FIG. 66.
IT is well known that when a light of any kind shines through a dense medium it appears larger, or rather gives a greater "glare," at a given distance than when it is seen through a lighter medium. This is more remarkable when the medium holds aqueous particles or vapour in solution, as in a damp or foggy atmosphere. Anyone may be satisfied of this by standing within a few yards of an ordinary street lamp, and noticing the size of the flame; on going away to many times the distance, the light or "glare" upon the atmosphere will appear considerably larger. This phenomenon may be noticed, to a greater or less degree, at all times; but when the air is moist and vapoury it is more intense. It is evident that at sunrise, and at sunset, the sun's light must shine through a greater length of atmospheric air than at mid-day; besides which, the air near the earth is both more dense, and holds more watery particles in solution, than the higher strata through which the sun shines at noonday; and hence the light must be dilated or magnified, as well as modified in colour. The following diagram, fig. 66, will show also that, as the sun recedes from the meridian, over a plane surface, the light, as it strikes the atmosphere, must give a larger disc.
Let A, B, represent the upper stratum of the atmosphere; C, D, the surface of the earth; and 1, 2, 3, 4, 5, the sun, in his morning, forenoon, noon, afternoon, and evening positions. It is evident that when he is in the position 1, the disc of light projected upon the atmosphere at 6, is considerably larger than the disc projected from the forenoon position, 2, upon the atmosphere at 7; and the disc at 7 is larger than that formed at 8, when the sun, at 3, is on the meridian; when at 4, the disc at 9 is again larger; and when at 5, or in the evening, the disc at 10 is again as large as at 6, or the morning position. It is evident that the above results are what must of necessity occur if the sun's path, the line of atmosphere, and the earth's surface, are parallel and horizontal lines. That such results do constantly occur is a matter of everyday observation; and we may logically deduce front it a striking argument against the rotundity of the earth, and in favour of the contrary conclusion, that it is horizontal. The atmosphere surrounding a globe would not permit of anything like the same degree of enlargement of the sun when rising and setting, as we daily see in nature.
