Physical Constitution of the Sun. 347 



not be assumed that Father Secchi has here allowed himself to 

 be misled to the inference of post hoc, ergo propter hoc. To enter 

 upon a closer consideration of the cyclone theory of the sun- 

 spots, recently set up by MM. Reye* and Fayef, I think I may 

 at present decline, since, in the animated discussion of these 

 views in the Paris Academy, Father Secchi and M. Tacchini 

 have brought out the contradictions to simple facts of observa- 

 tion into which the theory falls. I take leave to address to its 

 advocates, without here discussing its physical bearings, the 

 question put by Tacchini : — " In the face of observations so 

 clear, so independent of hypotheses or preconceived ideas, is it 

 possible to accept a theory which makes cyclones the sole cause 

 of sun-spots?^!. I am moreover bound to thank Professor 

 Reye for the correction (in p. 177 of his work) he has made of 

 an error in my memoir " On the Law of Rotation of the Sun 

 and the Great Planets " (p. 91), relative to the transformation 

 of the sun-spots into streaks, after the analogy of the formation 

 of streaks of cloud-like products observed on Jupiter and Saturn. 

 From Carrington's Table given by me in that memoir it is im- 

 mediately seen that the difference between the velocities of 

 diurnal rotation of two points of the sun's surface, the helio- 

 graphic latitudes of which differ 1°, amounts on the average not 

 to 1°*6 (as incorrectly given by me), but to V'6. 



If at the conclusion of these considerations I once more reca- 

 pitulate the view I have invariably, since the year 1865, advo- 

 cated, of the nature of the sun-spots as scoria-like products of 

 cooling resulting from radiation at the incandescent-liquid sur- 

 face of the sun§, it is only for the purpose of showing how 

 simply, if it be adopted, all those phenomena can be explained 

 by which the opponents of this theory have hitherto seen them- 

 selves necessitated to continual modifications of their views. 



As the briefest possible expression of the essential point of 

 my theory, I select the representation given in 1869, in the 

 Vierteljahrschrift der Astronomischen GeseIIschaft\\ } Qind repro- 

 duce it verbatim : — 



11 The masses of scoria possess, as products of cooling, a con- 

 siderably lower temperature than the glowing liquid mass, sur- 

 rounding them on all sides, of the bright surface of the sun. 

 The differences of temperature hereby determined must generate 



* * Die Wirbelstiirme, Tornado's und Wettersaulen in der Erd-Atmo- 

 sph'are mit Beriicksichtigung der Sturme in der Sonnen-Atmosphare, dar- 

 gestellt und wissenschaftlich erklart von Dr. Theodor Reye, ord. Professor 

 a. d. Universitat Strassburg. 5 Hannover, 1872. 



t Comptes Rendus, vol. lxxv. p. 1664. 



% Comptes Rendus, vol. lxxvi. p. 827. 



§ Photometrische Untersuchungen (Leipzig, 1865), p. 246 sqq. 



!| Jahrgang iv, Heft 3, pp.. 165-179. 



