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.
Aqueous vapor is contained in the air in quantities varying in different localities and at different times, and depending mainly on the temperature of the air. Air at a given temperature cannot contain more than a certain quantity of moisture in solution; and when it has taken up its maximum quantity, it is said to be saturated with aqueous moisture. The higher the temperature of the air the more water can it retain as vapor; and when air saturated with moisture is cooled, the water is deposited in liquid form in very small globules, forming a mist, fog, or cloud. This is the cause of the fall of rain, snow, and hail; when warm air heavily laden with moisture from the ocean passes into a higher and colder position, or meets with a stratum of air of lower temperature, it cannot any longer retain so much aqueous vapor, and a large quantity assumes a liquid form, falling as rain when the temperature is above the freezing-point, or crystallizing as snowflakes if the temperature is below that point. Hail is caused by the congelation of raindrops in passing through a stratum of air below the freezing-point. The deposition of dew is caused by the rapid cooling of the earth's surface by radiation after sunset, and by the consequent cooling of the air near the ground below the temperature at which it begins to deposit moisture. In general the air contains from 50 to 70 per cent. of aqueous vapor of the quantities necessary to saturate it. If the quantity be not within these limits the air is either unpleasantly dry or unpleasantly moist.
The air contains, besides the gases of oxygen and nitrogen, carbonic acid, ammonia, accidental impurities, and volatile organic matter, which latter is the most important, as it probably influences to a great extent the healthfulness of the special situation. We become aware of the existence of such organic putrescent substances when entering a crowded room from the fresh air; and it is probable that the well-known unhealthiness of marshy and other districts is owing to the presence of some organic impurities.
We may have occasion to refer to this when speaking of the deluge, etc.