The heat and light of the sun are indispensable to the production and growth of all vegetation. Age after age the genial rays of the sun nourished on this earth an improving and enlarging vegetable growth, which on its decay left added soil for generations of plants to come. Thus the sun was not only the parent but the cherishing nurse of our little globe. We have seen that in the coal period of earth-building, the atmosphere of the world was heavily charged with carbonic acid gas, but under the stimulating effects of sunshine, vegetation continually increased, used up most of the carbonic acid gas, and the atmosphere became constantly richer in oxygen,[21] and more and more fitted for the use of animals. Thus we see the sun, as a prudent householder, storing up in the world supplies of coal, and purifying at the same time the air. Meanwhile, by the action of sun rays on the salt, shallow seas, vast salt beds were produced and buried, providing a mineral especially needful to man. Our dependence upon the sun awakens a desire to know something about the orb itself; but how can a body so intensely bright be studied? Smoked and colored glass plates are used, through which the sun can be observed with comfort, and there are certain occasions when the study of this vast, glowing, and distant body can be favorably pursued. These especial occasions favorable to sun-study are during an eclipse, and when the planets Mercury and Venus cross the sun’s disk. Such a crossing is called a transit. It was once supposed that the sun was no larger than the moon is, and as near to the earth as that planet is; both sun and moon were believed to be much smaller than the earth, and very near to it, and revolving in an orbit about it. To us the sun appears to rise above the horizon, move across the heavens, and sink beneath the horizon, and return after a few hours’ absence to pursue the same path. Meanwhile, the earth does not appear to us to move. This is not the only case in which our eyes deceive us until we call reason to their aid. When we are on a swiftly moving train of cars, it seems to us that trees, posts, telegraph-poles, go rushing by us, while we remain at rest. Although no one now questions that the earth moves, and that with regard to the earth the sun remains fixed in one place, we still use the language that fits appearances rather than facts, and we say, “the sun rises, the sun sets,” when in truth the earth is merely turning over and over, and so constantly bringing a different part of its surface under the sun’s rays.