Kepler’s physical reasonings, therefore amount, in fact, to the doctrine of vortices around the central bodies and are occasionally so stated by himself; though by asserting these vortices to be ‘an immaterial species,’ and by the fickleness and variety of his phraseology on the subject, he leaves his theory in some confusion; a proceeding, indeed, which both his want of sound mechanical conceptions and his busy and inventive fancy might have led us to expect. Nor, we may venture to say, was it easy for any one at Kepler’s time to devise a more plausible theory than the theory of vortices might have been made. It was only with the formation and progress of the science of mechanics that this theory became untenable.” REFERENCES.--“Kepler, sa vie et ses ouvrages,” in the “Journal des Savants” for June, July and August 1847; Kepler’s manuscripts, “Phil. Trans.,” Vol. XI. p. 27; Wm. Whewell, “Phil. of the Ind. Sc.,” London, 1840, Vol. II. pp. 383–386; “Epistolæ ad J. Keplerum,” published by M. G. Hansch in 1718; Houzeau et Lancaster, “Bibliogr. Générale,” 1887, Vol. I. part i. pp. 612–614, detailing the contents of Kepler’s “Opera Omnia,” also Vol. I. part ii. pp. 1315–1316, 1330–1331, 1383, and Vol. II. pp. 175–176, 456–462 and 1581; Robert Small, “An Account of the Astronomical Discoveries of Kepler,” London, 1804; Humboldt, “Cosmos,” 1860, Vol. II. p. 710, notes, for Laplace, Chasles and Brewster on the writings and theories of Kepler; “Jour. des Savants” for June, July and August 1847; “Geschichte der Mathem.,” Vol. III. p. 318, and Vol. IV. pp. 216, 311; Dr. Geo. Miller, “Hist. Phil. Ill.,” London, 1849, Vol. III. notes at pp. 134–135; Fourth Dissert. of “Encycl. Brit.”; Whewell, “Hist. of the Ind. Sc.,” 1859, Vol. I. pp. 291–311, 320, 386, 387, 415, 462, 532–534, and Vol. II. pp. 55, 56.