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HISTORY OF PHYSICAL ASTRONOMY.

to his acute perception the fact of the sun’s motion round a fixed axis. Inthe remarks on the spots, which appear in the beginning of his “Disserta-tion on Floating Bodies,” he makes the period of rotation to be about alunar month*. In his third letter to Welser his language is moreprecise. He states that, having observed a great number of spots withmuch attention, he came to the conclusion, that the time during whichthey remained on the disk was somewhere about fourteen daysf. Thisassigns rather more than twenty-eight days to the period of the sun’sapparent rotation. Modern observation makes it 27 d 8 h . He also re-marked that the axis of rotation is not perpendicular to the plane of theecliptic, but that it deviates from the pole of that circle only by a smallangle. Although Scheiner formed very erroneous views of the nature ofthe solar spots, lie studied with great assiduity all the circumstances con-nected with their appearance. In 1630 he published his Rosa Ursina +,an immense work, devoted exclusively to the subject of the solar spots,

* Che (il sole) in un mese lunare in circa finisce il suo periodo. Opere di Galileo ,Edit Pad. tome i. p. 189.

+ M. Arago, while engaged in discussing the relative merits of Galileo and Scheiner,with reference to their respective researches on the subject of the* sun’s rotation (An-nuaire , 1842), appears to have been hurried, by the ardour of dispute, into the commissionof an act of injustice against the illustrious Italian . The following is a brief statement ofthe facts connected with this question. Scheiner, in one of his letters, had contendedthat the spots could not be attached to the surface of the sun, alleging in support of hisassertion, that the time occupied by a spot in traversing the sun’s disk was greater in somecases than it was in others. Galileo denied the existence of any difference in the durationof the spots on the disk, and proceeds in the following terms to state the grounds of hisdisbelief:—“ Perche havendo io circa questo particolare fatte molte, et molte diligentis-sime osservazioni non ho trovato incontro alcuno, onde si possa concluder altro, se nonche le macchie tutte indifleramente dimorano sotto ’I solar disco tempi eguali che al miogiudizio sono qualche cosa piii di giorni 14.” (Istoria, &c., alle Macchie Solari, p. 116.)It appears by this passage, and also by that cited in the foregoing note, that as early as1612 Galileo had watched with great assiduity the various circumstances connected withthe solar spots, and that as nearly as he could possibly judge the period of the sun’s rota-tion was 28 days. In 1630, Scheiner published his Rosa Ursina , in which he fixedthe period of rotation between 26 and 27 days. Let the reader compare these factswith the following assertion of M. Arago’s. “ Galileo has never assigned the period ofthe sun’s rotation, whether apparent or real, otherwise than in a vague manner. Withrespect to the apparent period he fixed it at about a month (nello spazio quasi d’unmese, Dialogues)....Besides, the Dialogues did not appear until 1632, two years after thepublication of the Rosa Ursina of Scheiner.” Thus it appears that M. Arago entirelyoverlooks the scientific statement made by Galileo in his letter to Welser, of December,1612, and assumes, as the basis of his reasoning, a remark on the same subject made bythe illustrious philosopher in his “ Dialogues on the System,” a work in which the authoravowedly expounds his views rather in the language of familiar explanation than in thestrict phraseology of science. Galileo ’s estimate of the period of rotation is also pre-sented in a very unfavourable point of view by placing it after the publication of Scheiner’swork, and twenty years subsequent to the stricter statement made by him in his letter toWelser. It may be urged that the language of the Italian philosopher is by no meansconsistent with the precision due to such researches. This is, no doubt, true, but it cer-tainly does not lose, in this respect, by comparison with the language of Scheiner. Indeed,when we take into account the discordances of the modern determinations of the period ofrotation (amounting to seven or eight hours), we cannot refrain from the conclusion thattoo much reserve cannot be employed in advancing the charge of vagueness against theoriginal explorer of the heavens with the telescope, whose observations were made with alittle instrument that magnified only thirty-three times.

X Rosa Ursina, sive Sol ex admirando facularum et maculamm suarum phenomenovarius, &e. Alluding to this enormous work, Delambre says, “ There are few books sodiffuse and so void of facts. It contains 784 pages: there is not matter in it for 50pages.”— Hist, Ast Mod. tome i. p. 690.