HISTOBY OF PHYSICAL ASTHOKOMY.

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admirable tact, cites the existence of these phenomena as an irrefragableproof that the spots are attached to the surface of the sun, instead of beingplanets, as Scheiner, in his “Letters to Welser,” supposed them to be;for, on the latter supposition, he remarks, that the faculae, in virtue of theirmotion from east to west, ought sometimes to appear as bright spots beyondthe limb of the sun, a conclusion which was totally at variance with obser-vation. The faculae generally present an extended luminous appearance,but occasionally they exhibit a round form. On the 3rd December, 1800,Sir William Herschel observed a facula, which measured 2' 45".9 in ap-parent length, and therefore extended over a linear space of 75,000 miles.Faculse are for the most part seen in the neighbourhood of spots, butsometimes they appear alone. In the latter case they are generally theprecursors of spots which appear on the disk the following day. Messierwas frequently enabled by this circumstance to predict the appearance ofspots twenty-four hours before they actually presented themselves on thedisk. The faculae are always brightest on the sun’s limb, and generallydisappear as they approach the centre of the disk. When they enter thedisk on the eastern limb they generally continue to be perceived for twoor three days, after which they cease to be visible until they reappear onthe opposite side of the disk, when they are again perceptible during twoor three days before disappearing at the western limb.

The facula; always appear in the region of the spots. Observers, however,have discovered that the whole surface of the sun is diversified with minuteluminous specks of different degrees of brightness and irregular streaks oflight of extreme tenuity, bordered by more obscure parts. These pheno-mena are termed luculi. They are always perceptible on the sun’s disk,and cause the whole surface to assume a mottled appearance. Sir William Herschel compares the corrugated state of the sun, arising from this cause,to the roughness of an orange. These phenomena are found to be in astate of constant change. The more obscure parts, when submitted to aclose examination by Sir William Herschel , presented small pores asblack as the nuclei of the spots.

We now proceed to notice the various explanations of the solar spotswhich have been advanced by different astronomers. Galileo supposedthem to be clouds, of a greater or less degree of opaqueness, which are con-stantly floating in the solar atmosphere, and, by their occasional interposi-tion, prevent the observer from viewing the luminous surface of the sun.This theory, however, is incompatible with certain appearances (to benoticed presently) which indicate that the spots are depressions in theluminous surface of the sun. It is also defective, inasmuch as it does notexplain the constant presence and well-defined outline of the penumbra,nor give any account of the existence of faculae. Scheiner, after he wasconvinced that the spots could not be planets, adopted the opinion thatthey were the indications of tumultuous movements occasionally agitatingthe ocean of liquid fire of which he supposed the sun to be composed. LaHire imamned the sun to be a fluid mass, which contained within it nu-merous oP?que bodies. Occasionally the latter approached the surface andgave rise to the appearance of spots. When floating on the surface theyattracted all the particles of a similar nature around them; and, conse-quently, when they disappeared the places that had been occupied by themwere brighter than the other parts of the surface : hence originated thesolar faculte. This explanation agrees with a remark of Cassini’s, to theeffect, that when a spot disappears it is generally succeeded by a facula.