Such extraordinary catastrophes are rare, but their possibility is always present to the minds of those responsible for the government and welfare of Egypt. We shall see what steps have been taken to guard against them, and first we shall examine the different systems of irrigation which have prevailed, and the nature of the services which the water, whose variations we have described, is made to perform before it finally reaches the sea. CHAPTER II BASIN IRRIGATION There are two kinds of irrigation in Egypt—basin and perennial irrigation. Basin irrigation is the ancient and historical method of the country. Tradition ascribes its invention to the first King of Egypt, and it is obviously designed to take full advantage of the annual flood. Practically the same as it was 7,000 years ago, it may be seen in Upper Egypt to-day. The cultivable land from Assouan to the Delta is, with the exception of the Fayoum, that altogether remarkable province, a narrow strip of country, varying in breadth from a few miles to nothing at all, sometimes on both sides of the river, sometimes only on one. Being of deltaic formation, the land is highest near the river, and slopes away towards the desert. As the river flows from south to north, there is, of course, also a general slope of the country in the same direction. Earthen dykes are run at right angles to the river as far as the desert, a dyke parallel to the river and close to its bank connects them, and so a basin is formed, enclosed on the fourth side by the desert. Thus the land is arranged in a series of terraces. Usually these basins are arranged in a series, one basin draining into its neighbour, the last of the series discharging back again into the Nile. Sometimes a second dyke, parallel to the river, divides the lower land near the desert from the higher; sometimes the arrangement is still further complicated by other dykes, making enclosures within the area of the original basin.