Note.--Baily's "History of Astronomy," Part I, page 31, § 124, and Part II, pp. 33, 39, maintains that India has existed as a nation, as the records show, 4,320,000 years. The Indians divide this time into four principal periods: First period, that of innocence or simplicity, 1,728,000 years; second period, 276,000; the third period, 864,000; and the ages of misfortune about 422,000--Cali-yon-gan period. Similar statements are made by Cicero ("De Divinat," I, 19), concerning the Chaldeans: "Condemnemus, inquam, hos aut stultitiæ aut vanitatis aut imprudentiæ qui 470 millia annorum ut ipsi dicunt monumentis comprehensa continent." THE ATMOSPHERE. The atmosphere is the gaseous envelope encircling the earth; and it constitutes the ocean of air at the bottom of which we live. We become aware of the existence of the air when we move rapidly and experience the resistance offered to the passage of our bodies, and also when the air is set in motion, giving rise to a wind. We notice the pressure of the atmosphere if we withdraw the air from beneath the hand by a powerful air-pump, for we then find that the hand is pressed down with a force equal to 1.033 kilos. on a square centimeter, or nearly 15 lbs. on every square inch. The total atmospheric pressure which the human body has to support hence amounts to several tons. But this pressure is not felt under ordinary circumstances, because the pressure exercised is exerted equally in every direction. The instrument used for measuring the pressure of the air is termed a barometer, and the average pressure at the sea level is equal to that exerted by a column of mercury 760 mm. high. The air being elastic and having weight, it is clear the lower layers of air must be more compressed than those above them, and hence the density of the air must vary at the different hights above the sea level. The density of the air being thus dependent on the pressure to which it is subjected, the higher strata of air become generally rarefied, and it is hence difficult to say whereabouts the air ceases, but it appears that the limit of the atmosphere is about 200 to 300 miles from the level of the sea. If the whole atmosphere were of the same density throughout as it is at the earth's surface, it would reach only to a height of a little more than five miles above the sea level.