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HISTORY OF PHYSICAL ASTRONOHY.

mation of the spots, Herscliel supposed a highly elastic gas to begenerated in the solid body of the sun. This gaseous substance ascends invirtue of its inferior specific gravity relative to the solar atmosphere,forcing a passage through both strata of clouds, and exposing to the eye ofan observer the dark body of the sun. When it reaches the luminousregions, it enters into combination with other gases, whence result decom-positions which give rise to faculse, and cause the solar surface generallyto assume a mottled appearance.

It is scarcely necessary to state that any theory which involves theagency of a principle, of whose existence we have no positive proof, cannotbe recognised as forming a part of the general body of science. Thehypothesis of an elastic gas to whose energy the formation of the solarspots is mainly ascribed in the foregoing explanation, may or may not betrue; but in the present state of our knowledge it can only be regarded as anarbitrary assumption, which possesses no stronger claims to our acceptancethan many other similar hypotheses that might be adduced.

It is necessary, therefore, to draw a broad line of distinction between thepurely speculative views of Sir William Herschel on this subject, and theimportant conclusions to which he has been conducted by reasoning fromestablished facts. Indeed, it is quite clear that the illustrious astronomerhimself did not attach any value to his hypothesis of the generation of thespots, beyond what it possessed as a convenient medium of connecting to-gether the various results at which he had arrived in the course of hismore rigorous researches *. The latter were pursued in the true spirit ofinductive investigation^ and the results constitute a valuable contributionto our knowledge respecting the interesting subject to which they refer.The facts upon which he has established the existence of two distinct strataof clouds in the solar atmosphere were exhibited by him in a very clearlight; and the rfeasoning by which he fortifies that conclusion appears tobe quite unexceptionable. This step had the effect of conducting him toan explanation of the penumbra of the solar spots—phenomena which allpreceding astronomers had signally failed to render a satisfactory ac-count of.

The geometrical relation of the nuclei of the solar spots with respect tothe luminous surface, was first pointed out by Wilson. For a similarexplanation of the penumbrse, astronomy is indebted to Herschel . Withrespect to the faculse, the just surmises of Wilson and Bode werefully established by Herschel , on facts derived from accurate observa-tion. A consistent view of the solar spots was thus obtained by a strictprocess of inductive reasoning. It still remained to render a legitimateaccount of the origin of these phenomena, by referring them to theoperation of some really existing physical cause. Some interesting spe-culations designed to accomplish this object appear in Sir John Herschel’srecent work, containing the results of his observations at the Cape ofGood Hope . That distinguished philosopher justly remarks that, what-ever be the cause of the spots, it is very evident that these phenomena

* In confirmation of this remark, the following passages may be cited from the papersof 1795 and 1801. “ They (the luminous clouds of the sun) plainly exist, because we seethem ; the manner of their being generated may remain an hypothesis, and mine, till abetter can be proposed, may stand good; but whether it does or not, the consequences Iam going to draw from what has been said, will not be affected by it.”— Phil. Trans.,1795, p. 61. “ I am, however, well prepared to distinguish between facts observed, and

the consequences that in reasoning upon them we may draw from them; and it will beeasy to separate them, if that should be hereafter required.”— Phil. Trans., 1801, p. 303.