Imágenes de páginas

the hypothesis of Cæsalpinus, which is in itself a curious one, and which clearly suggested to Galileo his own explanation of the cause 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 lec

, tionis” 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.

1 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.

Another theory, which was propounded by Sfondratus, in a tract published in 1590, and entitled Causa Astûs Maris, explains the reciprocating motion of ebb and flow (as owing] to the effect produced by the continent of America. The water under the influence of the sun moves in accordance with the motion of the heavens from east to west. But it is reflected and made to regurgitate eastward by impinging on the coast of America, which was supposed to extend indefinitely southward (Cape Horn was not discovered until (1615]) and which permits only a portion of it to pass through the Straits of Magellan. Between this theory, of which Patricius speaks contemptuously and without mentioning the name of its author, and that which J. C. Scaliger had put forth in the Exercitationes adversum Cardanum, 52., there is no essential difference, though Scaliger ascribes the general westward motion of the ocean to its sympathy with the moon. But in both theories the change of direction of the motion is ascribed to the action of the coast of America; and both were doubtless suggested by the current which flows from east to west through the Straits of Magellan.

Bacon himself, as we perceive from the following tract, was inclined to adopt the same view. He compares the Straits of Dover with those of Magellan, and conceives that the German Ocean exhibits on a small scale the same phenomena of a stream tending in one

direction, and compelled to regurgitate in the opposi* one by the obstacles which it meets with, as the grea. Atlantic. This at least appears to be the import of the expressions of which he makes use. That the period of the revolution of the waters round the earth is greater than twenty-four hours, appeared to Bacon to be in entire accordance with the retardation of the diurnal motion of the planets. All the inferior orbs

. lag behind the starry heaven, and that of the moon most of all; wherefore the moon's diurnal period is more nearly the same as that of the waters than any other.

In these views there is an absolute confusion between the bodily motion of water as in a current, and the propagation of an undulation; a confusion not unnatural, seeing that to conceive the motion of an undulation apart from that of the matter of which it is composed is by no means easy. Scaliger however might have learned from Cardan, notwithstanding the arrogance with which he treats him, to distinguish between them. For Cardan, after saying that high water follows the moon, inquires why the motion of the flood current is so much slower than the moon's. He answers : “ Causa est, quod non tota aqua, nec una pars lunam sequitur, sed proximæ in proximas transferuntur, velut si quis carnem primens tumorem elevet, caro quidem parum loco movebitur, celerrimè tamen tumor per totum crus transferetur.” 1

It became necessary, when the flood current was confounded with the motion of the tide wave, to assign a cause for the reciprocating motion of ebb and

i De Subtilit. ii. p. 408.



« AnteriorContinuar »