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 Posidonius tells us that Seleucus, a native of the country next the Erythræan Sea,1 states that the regularity and irregularity of the ebb and flow of the sea follow the different positions of the moon in the zodiac; that when she is in the equinoctial signs the tides are regular, but that when she is in the signs next the tropics, the tides are irregular both in their height and force; and that for the remaining signs the irregularity is greater or less, according as they are more or less removed from the signs before mentioned. Posidonius adds, that during the summer solstice and whilst the moon was full, he himself passed many days in the temple of Hercules at Gades, but could not observe any thing of these annual irregularities. However, about the new moon of the same month he observed at Ilipa2 a great change in the reflux of the water of the Guadalquiver, as compared with previous flood-tides, in which the water did not rise half as high as the banks, and that then the water poured in so copiously, that the soldiers there dipped their supply without difficulty, although Ilipa is about 700 stadia from the sea. He says, that the plains next the sea were covered by the tides to a distance of 303 stadia, and to such a depth as to form islands, while the basement of the temple in the enclosure dedicated to Hercules, and the top of the mole in front of the harbour of Gades, were not covered higher than 10 cubits, as observed by actual soundings; but if any one should add the double of that for the occasional risings of the tide which occur, [neither] thus would he be able to estimate the violence with which the full force of the high tide rushes over the plains. Posidonius informs us that this violence [of the tide] is common to all the coasts of Spain on the Atlantic,4 but what he relates concerning the Ebro is unusual and peculiar to itself, for he says that it sometimes overflows after continued north winds, although there may have been neither rains nor snows. The cause of this [he supposes] to be the lake through which the Ebro flows, its waters being driven by the winds into the current of the river.5
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