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would have a greater specific gravity, and consequently a greater scour. And we may therefore conclude, that, if any branch of the Nile at any time during its mean flood is able to maintain a navigable channel and mouth, a sea river from Suez would have three times the capability of doing so, supposing the hydraulic depth of each to be the same; but the Red Sea offers an inexhaustible supply of water, which can be drawn upon to any desirable amount.
For scouring a channel and maintaining a navigable mouth, I am disposed to place great stress on the superior efficacy of a salt water stream over a fresh water one; as each of them, in coming into collision with their recipient waters, will be materially biassed in the direction of their currents by their comparative specific gravities. Thus, if the water of the Nile, having a specific gravity of 100, falls into the Mediterranean Sea, having a specific gravity of 103, it will naturally be deflected upwards, and lose its useful scour on the bottom; whereas, if the Red Sea water has a trifle more of specific gravity than that of the Mediterranean, however small the difference, its bias on meeting would be downwards, and tend to preserve its scouring force. And although I am not aware of the fact, we have every ground to infer that the water of the Red Sea is more saline, and consequently heavier, than that of the Mediterranean.
We may now proceed to discuss the question of the comparative merits of a navigable channel across the Isthmus, formed in one case chiefly by the operations of nature, and in the other by those of art: the first being obviously an incontrollable stream; the latter, a channel completely under subjection to man.
Some have proposed as the most effective channel one