=A.D. 1644.=--Descartes (René), a prominent French philosopher and mathematician, publishes his “Principia Philosophiæ,” divided into four parts; the first giving an exposition of the principles of all human knowledge, the second treating of the principles of natural things, and the third and fourth parts developing his theory of vortices. His main idea was that a rush of subtle matter passes very rapidly through the earth from the equator towards each pole, being opposed by magnetic substances throughout its passage and that the sun is the centre of a vortex of an ethereal fluid, whose whirling motion produces the revolution of planets about the sun, or around the fixed stars. Moreover, as Noad states it, “the vortex moves with the greatest facility in a particular direction, one of its ends being always turned toward the north.” One of the most prominent fellow-students of Descartes was Marin Mersenne, who joined the religious Order of “Minimes,” and who, after publishing in 1634 and 1639 “Les Mécaniques de Galilée” and “Nouvelles Découvertes de Galilée,” brought out, during the years 1644 and 1647, his well-known “Cogitata physico-mathematica,” which, Montucla says, contains _un océan d’observations de toutes espèces_ ... and embraces a very interesting treatise on navigation besides many letters from leading scientists of that period not elsewhere to be found. REFERENCES.--“La grande Encyclopédie,” Vol. XXIII. pp. 730–731; Larousse, “Dict.,” Vol. XI. p. 94; “Biographie Générale,” Vol. XXXV. pp. 118–123; “The English Cyclopædia,” Vol. IV. p. 206; Alex. Chalmers, “Gen. Biog. Dict.,” London, 1811, Vol. XXII. pp. 81–83; “Biographie Universelle,” Vol. X. pp. 465–473; Whewell, “Hist. of the Ind. Sc.,” Vol. I. pp. 323, 328, 338, 339, 343, 354, 387, 423, 429, 430; Vol. II. p. 220; likewise pp. 320 and 390 of Vol. I. relative to Le Père Marin Mersenne and pp. 391 and 423 concerning the “Traité de Physique” of James Rohault; Playfair’s Fourth Dissertation in the eighth edition of the “Encycl. Britann.”; “Essai théorique ... des connaissances humaines,” par G. Tiberghien, Bruxelles, 1844, Vol. I. pp. 472–495; Dr. W. Windelband, “History of Philosophy,” New York, 1893, pp. 380–381, 391–396; Dr. F. Ueberweg, “History of Philosophy,” New York, 1885, Vol. II. pp. 41–55; Alfred Weber, “History of Philosophy,” translated by Frank Thilly, New York, 1896, pp. 305–323; Ruard Andala, “Descartes in reality the overturner of Spinosism and the architect of experimental Philosophy”; Erasmus Bartholinus, “De Cometis,” Copenhagen, 1664–1665 (“Biog. Univ.”; Weidler, p. 508) Mahaffy, 1880; Houzeau et Lancaster, “Bibl. Gen.,” Vol. II. for Descartes, p. 119, and for Mersenne, p. 204; “Journal des Savants” for Feb. 1826, p. 103, for Feb. 1827, p. 110, also for Aug.-Oct. 1850, Dec. 1860, Jan.-Feb. 1861, Oct.-Nov. 1869, Feb., April and July 1870, Mar.-April 1880, Aug. 1884, April 1898, Feb. 1899.