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, Huggins, and Secchi, they say: "It follows conclusively that the spots are regions of increased absorption. This accords with Mr. Herbert Spencer'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
Fig. 6.
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 peripheral parts, and therefore more opaque. This, too, corresponds with observation. Nor are we, on this hypothesis, without some interpretation of the penumbræ. If we may suppose the so-called 'willow--
