Intimately connected with this particular subject is the application of the phenomena of twilight, or rather of the amount of depression of the sun at the close of twilight, to determine the height of the atmo sphere, first proposed by Kepler, and which, in fact, gives a height nearly agreeing with that inferred from the law of elasticity of the I air, of between forty and forty-five miles. But the argument from the observed depression of the sun is inconclusive, because we do not know when twilight has ceased, nor, indeed, whether it ever ceases. According to Leslie, admitting, from the fact that in clear weather in no climate is there total darkness, even at midnight, "that the body of air extends to such an altitude, as to receive the most dilute glimmer, after the sun has attained his utmost obliquity, and sunk ninety degrees below the horizon," the elevation of the atmosphere must be equal at least to 1638 miles. In this reasoning, however, no account is taken of the necessary limitation of the atmosphere by the cold of interplanetary space, which must have effect at a point greatly nearer the earth. On the other hand, the actual demonstration of the ex istence of a solid or liquid stratum at the summit of the atmosphere, by the means suggested abovo, would probably involve also, the deter mination of its altitude, and, conversely would enable us to fix, definitively, the extent of twilight upon the earth's surface. [ATMO
SPHERE; METEOROLOGY.] A question here arises as to the photographic intensity and proper ties of the reflected light from which the phenomenon of twilight proceeds, as compared with those of the direct sun-light itself. But this comparison does not appear to have been made in any express manner. According to the photo-chemical researches of Bunsen and Roscoe PhiL Trans.' 1859, p. 898), the chemical illumination, that is, the affection by the chemical rays, of the earth's surface, is merely a function of the sun's zenith-distance ; all the elements of the sun's radiation (so to call them), light, heat, and chemical action, diminishing equally with his altitude. As the diffused light of day is all, iu its immediate origin, reflected, and as it, in common with that reflected from the clouds, possesses normal chemical action, differing only in its intensity from that of the direct light of the sun, we may infer that a similar difference only exists in the case of twilight.