The _transparency_ of the air is a very beneficial property it possesses. According to Dr. Keill, and other writers on astronomy, it is entirely owing to the atmosphere that the heavens appear bright in the day-time. For, without an atmosphere, that part of the heavens only would shine in which the sun is placed: and if we could exist without air, and should turn our backs toward the sun, the whole heavens would appear as dark as in the night, and the stars would be seen as clear as in the nocturnal sky. In this case we should have no twilight; but a sudden transition from the brightest sunshine to the blackest darkness immediately after sunset; and from the blackest darkness to the brightest sunshine at sun-rising; which would be extremely inconvenient, if not fatal to the sight of men. But, by means of the atmosphere, we enjoy the sun’s light, reflected from the aërial particles, for some time before he rises, and after he sets. For, when the earth by its rotation has prevented us from seeing the sun, the atmosphere, being still higher than we, has the sun’s light imparted to it, which gradually decreases until he has descended 18 degrees below the horizon; and then, all that part of the atmosphere which is above us becomes dark. The atmosphere refracts the sun’s rays so, as to bring him in sight every clear day, before he rises in the horizon; and to keep him in view for some minutes after he is really set below it. For, at some times of the year, we see the sun ten minutes longer above the horizon, than he would be if there were no refractions; and about six minutes every day at a mean rate. We cannot but perceive the wisdom of God displayed in this contrivance, to prevent the sudden transition from light to extreme darkness, and his goodness manifested therein to man.