The illumination of the heavens which precedes the rising of the sun, and continues sometime after he is set--or, what is commonly called the morning and evening _twilight_--is likewise produced by the atmospherical refraction--which circumstance forms a very pleasing and beneficial arrangement in the system of nature. It not only prolongs to us the influence of the solar light, and adds nearly two hours to the length of our day, but prevents us from being transported all at once from the darkness of midnight to the splendour of noon-day, and from the effulgence of day to the gloom and horrors of the night--which would bewilder the traveller and navigator in their journeys by sea or land, and strike the living world with terror and amazement. The following figure will illustrate the position now stated, and the manner in which the refraction of the atmosphere produces these effects. Let A _a_ C, fig. 4, represent one half of our globe, and the dark space between that curve and B _r_ D, the atmosphere. A person standing on the earth’s surface at _a_ would see the sun rise at _b_, when that luminary was in reality only at _c_--more than half a degree below the horizon. When the rays of the sun, after having proceeded in a straight line through empty space, strike the upper part of the atmosphere at the point _d_, they are bent out of their right-lined course, by the refraction of the atmosphere, into the direction _d a_, so that the body of the sun, though actually intercepted by the curve of the earth’s convexity consisting of a dense mass of land or water, is actually beheld by the spectator at _a_. The refractive power of the atmosphere gradually diminishes from the horizon to the zenith, and increases from the zenith to the horizon, in proportion to the density of its different strata, being densest at its lower extremity next the earth, and more rare towards its higher regions. If a person at _a_ had the sun, _e_, in his zenith, he would see him where he really is; for his rays coming perpendicularly through the atmosphere, would be equally attracted in all directions, and would therefore suffer no inflection. But, about two in the afternoon, he would see the sun at _i_, though, in reality, he was at _k_, thirty-three seconds lower than his apparent situation. At about four in the afternoon he would see him at _m_, when he is at _n_, one minute and thirty-eight seconds from his apparent situation. But at six o’clock, when we shall suppose he sets, he will be seen at _o_, though he is at that time at _p_, more than thirty-two minutes below the horizon. These phenomena arise from the different refractive powers of the atmosphere at different elevations, and from the obliquity with which the rays of light fall upon it; for we see every object along that line in which the rays from it are directed by the last medium through which they passed. [Illustration: _figure 4._]