In these conditions Egypt, instead of being the cradle of civilization, would have been a desert ravine like one of those which compose the Sahara, if the waters of the sacred Nile had not brought life to it annually. From the last days of June till the end of September the Nile swells and inundates almost all Egypt; from the end of October to the last days in May the year following it falls and exposes gradually lower and lower platforms of land. The waters of the river are so permeated with mineral and organic matter that their color becomes brownish; hence, as the waters decrease, on inundated lands is deposited fruitful mud which takes the place of the best fertilizer. Owing to this mud and to heat, Egyptian earth-tillers, fenced in between deserts, have three harvests yearly and from one grain of seed receive back about three hundred. Egypt, however, is not a flat plain, but a rolling country; some portions of its land drink the blessed waters during two or three months only; others do not see it every year, as the overflow does not reach certain points annually. Besides, seasons of scant water occur, and then a part of Egypt fails to receive the enriching deposit. Finally, because of heat the earth dries up quickly, and then man has to irrigate out of vessels. In view of all these conditions people inhabiting the Nile valley had to perish if they were weak, or regulate the water if they had genius. The ancient Egyptians had genius, hence they created civilization. Six thousand years ago they observed that the Nile rose when the sun appeared under Sirius, and began to fall when it neared the constellation Libra. This impelled them to make astronomical observations and to measure time. To preserve water for the whole year, they dug throughout their country a network of canals many thousand miles in length. To guard against excessive waste of water, they built mighty dams and dug reservoirs, among which the artificial lake Moeris occupied three hundred square kilometres of surface and was fifty-four metres deep. Finally, along the Nile and the canals they set up a multitude of simple but practical hydraulic works; through the aid of these they raised water and poured it out upon the fields; these machines were placed one or two stories higher than the water. To complete all, there was need to clear the choked canals yearly, repair the dams and build lofty roads for the army, which had to march at all seasons. These gigantic works demanded knowledge of astronomy, geometry, mechanics, and architecture, besides a perfect organization. Whether the task was the strengthening of dams or the clearing of canals, it had to be done and finished within a certain period over a great area. Hence arose the need of forming an army of laborers, tens of thousands in number, acting with a definite purpose and under uniform direction,--an army which demanded many provisions, much means, and great auxiliary forces.