54 



THE POPULAR SCIENCE MONTHLY. 



posed, the view of Mr. Spencer is presented last, as more in conformity 

 to the facts than any other. After showing how the hypothesis of M. 

 Faye is discredited by the spectroscopic observations of Lockyer, Hug- 

 gins, and Secchi, they say : " It follows conclusively that the spots are 

 regions of increased absorption. This accords with Mr. Herbert Spen- 

 cer's theory, with which also the observations of Mr. De La Rue and 

 Dr. Balfour Stewart are in satisfactory agreement. Let us now," say 

 they, " present the theory of Mr. Spencer, whose suggestions as to the 

 possible causes of solar spots are very valuable. 



" Mr. Spencer, basing his reasoning on terrestrial analogies, thus 

 accounts for the spots : ' The central region of a cyclone must be a 

 region of rarefaction, and consequently a region of refrigeration. In 

 an atmosphere of metallic gases rising from a molten surface, and 

 reaching a limit at which condensation takes place, the molecular 



Solar Spot, with Three Bridges of Light. (Nasmyth.) 



state, especially toward its upper part, must be such that a moderate 

 diminution of density and fall of temperature will cause precipitation ; 

 that is to say, the rarefied interior of a solar cyclone will be filled with 

 cloud ; condensation, instead of taking place only at the level of the 

 photosphere, will here extend to a great depth below it? It will be seen 

 that Mr. Spencer, as opposed to Kirchhoff, not only accounts for the 

 formation of a cloud, but places it where the objections made to 

 Kirchhoff's clouds do not hold good. He next shows that a cloud thus 

 occupying the interior of a cyclone will have a rotary motion, and this 

 accords with observation. Being funnel-shaped, as analogy warrants 

 us in assuming, its central parts will be much deeper than its periph- 

 eral parts, and therefore more opaque. This, too, cori-esponds with 

 observation. Nor are we, on this hypothesis, without some interpreta- 

 tion of the penumbrse. If we may suppose the so-called ' willow* 



