direction having been formed by noticing that it passed clearly above y Eridani, and as clearly below and parallel to the direction of d, e Eridani, which two stars being dimmed by the vapours of the horizon and the twilight, were so little conspicuous as perfectly to account for tt not having been noticed. At the point of its pass- age between y and ^ it was still considerably bright, and as it ter- minated with somewhat more abruptness at a point beyond e (then about 12° high) than at its upper extremity, I am rather disposed to consider this end as somewhat curtailed by the vapours. Making no allowance, however, for this, and estimating its visible termina- tion at a point on a celestial globe nearly opposite ^ Eridani (which star however was not noticed at the time), the length above assigned to the luminous band (30°) has been concluded by measurement on the globe.

I am thus particular in describing the course, situation and di- mensions of the band, not only as terms of comparison with other observations of it, should any have been made, but for another rea- son, in which consists the peculiarity of the phenomenon, and which is my sole motive for making this communication. The above situ- ation and course, relatively to those stars, remained perfectly unal- tered the whole time it remained visible at all, which it did for up- wards of an hour from the time I first saw it, accompanying the stars in their diurnal motion, until the preceding end at length was extin- guished in the horizon vapours with the stars adjacent, and until the light of the rising moon dimmed and at length effaced the rest, though 1 apprehend its intrinsic lustre to have been in progress of diminution during the last quarter of an hour or twenty minutes.

I should not forget to mention, that neither in the north-west, nor elsewhere, were any streamers or other appearances of Aurora Borealis perceptible during any part of the evening. The only other luminous appearance, the milky way excepted, was that of the zodiacal light, which I have seldom seen to greater advantage in this climate, and which extended high enough to involve the Pleiades, then about 55° from the sun.

I have said that the general aspect of the phenomenon was that of a bright white cloud. In fact, my first impression was that such was its nature ; an impression immediately dissipated and ultimately converted into the contrary certainty by the following considerations and observed facts. For, in the first place, no ordinary cloud at such an angular elevation above the horizon could have received from the sun, even at the earliest hour when it was observed, any- thing like sufficient illumination to have presented so luminous an appearance; that luminary being then between 9° and 10° below the horizon, and the moon not yet being risen, even at eight o'clock, when I judged the light of the band by contrast with the increasing darkness of the ground of the sky to have attained its maximum, at which hour the depression of the sun was nearly 12°.