284 SCIENCE PROGRESS 



solar light, and hence to appear dark. The temperature of the 

 sun's surface is about 6,000 degrees on the absolute Centigrade 

 scale, and that of sun-spots, as is indicated by a lowering of 

 temperature sufficient to allow of the formation of chemical 

 compounds, such, for instance, as titanium oxide, magnesium 

 hydride, and possibly superheated steam, is about 3,500 

 degrees on the same scale. Hence they exercise an absorbing 

 effect on the higher temperature vapours of the solar core. 



In the year 1769 Dr. Wilson of Glasgow broached the idea 

 that sun-spots were depressions in the solar surface. His de- 

 monstration was founded upon the appearance of the penumbra 

 of spots, seen in perspective, as the spots travel from east to 

 west across the visible solar hemisphere. When the spot is 

 near the east limb, the penumbra only appears on the side turned 

 towards the limb, and appears to be wanting on the other 

 side. As the spot approaches the centre of the disc the westerly 

 edge of the penumbra gradually comes into sight. As the 

 spot moves towards the west limb carried round by the rotation 

 of the sun, the eastern part of the penumbra begins to disappear, 

 so that when it is near the west limb, the western side of the 

 penumbra is alone seen. The interpretation of these pheno- 

 mena is that the spot is a saucer-like depression, with the umbra 

 corresponding to the bottom of the saucer. But there are rela- 

 tively few round symmetrical spots which can be observed as 

 tests of this theory. Moreover, when the spots are very near 

 the sun's limb, the penumbrae are not seen at all, but only a 

 foreshortened dark line, presumably representing the umbra. 

 The umbra also, when a large spot is on the limb, seems to 

 block out the view of the edge of the sun, and would be better 

 described as an elevation above the level of the general surface 

 of the sun, than as a depression below it. This was noticeably 

 the case when the big spot of February 15, 191 7, was crossing 

 the sun's western limb. Visually a sun-spot is a shallow 

 depression represented by the penumbra, in which rises an 

 elevation seen as the umbra. 



There are relatively very few spots in which a gyratory 

 or cyclonic motion can with certainty be detected. Neverthe- 

 less there are such instances observed, as for instance the rota- 

 tion of the umbra and penumbra of a spot around a nucleus 

 described by Dawes, who recorded in one case a rotation of 

 100 degrees in six days, and on December 23, 1851, observed in 



