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 This obscure expression is taken to relate to the tides, and probably does so. pinus his theory of their cause. 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 κἀκεῖσε. It suggested to Cæsal- 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 Cæsalpinus 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 Cæsalpinus,) 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 ·66 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 servation that the hours of high water correspond, speaking roughly, on the European and American coasts, Bacon quotes in the De Fluxu et Refluxu Maris no authority; but in the Novum Organum he ascribes it to Acosta and others. But it is very remarkable that Acosta does not say what Bacon makes him say, namely that the times of high water are the same on the coast of Florida and that of Europe, and that he does say what Bacon admits would be fatal to his theory, namely that there is high water at the same time in the Atlantic and Pacific oceans. In his Natural History of the Indies, iii. 14., he speaks of the tides, and of the two theories by which they had been explained. There are some, he says, who affirm that the ebb and flow of the sea resembles a caldron of water moved to and fro, the water rising on one side when it falls on the other, and reciprocally; while others liken it to the boiling over of a pot, which rises and falls on all sides at once. The second view is in his judgment the true one. He says that he had inquired from a certain pilot, Hernandez Lamero,1 who had sailed through the Straits of Magellan about the year 1579, how he had found the tides there, and particularly if the tide of the South Sea or Pacific flowed when that of the North Sea or Atlantic ebbed, and vice versa. Lamero made answer that it was not so, that both tides ebb and flow together, and that they meet about seventy leagues from the Atlantic and thirty from the South Sea. With this statement Acosta is altogether satisfied; and so far from trying to compare the time of high water on the opposite shores of the Atlantic, he remarks that but for the Straits of Magellan it would be impossible 1 See Acosta, iii. 11. |
