Читать книгу Is a Ship Canal Practicable? онлайн

32 страница из 34

The Nile brings down a prodigious quantity of sand, which is swept into the river by the Libyan winds, and borne by the current to the sea, mingled with fragments of pottery from the villages on the banks. The quantity of sand brought into the sea has excited the astonishment of the most experienced students of delta formations. The Ganges, the Indus, the Dneipper, the Danube, and the Mississippi, the Yang-Tse-Kiang, and the Hoang Ho bring down annually millions of tons of solid matter to add to the accretions at their mouths.

[Click image to enlarge.]

The whole amount carried yearly into the Gulf of Mexico by all the passes of the Mississippi is seven hundred and fifty millions of cubic feet, or a mass of one mile square and twenty-seven feet thick. “As the cubical contents of the whole mass of the bar at the South-west pass is equal to a solid of one mile square and four hundred and ninety feet thick, it would require fifty-five years to form the bar as it now exists.”[3]

Since the time of Strabo the Nile has advanced the coast line of Egypt, by its yearly contributions of sand, from four to six miles into the sea. Any interruptions of the littoral currents greatly accelerates this result. Such is the well-known effect of jetties and moles. Since the construction of the mole at Port Said, the shore line has advanced 1213 feet in eight years. Eighty-eight feet of this distance was made in the last six months. “If the shore line continues to advance,” Mr. Mitchell remarks, “at any thing like its present rate, the dry land will extend to the end of the mole in forty years. The shoaling of the entrance to the harbor will keep pace with the advance of the shore line, and before the end of twenty years an extension of the mole will be necessary.”