It requires the river to attain a medium rise in order to benefit the country: too little, involving scarcity and famine; too much, compromising the safety of the people and their dwellings. Wilkinson calls a rise of 19 cubits, tolerable; 20, good; 21, sufficient; while a rise of 22 cubits is abundant enough to fill every canal, and a rise of 24 cubits would overwhelm and ruin the villages. A cubit is rather more than 21 inches; so that, in order fully to meet the wants of the country, a perpendicular rise of 38 feet is necessary. The Nile is also distinguished among rivers for the pleasant taste and salubrity of its waters when not in flood; properties highly extolled by the ancients, and acknowledged to belong to it by modern travelers. It is a common saying with the Egyptians, that if Mahomet had tasted of its stream, he would have sought a terrestrial immortality in order to enjoy it forever. The physical circumstances of the river easily account for the possession of this attribute. The air above is pure and serene. But little rain falls upon the country through which the greater part of its course is prosecuted, and no snow or hail. Hence there is little drainage into it from the surrounding land, and its waters are kept free from any noxious taint derived from earths and minerals, except from those in its immediate channel. The same property of being remarkably pure and salutary is ascribed by Herodotus to one of the Susianic rivers, of which alone, according to tradition, none but the kings of Persia drank. “There Susa, by Choaspes’ amber stream, The drink of none but kings.” The Susianic streams, along with the Nile, may not improperly be styled the oldest rivers of the globe, because of their place in its most ancient traditions and histories; and however subordinate to the gigantic currents of the western hemisphere, those of the eastern, in general, present higher points of interest, in their long-known identification with the destinies of mankind. If not the actual birthplace of man, the great plains on the banks of the Tigris and the Euphrates were the abode of the founders of the diluvian race. There, the two greatest cities of the ancient world--Nineveh and Babylon--rose into magnificence. There, a supernatural finger traced the doom of the latter upon the palace wall of its trembling monarch, while an exiled Jew, in the majesty of inspiration, gave him the interpretation of the mystic writing. There, too, the splendid empire of the Medes and Persians fell a prey to the Macedonian on the field of Arbela, while, in later ages, the same neighborhood witnessed the catastrophe of Cunaxa, and the bold bearing of the indomitable ten thousand--the defeat and death of Crassus--the retreat of Mark Antony--the fall of the apostate Julian--and the short-lived glory of Bagdad. How different the associations connected with the Arkansas and the Osage to those of the Euphrates and Tigris!