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CHAP. 99. (97)—CONCERNING THE CAUSE OF THE FLOWING AND EBBING OF THE SEA.

Much has been said about the nature of waters; but the most wonderful circumstance is the alternate flowing and ebbing of the tides, which exists, indeed, under various forms, but is caused by the sun and the moon. The tide flows twice and ebbs twice between each two risings of the moon, always in the space of twenty-four hours. First, the moon rising with the stars1 swells out the tide, and after some time, having gained the summit of the heavens, she declines from the meridian and sets, and the tide subsides. Again, after she has set, and moves in the heavens under the earth, as she approaches the meridian on the opposite side, the tide flows in; after which it recedes until she again rises to us. But the tide of the next day is never at the same time with that of the preceding; as if the planet was in attendance2, greedily drinking up the sea, and continually rising in a different place from what she did the day before. The intervals are, however, equal, being always of six hours; not indeed in respect of any particular day or night or place3, but equinoctial hours, and therefore they are unequal as estimated by the length of common hours, since a greater number of them4 fall on some certain days or nights, and they are never equal everywhere except at the equinox. This is a great, most clear, and even divine proof of the dullness of those, who deny that the stars go below the earth and rise up again, and that nature presents the same face in the same states of their rising and setting5; for the course of the stars is equally obvious in the one case as in the other, producing the same effect as when it is manifest to the sight.

There is a difference in the tides, depending on the moon, of a complicated nature, and, first, as to the period of seven days. For the tides are of moderate height from the new moon to the first quarter; from this time they increase, and are the highest at the full: they then decrease. On the seventh day they are equal to what they were at the first quarter, and they again increase from the time that she is at first quarter on the other side. At her conjunction with the sun they are equally high as at the full. When the moon is in the northern hemisphere, and recedes further from the earth, the tides are lower than when, going towards the south, she exercises her influence at a less distance6. After an interval of eight years, and the hundredth revolution of the moon, the periods and the heights of the tides return into the same order as at first, this planet always acting upon them; and all these effects are likewise increased by the annual changes of the sun7, the tides rising up higher at the equinoxes, and more so at the autumnal than at the vernal; while they are lower8 about the winter solstice, and still more so at the summer solstice; not indeed precisely at the points of time which I have mentioned, but a few days after9; for example, not exactly at the full nor at the new moon, but after them; and not immediately when the moon becomes visible or invisible, or has advanced to the middle of her course, but generally about two hours later than the equinoctial hours10; the effect of what is going on in the heavens being felt after a short interval; as we observe with respect to lightning, thunder, and thunderbolts.

But the tides of the ocean cover greater spaces and produce greater inundations than the tides of the other seas; whether it be that the whole of the universe taken together is more full of life than its individual parts, or that the large open space feels more sensibly the power of the planet, as it moves freely about, than when restrained within narrow bounds. On which account neither lakes nor rivers are moved in the same manner. Pytheas11 of Massilia informs us, that in Britain the tide rises 80 cubits12. Inland seas are enclosed as in a harbour, but, in some parts of them, there is a more free space which obeys the influence13. Among many other examples, the force of the tide will carry us in three days from Italy to Utica, when the sea is tranquil and there is no impulse from the sails14. But these motions are more felt about the shores than in the deep parts of the seas, as in the body the extremities of the veins feel the pulse, which is the vital spirit, more than the other parts15. And in most estuaries, on account of the unequal rising of the stars in each tract, the tides differ from each other, but this respects the period, not the nature of them; as is the case in the Syrtes.

1 "Mundo;" the heavens or visible firmament, to which the stars and planets appear to be connected, so as to be moved along with it.

2 "Ancillante; ""Credas ancillari sidus, et indulgere mari, ut non ab eadem parte, qua pridie, pastum ex oceano hauriat." Hardouin in Lemaire, i. 427.

3 Not depending on the time of the rising and setting of the sun or the latitude of the place, but determinate portions of the diurnal period.

4 By a conjectural variation of a letter, viz. by substituting "eos "for "eas," Dalechamp has, as he conceives, rendered this passage more clear; the alteration is adopted by Lemaire.

5 "In iisdem ortus occasusque operibus;" "Eodem modo utrinque orientibus occidentibusque sideribus," as interpreted by Alexandre in Lemaire, i. 428.

6 It is scarcely necessary to remark, that both the alleged fact and the supposed cause are incorrect. And this is the case with what our author says in the next sentence, respecting the period of eight years, and the hundred revolutions of the moon.

7 "Solis annuis causis." The circumstances connected with the revolution of the sun, acting as causes of the period and height of the tides, in addition to the effect of the moon.

8 "Inanes;" "Depressiores ac minus tumentes." Hardouin in Lemaire, i. 429.

9 According to the remark of Alexandre, "Uno die et dimidio altero, 36 circiter horis, in Gallia." Lemaire, i. 429.

10 Alexandre remarks on this passage, "Variat pro locis hoc intervallum a nullo fere temporis momento ad undecim horas et amplius;" Lemaire, i 429.

11 Our author has already referred to Pytheas, in the 77th chapter of this book.

12 It is scarcely necessary to remark, that the space here mentioned, which is nearly 120 feet, is far greater than the actual fact.

13 "Ditioni paret;" "Lunæ solisque efficientiæ, quæ ciet æstum." Hardouin in Lemaire, i. 430.

14 The effect here described could not have depended upon the tides, but upon some current, either affecting the whole of the Mediterranean, or certain parts of it. See the remarks of Hardouin in Lemaire.

15 Pliny naturally adopted the erroneous opinions respecting the state of the blood-vessels, and the cause of the pulse, which were universally maintained by the ancients.

The Natural History. Pliny the Elder. John Bostock, M.D., F.R.S. H.T. Riley, Esq., B.A. London. Taylor and Francis, Red Lion Court, Fleet Street. 1855.

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