In consequence of this refractive power of the atmo sphere, all the heavenly bodies appear higher than they really are ; and hence they often appear above the hori zon when they are actually below it. A very interest ing phenomenon of this kind was seen at Paris on the 19th of July, 1750. The moon was seen eclipsed above the horizon, while the sun itself was distinctly visible. This was evidently owing to refraction; for at full moon the sun and moon are diametrically opposite, and there fore could not both be seen at the same time.
The oval appearance of the sun and moon near the horizon arises from the same cause. As the lower limb of the sun is nearer the horizon than his upper limb, it is raised more by the refraction of the atmosphere; and consequently the vertical diameter of the sun is con tracted, while the horizontal diameter remains the same.
In order to construct tables containing the refraction for different heights, astronomers observe the altitudes of circumpolar stars, when they come to the meridian above and below the pole. When they come to the meridian below the pole, the refraction is much greater than when they come to the meridian above it, and therefore the true height of the pole is not an arith metical mean between these two altitudes of the star. By taking stars at different distances from the pole, the refraction may be determined for different altitudes. A Table containing the results of these observations will be found at the end of this article. As the state of the atmosphere is perpetually changing, and as the density of the air varies by the effect of heat, and by the pre sence of water and vapours, the numbers in the Table must be corrected according to the state of the barome ter and thermometer at the time when the refraction is wanted.
Similar effects are produced by refraction, when the body which emits the light is placed on the earth's surface at a distance from the observer. A curious phenomenon of this kind will be mentioned under ATMOSPHERE, and many others have been noticed by observers. Inverted images of ships often appear im mediately above the ships themselves. The spire of a distant church sometimes appears crooked, and distant coasts and islands are often distinctly seen, and at other times seem to be plunged in the sea. A very remark able case of refraction was observed by the French in Egypt. The flat surface of lower Egypt is occasionally diversified with little eminences, on which villages are built. When the sun rises in the morning, the vapours
generated by the heat are dispersed over the level sur face, and the villages appear like little islands situated in the middle of an immense sea. Under each village, an inverted image of it is distinctly seen, as it would appear, if reflected from the surface of water. In ap proaching to these eminences, the limits of this sea of vapours recede, and the image of the village disappears; but a new eminence comes into view, and the same phe nomena are repeated.
While the earth's atmosphere refracts the rays of light, it has also a reflective power, by which the light of the sun is returned to the earth in an enfeebled state after his direct rays have been intercepted by the hori zon. This phenomena is called the twilight, or crepus cular light, and generally begins in the morning when the sun is within 18° of the horizon, and terminates in the evening when the sun has sunk 18° below it. When the ecliptic, or the sun's apparent path in the heavens, is oblique to the horizon, as it is in winter and summer when his declination is great, the twilight contintifs longer, because his perpendicular distance from the horizon increases slowly ; and when the ecliptic cuts the horizon more perpendicularly, as it does in the au tumn and spring when he is near the equator, the twi light is of short duration, as the sun's distance from the horizon increases with great rapidity. Hence, if the earth were viewed from a distance, the boundary be tween light and darkness would not be distinctly mark ed like the line ozzjz, Plate XXXV. Fig. 6. but it would be accompanied with a faint light cmd, increasing to wards the poles.
From a combination of the refractive and reflective power of the atmosphere, the light which penetrates it undergoes many remarkable modifications. The red and orange colour of the morning and evening clouds ; the ruddy hue of all the celestial bodies when near the horizon ; the blue colour of the sky, and the bright azure of the distant landscape, are all the effects of the refractive and reflective power of the atmosphere. See ATMOSPHERE, where these interesting phenomena are explained. See also Latham, Phil. Trans. 1798, vol. lxxxviii. p. 357. Vince, Phil. Trans. 1799, vol. lxxxix. p. 13. Wollaston, Phil. Trans. 1800, vol. xc. p. 239. Biot, Traite Element. D'?lstron. Physique, tom. I. p. 40. 1\Ionge Decad. Egypt, vol. i. Phil. Mag. vol. xxxv. p. 287. See also REFRACTION.