472 SUN and he finds that though the nucleus looks per- fectly black by contrast with the general sur- face, it shines in reality with a light unbearably brilliant when viewed alone, while his thermal measurements show that the heat from the nucleus is even greater proportionately than the light, and not very greatly below the heat of the surrounding surface. The boundary between the umbra and the penumbra is in general well defined ; and commonly the inner part of the penumbra nearest to tbe umbra is brighter than the exterior portion. Many spots are of enormous size, so as to be visible with the naked eye. Sir W. Herschel saw one in 1779 which had a diameter exceeding 50,- 000 m., and many far larger than this have since been seen. The spots are not scattered over the whole surface of the sun, but are for the most part confined to two belts between lat. 5 and 30 on either side of the solar equa- tor. An equatorial zone 6 wide is almost entirely free from spots. Owing to this pe- culiarity of arrangement, Sir J. Herschel sug- gested the existence of motions in the solar atmosphere corresponding to our trade winds ; but the circumstances of the solar orb and atmosphere differ so entirely from those of our earth and air, that such comparisons are unsafe. Dr. Wilson of Glasgow was the first to show that the umbra of a spot is below the level of the penumbra. He observed that a spot, visible in 1769, changed in shape as it traversed the solar disk, precisely as it would if the spot had been a depression below the general surface of the sun. The penumbra was markedly wider on the side nearest the edge of the solar disk than on the other side, whereas the reverse should have been the case if the spot had been a surface marking. Sir W. Herschel in 1777 began a series of solar observations which before long confirmed Wil- son's views. He was led to explain the spots by the theory that the sun's globe is surround- ed by two layers of clouds, suspended in an atmosphere at different elevations. He sup- posed the upper cloud stratum to be self- luminous, and to be the source of the solar light, or the true photosphere (to use a conve- nient term invented by Schroter). The lower layer he regarded as opaque, and as owing whatever light it appears to possess to the re- flection of light received from the upper layer. He supposed that when an opening is formed in the outer layer we see merely a penumbral spot ; but that when the inner layer also is displaced we see the true surface of the sun, which he supposed to be solid, and not neces- sarily so heated as to be unfit for habitation. Modern researches show this part at least of Herschel's theory to be wholly untenable, every- thing tending to prove that the whole mass of the sun to its innermost core is intensely heated. The recognition of a nucleus within the umbra would seem to indicate that a third cloud layer exists within the second or inter- nal layer of Herschel's theory. But the obser- vations of Prof. Langley show that most prob- ably all the features of the solar photosphere yet observed are phenomena of cloud enve- lopes, since he has been able to recognize cloud farms at one level floating over cloud forms at a lower level, while even in the (relatively) dark- est depths of the nucleus clouds are still to be perceived, though so deep down that their out- lines can be barely discerned. The study of the solar spectrum (see SPECTRUM ANALYSIS), while revealing much respecting the constitu- tion and physical condition of the solar orb, has thrown some light also on the nature of sun spots. Mr. Huggins, for instance, has found that several of the absorption bands be- longing to the solar spectrum are wider in the spectrum of a spot, a circumstance indicative of increased absorption so far as the vapors corresponding to such lines are concerned. Spots are more numerous in some years than in others, and occasionally no spots are visible for many successive days. Schwabe of Des- sau began to study this peculiarity in 1826, and after many years recognized a remarkable periodicity in the frequency of sun spots. They are found gradually to increase in num- ber during a certain period, and then to de- crease until at length there are no spots ; then they increase again, and so on. According to Schwabe's earlier investigations, the cycle lasts 10 years ; but Wolf of Zurich has found by examining earlier observations that the true average period is about 1 1 '1 1 years. (See MAG- NETISM, TERRESTRIAL.) Various minor cycles have been suspected, besides a long cycle of about 56 years. Wolf in 1859 presented a formula by which the frequency of spots is connected with the motions of the four bod- ies, Venus, the earth, Jupiter, and Saturn. Prof. Loomis of Yale college has since advo- cated a theory (suggested by the present wri- ter in 1865, in " Saturn and its System," p. 168, note) that the long cycle of 56 years is related to the successive conjunctions of Sa- turn and Jupiter. But the association is as yet very far from being demonstrated, to say the least. Besides the spots, the telescope re- veals minute dark dots or pores mottling the surface, which have been lately found to be the intervals separating numberless cloud-like forms, apparently minute, but in reality from 200 to 1,000 m. in diameter, the brilliancy of which so greatly exceeds that of the interve- ning spaces that they must be recognized as the principal radiators of the solar light and heat. These are found to be in constant fluc- tuation, and Sir J. Herschel compares their appearance to the slow subsidence of some flocculent chemical precipitates in a transparent fluid when viewed perpendicularly from above. Near the great spots or groups of spots there are often seen streaks more luminous than the neighboring surface, called faculce. They are oftenest seen toward the borders of the disk. Mr. Dawes saw, on Oct. 22, 1859, in a large mass of faculee, one bright streak forming the
