6a 



NATURAL-PHILOSOPHICAL COLLECTIONS. 



Supposed Influence of the Spots of the Sun on Temperature. — M. Arago 

 comprises, in his annual epitome of meteorology, a table of the changes which 

 the surface of the sun has undergone during the preceding year. His object is to 

 furnish by these means data which may, at some period, serve to decide the ques- 

 tion whether the solar spots exercise any sensible influence on terrestial tempera- 

 ture. It is unfortunate that M. Arago has not been able to fill up, from observa- 

 tions made in other observatories, the blanks which the inconstancy of the cli- 

 mate of Paris has obliged him to leave in his tables. 



The difficulty of observing the sun, without being injured by the brilliance of 

 its light, has long prevented astronomers from discovering the changes which take 

 place at the surface of this planet, and until 1611 its spots had never been seen. 

 Various explanations have since been advanced to account for this phenomenon. 

 Lahire and Cassini supposed that they were the result of the exposure of the sum- 

 mits of many mountains in the substance of the sun, which became occasionally 

 visible, in consequence of the fluctuations of the inflamed fluid which covers the 

 surface of this star. In this view, the appearance of the spots would necessarily 

 coincide with a diminution of the activity of the combustion of the sun. This 

 was admitted by the astronomers whom we have named ; but this explanation 

 cannot now be sustained ; it cannot account for the general and detailed observa- 

 tions which we now possess. It does not, for instance, explain how it happens 

 that when we are observing two neighbouring spots, the one often increases whilst 

 the other diminishes ; for, according to the hypothesis of Lahire and Cassini, 

 two adjoining spots should increase and diminish at the same time. 



It has been supposed, also, that these spots were scorias thrown from volcanoes 

 situated on the surface of the sun ; others have said that the spots were placed 

 at the bottom of a funnel-shaped cavity, which was formed in the luminous part 

 of the sun. This was a much more satisfactory opinion ; but it also must be 

 renounced. 



Herschell, at length, gave such an explanation of these spots, as is consistent 

 with all the appearances which have been observed. He supposes the sun to be 

 formed of a solid and opaque kernel, surrounded by an atmosphere of little trans- 

 parency and reflective, which is itself enveloped by a luminous gas. If in a star, 

 thus constituted, an opening were to occur in such a manner, that the aperture 

 of the luminous atmosphere would be of the same dimensions as that of the at- 

 mosphere which immediately surrounds the obscure mass, we should see in the 

 centre a black spot without any penumbra, which sometimes occurs ; but if the 

 aperture be larger in the gaseous atmosphere than in the first envelope, a part of 

 the inferior atmosphere would be seen forming a penumbra, with aU the^ pecu- 

 liarities which we observe in the spots of the sun. 



But what can be the cause of this opening, which we suppose to be found in 

 the incandescent matter of the sun ? It may be the result of an ascending cur- 

 rent, which first breaks across the first atmosphere, and then the second, if sufS- 

 cient force remain. 



A very important thing to be remarked is, that, in almost every case, the for- 

 mation of a new spot is preceded by a brilliant spot, called faculum, whose exist- 

 ence Galileo first remarked. Generally, also, when a spot closes, a faculum is 

 formed in the place which it occupied. The appearance of facula seems then only 

 to indicate the formation of spots, and far from announcing a diminution of heat 

 in the sun, coincides with an increased activity in the gaseous matter of this star. 



The explanation given by Herschell of the formation of spots in the sun, 

 moreover, accords perfectly with the latter supposition, which has been adopted 

 by this celebrated astronomer. He thinks that the appearance of a spot indicates 

 a more active fermentation in that portion of the sun which is susceptible of incan- 



