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DE FLUXU ET REFLUXU MARIS
BY ROBERT LESLIE ELLIS.
It was a natural result of the progress of maritime discovery in the sixteenth century, that much was thought and written on the subject of the tides. The reports continually brought home touching the ebb and flow of the sea on far distant shores, not only excited curiosity, but also showed how little the philosophers of antiquity had known of the phenomena which they attempted to explain. Men who dwelt on the shores of an inland sea, and whose range of observation scarcely extended beyond the Pillars of Hercules, were in truth not likely to recognise any of the general laws by which these phenomena are governed. Their authority accordingly in this matter was of necessity set aside; and a number of hypotheses were proposed in order to explain the newly discovered facts. Of these speculations an interesting account is given in the twenty-eighth book of the Pancosmia of Patricius. It is not, however, complete; no mention being made of the hypothesis of Cæsalpinus, which is in itself a curious one, and which clearly suggested to Galileo his own explanation of the calise of the tides. Otto Casmann, the preface to whose Problemata Marina is dated in 1596, gives a good deal of information on the same subject, some of which however seems to be simply copied from Patricius; but he mentions Cæsalpinus, whom, as I have said, Patricius omits. Patricius, it may be remarked, is a scrupulously orthodox philosopher, and dedicates his work to Gregory XIV. with many expressions of reverence and submission.
It is perhaps on this account that he has said nothing of Cæsalpinus, whose works were “ improbatæ lectionis” and who seeks to explain the tides, and also certain astronomical phenomena, by denying the orthodox doctrine of the earth's immobility.
The earliest modern writer whom Patricius mentions is Frederick Chrysogonus, whose work on the tides must have been published in 1527. To his account of the phenomena little, according to Patricius, was added by subsequent writers; nor are his statements contradicted by the reports of seafaring men, who however mention certain matters of detail which he had omitted. Of seamen Patricius particularly mentions Peter of Medina and Nicolaus Sagrus, the latter with especial commendation. From Sagrus (but probably through Patricius) Bacon derived some of the statements of the following tract ; those, namely, which relate to the progress of the tide-wave from the Straits of Gibraltar to Gravelines. On the day of new moon, according to Sagrus, there is high water along the coast from Tarifa to Rota at an hour and a half after midnight. After mentioning several intermediate places, he says that along the coast of Normandy as far as Calais and Nieuport there is high water at nine, and after a not very distinct statement as to the time of high water in the middle of the channel, goes on to state that from Calais to Gravelines the water is high off shore (in derotâ) at an hour and a half after midnight, that is at the same time as at Rota, and at Zealand at the same time as on the coast of Portugal. These statements are scarcely sufficiently accurate to make it worth while to compare them with modern observations; but it is necessary to remark that Sagrus, though he mentions it as a remarkable circumstance that the time of high water should be the same at Gravelines and at Rota, does not mean to assert that there is any discontinuity in the progress of the tide along the shores of France and the Netherlands. The tide gets progressively later and later until we come to a place where there is high water about one in the afternoon, and therefore also high water about half-past one after the succeeding midnight. In order to compare Gravelines and Rota, he takes (but without mentioning that he does so) two different tide-waves, - the statement with reference to Gravelines appearing to relate to a later wave than the other. Bacon however does
not appear to have understood this; and consequently, after saying that the hour of high water becomes later and later from the Straits of Gibraltar to the coast of Normandy, proceeds thus :"Hucusque ordinatim ; ad Gravelingam vero, verso prorsus ordine, idque magno saltu, quasi ad eandem horam cum ostio freti Herculei.” This notion of a reversal of the order of the tides as we proceed along the French and Dutch coast is not justified either by Sagrus's statements or by the phenomena to which they relate.'
Sagrus is probably the first writer who remarks that the time of high water is not always the same as that of slack water. “Et illud adnotat Sagrus,” says Patricius, " non minus
. mirum” (he has been speaking of the coincidence as to the time of high water between the Dutch and Portuguese coasts) “ si a Selandiâ quis ad caput Angliæ Dobla [Dover?] naviget, mare plenum erit a medinoctio tertiâ quidem horâ, sed eodem itinere, fluxus aquæ obvius fiet per horas duas cum dimidiâ donec flaccescat, quod nautæ dicunt aquam fieri stancam.” Patricius rightly compares this with the phenomenon observed at Venice, namely that when the water has already sunk half a foot at the entrance of the harbour it is still rising in the harbour itself.
With respect to theories of the cause of the tides, it may be observed that a connexion of some kind or other between the tides and the moon has at all times been popularly recognised. But the conception which was formed as to the nature of this connexion long continued vague and indefinite; and in Bacon's time those who speculated on the subject were disposed to reject it altogether. One theory, that of Telesius and Patricius, compares the sea to the water in a caldron ; that is to say it rises and tends to boil over when its natural heat is called forth under the influence of the sun, moon, and stars, and then after a while subsides. But why should this alternate rise and fall have a definite period of six hours ? Patricius calmly answers, “nimirum quia omnis motus fit in tempore,” and that there is no better reason for asking the question than for asking why certain other motions have periods of seven or fourteen days, of six months or twelve.
Another theory, which was propounded by Sfondratus, in a
' I have given Sagrus's statements in extenso in a note on the passage in the text. He seems to have forgotten that Nieuport is farther from Calais than Gravelines.