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HISTORY OF PHYSICAL ASTRONOMY.

of Sir 'William Herschel have shewn that this is not universally true. Onthe 18th February, 1801, that astronomer observed a spot on the sun’sdisk, from the nucleus of which there issued a branch, jutting out sharplyupon the penumbra, while on the same side of the spot there appeared asimilar projection of the penumbra upon the luminous region around it.In the course of a little more than two hours he found that the nucleushad thrown out three branches ; and, in this case also, he found that therew r ere three exactly corresponding branches of the penumbra. A conclusionmanifestly deducible from these curious facts was, that the cause whichacted upon the nucleus acted also in a similar manner upon thepenumbra.

The magnitude of some of the solar spots is immense. In 1754, Mayerperceived a spot, the dimensions of which amounted to ^th of the sun’sapparent diameter. This gives above 45,000 miles for the absolutediameter of the spot. In 1779 there appeared an immense spot on thesun which Herschel was enabled to discern with the naked eye. Whenobserved by him with a seven feet reflector, and a high magnifying power,it v r as found to be divided into two parts. The larger of the two measured1' 8" in diameter, which indicates an absolute length of 31,000 miles.The rapid changes which these phenomena undergo is very astonishing.Herschel states that, while engaged in observing a spot on the 19thFebruary, 1800, he fixed his attention on several places, but on lookingoff, even for a moment, the spots he had marked could not be found again.Sir John Lubbock , in a recent communication, states that he has observedspots visible to the naked eye, of which, on the following day, not a tracecould be found, even with the aid of a good telescope *.

A remarkable circumstance connected with the solar spots is their con-stant appearance near the equator. Galileo remarked that their distancefrom that circle never exceeded 29°. Schemer found by his own observa-tions, that they were all confined to a zone extending 30° on each side ofthe equator, which was termed by him, on this account, the royal zone.Subsequent astronomers have enlarged the region of the spots, so as toembrace a zone of about 35° north and south of the equator. Occasionally,indeed, spots are observed in the regions exterior to this zone. A largespot, which appeared on the sun’s disk in 1783, was found by Mechain tobe distant about 41° 30' from the solar equator. This is, however, an in-stance of very rare occurrence. Sir John Herschel carefully observed thespots on the sun’s disk, towards the close of the year 1836 and the begin-ning of 1837; and he has remarked that, during the whole period eimbraced by his observations, the most unpractised eye could not fail to per-ceive by the mere allineation of the spots, the situation of the poles andequator of the sun, without watching from day to day their progress acrossthe diskf.

Besides the phenomena of the spots, the telescope has disclosed otherinteresting appearances on the sun’s disk. Some parts are perceived tobe brighter than the rest of the surface, and hence have derived the ap-pellation of faculse. These phenomena were first noticed by Galileo inhis third letter to Welser on the spots The illustrious philosopher, with

* Phil. Mag. vol. xxxii. p. 171.

t Results of Astronomical Observations at the Cape of Good Hope , &c., &c. London ,1847.

7 Posso aggiungere che nella medisima faccia del sole si veggono tal volta alcunepiazzette piu chiare delresto. Istoria, &c., delle Macchie Solari, p. 132.