[136] Manilius supposes that comets are produced and rendered luminous by an operation very similar to the one described in the text; i. 815 _et seq._ Seneca, in the commencement of his Nat. Quæst., and in other parts of the same treatise, refers to this subject. His remarks may be worth perusing by those who are curious to learn the hypotheses of the ancients on subjects of natural science. We may remark, that Seneca’s opinions are, on many points, more correct than our author’s. [137] The author probably refers to that part of his work in which he treats on agriculture, particularly to the 17th and 18th books. [138] The æra of the Olympiads commenced in the year 776 before Christ; each olympiad consists of 4 years; the 58th olympiad will therefore include the interval 548 to 544 B.C. The 21st vol. of the “Universal History” consists entirely of a “chronological table,” and we have a useful table of the same kind in Brewster’s Encycl., article “Chronology.” [139] “rerum fores aperuisse ... traditur.” An account of the astronomy of Anaximander is contained in Brewster’s Encycl., article “Astronomy,” p. 587, and in the article “Anaximander” in the supplement to the same work by Scott of Aberdeen. I may remark, that these two accounts do not quite agree in their estimate of his merits; the latter author considers his opinions more correct. We have also an account of Anaximander in Stanley, pt. 2. p. 1 _et seq._, and in Enfield, i. 154 _et seq._ [140] In the translation of Ajasson, ii. 261-7, we have some valuable observations by Marcus, respecting the origin and progress of astronomy among the Greeks, and the share which the individuals mentioned in the text respectively had in its advancement; also some interesting remarks on the history of Atlas. Diodorus Siculus says, that “he was the first that discovered the knowledge of the sphere; whence arose the common opinion, that he carried the world upon his shoulders.” Booth’s trans. p. 115. [141] “nunc relicto mundi ipsius corpore, reliqua inter cœlum terrasque tractentur.” I have already had occasion to remark upon the various modes in which the author uses the word _mundus_; by _cœlum_, in this passage, he means the body or region beyond the planets, which is conceived to contain the fixed stars. _Sphæra_, in the preceding sentence, may be supposed to mean the celestial globe. [142] “ac trigesimo anno ad brevissima sedis suæ principia regredi;” I confess myself unable to offer any literal explanation of this passage; nor do the remarks of the commentators appear to me satisfactory; see Hardouin and Alexandre in Lemaire, ii. 241, 2. It is translated by Ajasson “en trente ans il reviens à l’espace minime d’où il est parti.” The period of the sidereal revolutions of the planets, as stated by Mrs. Somerville, in her “Mechanism of the Heavens,” and by Sir J. Herschel, in his “Treatise on Astronomy,” are respectively as follows:—