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Page:Popular Science Monthly Volume 72.djvu/113

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PSM V72 D113 A crude bridge.png

Crude Bridge.

called "bore." Professor G. H. Darwin noted on the banks of the Severn during the spring tide in September, 1897, that there was no proper bore, but only a succession of waves up-stream, and a rapid rise of water-level.

In the case of the River Seine, which has been dyked as far as Rouen to admit vessels of twenty feet draught, it is said that there is a bore (barre or mascaret) at every tide, ranging usually from eight to ten feet. This is probably accounted for by the fact that after Candebec and Quillebœuf, the estuary is set with extensive sand-banks between which flows a narrow navigable channel.

These bores are relatively small compared with that in the Ch'ientang Kiang, while the destructive bore of the great Amazon is robbed of its impressiveness because it can not be well observed on account of its very magnitude; moreover, with it as well as with the other rivers it is only at spring tides and with certain winds that the phenomenon is at all striking. On the other hand, the Hangchow Bore occurs at every tide to a remarkable extent in any season and at certain times assumes colossal proportions and is always easily observable. For comparison it may be interesting to note the following description of the Amazon's bore, or proroca, by La Condamine:

During three days before the new and full moons, the period of the highest tides, the sea, instead of occupying six hours to reach its flood, swells to its highest limit in one or two minutes. The noise of this terrible flood is heard five or six miles off, and increases as it approaches. Presently you see a liquid promontory twelve or fifteen feet high, followed by another, and another, and

PSM V72 D113 A common artistic bridge.png

An Artistic Bridge—a Common Type.