HISTOKY OF PHYSICAL ASTEONOMY.

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quence exhibited a fainter lustre, whence originated the penumbra. Hesupposed the faculse to be vast undulations in the luminous atmosphere,in proof of which he remarked that they were most conspicuous towardsthe limbs of the sun, and ceased to be visible when they approached thecentre of the disk. This explanation of the solar spots does not differmaterially from that which had been already advanced by Wilson; nordoes it remove any of the objections to which the latter is liable.

The next astronomer whose researches on the solar spots demand a briefnotice is Sir William Herschel . That illustrious individual devoted con-siderable attention to this subject, and has presented the result of hislabours in two papers, which appear in the volumes of the Royal Society *.In these papers the author has given a detailed statement of a multitudeof interesting facts derived from his observations of the solar spots, andhas exhibited a complete view of these phenomena which is in the highestdegree instructive. The conclusions to which he was conducted by hisresearches on this subject are entitled in some respects to be ranked withthe many other great achievements of his genius. He demonstrated, bymeans of his admirable observations, that the spots are real depressions inthe surface of the sun, and that the faculse are elevations above theordinary level. He supposed, also, in common with several astronomerswho preceded him, that the sun is an opaque mass surrounded by aluminous atmosphere. He maintained, however, that this atmospherecannot be either gaseous or fluid; for on either of these suppositions, theluminous matter would flow with great velocity into the depressions, andwould occasion an almost instantaneous disappearance of the spots, insteadof a gradual diminution as observation indicated. He remarked, also,that the existence of faculse is incompatible with the mobility of a fluid orgaseous substance; for these luminous ridges sometimes continue to main-tain themselves for several days at a considerable elevation above theordinary level of the solar surface. In order to account for the variousappearances of the spots, he supposed the sun to be surrounded by a tran-sparent atmosphere, in which are suspended two distinct strata of cloudsat different elevations. The upper stratum is composed of self luminousclouds, which constitute the source of the solar light. The lower stratumis composed of opaque clouds, which shine only by the reflexion of theluminous regions above them. They are denominated by Herschel theplanetary clouds, on account of their supposed resemblance to the cloudsof the terrestrial atmosphere. The solid nucleus of the sun is effectuallyprotected from the intense light of the luminous regions by the inter-position of the planetary clouds. When corresponding apertures areformed in both strata of clouds, the dark body of the sun is perceptiblethrough them, and gives rise to the appearance of a spot. If the aperturesbe of equal magnitude, the substratum will present an appearance ofuniform blackness, and will therefore indicate a nucleus without a penum-bra. If the aperture in the upper' stratum be wider than that in thelower, a portion of the latter will be seen surrounding the dark body ofthe sun; but as it shines only by reflexion, it will exhibit a fainter lustrethan the luminous regions, and therefore the appearance in this case willbe that of a nucleus surrounded by a penumbra. If there be no aperturein the stratum of the planetary clouds, the appearance will manifestly bethat of a penumbra without a nucleus. In order to account for the for-

* Phil. Trans., 1795-1801.