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swers, "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 tract published in 1590, and entitled Causa Estûs 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 ) 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 opposite one by the obstacles which it meets with, as the great 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 comprimens 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
1 De Subtilit. ii. p. 408.
flow; and this cause was sought for in the configuration of land and sea.
It seems as if Aristotle, if he had developed any theory of the tides, would have had recourse to some similar explanation. Thus Strabo says, (I quote from Xylander's translation,) "Jam Aristotelem Posidonius ait æstuum marinorum qui fiunt in Hispaniâ causas non recte ascribere litori et Mauritania" (by litori is probably meant the coast of Spain itself), "dicentem mare ideo reciprocare, quia extrema terrarum sublimia sint et aspera, quæ et fluctum duriter excipiant et in Hispaniam repercutiant, cum pleraque litora sint humilia et arenæ tumulis constent."1 With this passage is to be compared what Aristotle says in the commencement of the second book of the Meteorologics, from which it appears to have been his opinion that the seas within the Pillars of Hercules flow continually outwards in consequence of differences of level, and that where the sea is girt in by straits its motion becomes visible in the form of a reciprocating libration : διὰ τὸ ταλαντεύεσθαι δεῦρο KȧKeiσe. This obscure expression is taken to relate to the tides, and probably does so. It suggested to Casalpinus his theory of their cause. At least he quotes it, and dilates on its meaning; and when the ebb and flow of the sea is conceived of as a libration, it is easily inferred that this libration ought to be ascribed not directly to the fluid itself but to that on which it rests. And this notion of the libration of the earth connected itself with his views of astronomy. For in order to
1 Strabo, iii. p. 153. It is worth remarking that this passage is quoted by Ideler in his edition of the Meteorologics, i. p. 501., in a way which makes it quite unintelligible, some words having been accidentally omitted.
get rid of the necessity of supposing the existence of a ninth and tenth heaven, the former to explain the precession of the equinoxes, and the latter the imaginary phenomenon of their trepidation, he ascribed the motion by which these phenomena are produced to the earth itself. The cause of this motion he sought in the action of the ambient air on the earth's surface. To explain trepidation, the earth's motion was supposed to be in some measure libratory and irregular; and by being so it produced the tides.1
From the theory of Casalpinus we pass naturally to that of Galileo, seeing that in both the tides are explained by the unequal motion of the earth. Galileo's theory was first propounded in a letter to Cardinal Orsino, dated 1616. He remarks that the libratory motion"che alcuno ha attribuito alla Terra," (alluding of course to Casalpinus,) is in several respects not such as to save the phenomena, and maintains that the true cause is to be sought in the combination of the earth's motion in its orbit with its rotation on its own axis. In consequence of this combination, the velocity of any point of the earth's surface varies, going through its different values in the space of twenty-four hours. The waters of the sea, not accommodating themselves to this varying velocity, ebb and flow at any place as their velocity is less or greater than that of their bed. The boldness of the assertions by which Galileo supports this theory is remarkable: thus he affirms that the ebb and flow is always from west to east, and vice versâ; and that the notion that, speaking generally, the interval between high water and low is six hours è stata un' ingannevole opinione la quale ha poi fatto 1 Quæstiones Peripat. iii. 4. and 5.
favoleggiare gli scrittori con molte vane fantasie." No refutation of a theory which altogether misrepresents the facts which it proposes to explain could ever have been needed; but the advance of mechanical science has long since made it easy to show that no reciprocating motion of the waters of the sea could be produced in the manner described by Galileo.
Bacon does not mention Galileo's theory in the present tract, which was therefore probably written before or not long after 1616. But in the Novum Organum [II. 46.] it is mentioned and condemned; one ground of censure being that it proceeds on the untenable hypothesis of the earth's motion, and the other that the phenomena are misrepresented.
Bacon, both in this tract and in the Novum Organum, ascribes the tides in the Atlantic to a derivative motion of the waters, caused by the obstacles which the form of the continents of the old and new worlds oppose to its general westerly movement. It is thus that he meets the objection which would arise from the circumstance that there is high water at the same time on corresponding points of the shores of Europe and America. This notion of a derivative tide is absolutely necessary in the detailed explanation of the phenomena, and I am not aware that any one had previously suggested it, at least in the distinct form in which Bacon puts it. He admits that, if the tides of the Pacific synchronise with those of the Atlantic, his theory that the tides depend on a progressive motion of the ocean must be given [up]. If it be high water on the shores of Peru and China at the same hours as on those of Florida and Europe, there are no shores left on which there can then be low water. For the important ob