HISTORY OF PHYSICAL ASTRONOMY.

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and comprehending observations of these phenomena, which extendedover a period of eighteen years. He determined the period of thesun’s rotation to be between 26 and 27 days. He also assigned 6° and8° as the two extreme limits of the angle at which the pole of rotation isinclined to the pole of the ecliptic. Modern observation makes it 7° 20'.

The subject of the solar spots is calculated, in a strong degree, to attractthe attention of those engaged in the study of physical science. When itis considered that the sun exercises so commanding an influence over theoperations of nature on the earth, it is impossible to repress an intensedesire of arriving at some knowledge of those mysterious changes whichare perpetually taking place at his surface. Notwithstanding its manifestimportance, however, there is, perhaps, no department of astronomicalscience in which less real progress has been made than in that relating tothe solar spots. In order that the reader may form a clearer conceptionof the various theories which have been devised with a view to explain theorigin of these interesting phenomena, we shall give a brief statement ofthe principal facts relating to them which have been established by theobservations of astronomers.

When a spot on the sun’s disk is closely examined, it is found not to beuniformly obscure. The central part is characterised by intense blackness,but it is surrounded on all sides by a contour of appreciable breadth, whichexhibits a semi-luminous appearance. The more obscure part of the spotis termed the nucleus; while that which is visible by a faint light hasbeen denominated the penumbra. The nucleus and the penumbra do notshade into each other, but are separated by a well-defined boundary. Thisimportant characteristic of the solar spots was first remarked by Scheiner,and has been fully established by the observations of subsequent astrono-mers. A similar remark holds good with respect to the exterior part ofthe penumbra; the bounding line between it and the wholly luminous re-gion around it being in general distinctly visible. Sir William Herschel made a series of photometrical experiments with a view to determine therelative quantities of light emitted by the nucleus, the penumbra, and thewholly luminous part of the sun. The conclusion he came to was that, ifthe full light of the sun be represented by a thousand, the brightness ofthe penumbra will be represented by four hundred and sixty-nine, andthat of the nucleus by seven.

The formation of a spot is generally indicated by the appearance of avery black pore, which gradually enlarges on all sides. This enlargementof the spot is effected by a simultaneous enlargement of the nucleus andpenumbra. When two or more spots appear very close together, theyfrequently expand towards each other and form one large spot. When aspot is diminishing, previous to its disappearance, the process of diminu-tion is accomplished by an irregular encroachment of the penumbra uponthe nucleus. This circumstance causes the form of the nucleus to be veryirregular, and not unfrequently leads to its breaking up into two or moredistinct nuclei. When a spot is disappearing, the nucleus generallyvanishes before the penumbra.

Sometimes a spot is observed in which the penumbra is wanting.' Smallspots are generally destitute of these appearances. On the other hand, apenumbra without a nucleus is occasionally seen. Scheiner, and the an-cient observers of the spots, were of opinion that the exterior boundary ofthe penumbra never contained any sharp angles, however irregular theboundary on the side of the nucleus might be. The accurate observations