135
The Sun has been seen in Nova Zemla, when it was 4 degrees below the horizon, its rays suffering a very extraordinary refraction in their oblique passage through the frozen vapours hanging over the icy sea.
The cliffs of the French coast have been seen from Hastings at the distant of 50 miles, though they are commonly concealed by the convexity of the Globe.[1]
178. In some instances the refraction of the air is as it were inverted by the great rarification of the lower strata of the atmosphere by heat. When the surface of the sea is very warm, the horizon sometimes appears lower than it naturally should, to the amount of 4 or 5 minutes of a degree.
It will readily occur to the reader, that when the lower part of the atmosphere is very much rarified, so that its density diminishes rapidly to the ground, it is possible for a ray of light tending downwards, to be reflected, as it is in glass, or water, when the angle of incidence exceeds a certain value.
It is also possible, that two rays proceeding from the same point, above the heated air, one horizontally, the other slanting downwards, may meet as in Fig. 202, and thus give to an observer the appearance of two distinct points.
The following account is taken with very little alteration, from that which M. Biot has given in his Astronomie, his Physique, and his Précis Elémentaire.
When a dry and sandy soil is strongly heated by the Sun, the air in contact with it is rarified, so that the density of the atmosphere increases from the surface of the Earth to a certain height, which is generally very small, then continues sensibly the same for a certain space, and decreases slowly, but continually, throughout its remaining extent. Now if the eye of an observer be placed in the
- ↑ In the Annals of Philosophy for Nov. 1818, there is an account of some barometrical measurements on the Jura mountains, in which it is stated, that a signal in form of a pyramid, and a large poplar tree which stood by it, presented in the different parts of the day, the extraordinary varieties of appearance represented in Figs. 198, 199, 200, 201.
