180 



I)Isc,()Vi:hy 



the detection of all the characteristic phenomena of 

 the Zeeman-effcct. As the only known way in which 

 such magnetic fields could be produced at the tempera- 

 ture of the sun, it thus appears probable that sun-spots 

 are electrical vortices." 



In 1909 a brilliant discovery was announced by 

 Mr. John Evershed, the able English astronomer who 

 directs the observatory at Kodaikanal, in India. He 

 detected displacements of lines in the sun-spot spectra, 

 which indicated, on Dopplcr's principle, the existence 

 of large radial motions in spots— the movement of 

 gases outward from the centre of the spots and parallel 

 to the photosphcric surface. In the summer of 1910, 

 Dr. C. E. St. John, at Mount Wilson, commenced a more 

 elaborate investigation, and after two years he con- 

 firmed Evershed 's discovery and put forward a hypo- 

 thesis of its significance. " We are dealing," he said, 

 " with an actual flow of the material of the reversing 

 layer out of spots and of the chromospheric material 

 into spots." In the light of these disclosures, Hale's 

 original sun-spot hypothesis was obviously untenable 

 or at least only partially true. Accordingly, in 1912, 

 he outlined what he called a " tentative working 

 hypothesis " as " a guide to further research." Accord- 

 ing to this theory a column of gas moves upward from 

 the interior of the sun towards the surface of the 

 photosphere. Owing to differences of velocity of 

 adjoining surfaces or irregularity of structure, a vortex 

 motion is set up. The circulation in the vortex is 

 vertically upward and then outward. As a result 

 of expansion in the central portion of the vortex, 

 cooling sets in, and a comparatively dark cloud — 

 the sun-spot umbra — is formed. " A rapid flow of 

 negative ions sets in towards the cooler gases at 

 the centre from the hotter gases without. These 

 ions, whirled in the vortex, produce a magnetic 

 field." 



The systematic study of the chromospheric regions — 

 comprising prominences, facula;. and the higher levels 

 of the solar atmosphere — has been carried on in recent 

 years by Deslandres, Evershed, and the Mount Wilson 

 observers. Evershed's work on prominences has supple- 

 mented the earlier work of Tacchini and the Italian 

 spectroscopists. From 1890 to 1906, observing in 

 England, Evershed recorded 11,000 prominences, and 

 from the commencement of his work in India until 

 1917, 60,000 were numbered and investigated. 

 Evershed confirmed the conclusion of the Italian 

 astronomers with regard to the general identity of the 

 prominence-period with the spot-period. He distin- 

 guishes two classes of prominences — " large high 

 prominences " and " prominences associated with sun- 

 spots." The large and massive prominences are not 

 found directly above spots, while prominences of the 

 second type arc invariably associated with these 



objects. The application of the spcctrohcliograph has 

 now made possible the observation of prominences not 

 only on the edge of the sun, but all over the disc. 

 Allied to the prominences are filaments and alignments 

 in the higher regions of the solar atmosphere to which 

 Deslandres has devoted much attention since 1906. 

 These form a kind of network over the disc. Again 

 and again. Evershed has noticed that at a point of the 

 limb where a prominence is visible, a filament or 

 alignment usually ends. Evershed, indeed, identifies 

 filaments with prominences projected on the disc of 

 the sun. Indeed, prominences, alignments, filaments, 

 faculEE, and flocculi seem to be different varieties of the 

 same species, at different depths of the solar atmo- 

 sphere and of varying degrees of permanence. Un- 

 doubtedly the most ephemeral of all solar phenomena 

 are the hydrogen " bombs," first noticed by Mr. 

 Ellerman at Mount Wilson on September 21, 1915. 

 The average duration of these is from one to three 

 minutes, and on rare occasions from five to ten. " The 

 appearance of the phenomenon," wrote Ellerman in 

 1917, "indicates something in the nature of an explosion, 

 in which hydrogen seems to be the only element pla\nng 

 a part. . . . The regions where the bombs are likely to 

 appear are around and among active spot-groups, 

 especially groups which are developing and composed 

 of many members." 



Research work on selected areas and different levels 

 has enormously increased our knowledge of the solar 

 atmosphere, and of its components and circulatory 

 processes. The rarer and heavier elements appear to 

 be confined to the lower levels and the lighter and more 

 abundant gases form the upper layers. St. John's 

 study of the distribution of the elements in the solar 

 atmosphere indicates that the lower solar atmosphere, 

 including the lowest region of the reversing layer, is a 

 highly disturbed region, in which are located the upper 

 parts of the sun-spot vortices, where the outflow of 

 material from the interior takes place. Higher than 

 this is the general reversing layer, and higher still the 

 chromosphere. Beyond the chromosphere is the 

 corona. Unlike the chromosphere, this outer envelope 

 is always screened from view except on the rare occasion 

 of a total eclipse of the sun. Man}- attempts have been 

 made, by various methods, to photograph the corona 

 in daylight, but these have been so far without success. 

 At an early stage, it became apparent that the shape of 

 the corona is governed by the solar cycle. At maxi- 

 mum the coronal radiance is equally distributed all 

 round the sun's disc, while at minimum long streamers 

 from the equatorial zones make their appearance. 

 The study of the corona has made much less progress 

 than that of other solar appendages, and accordingly 

 its nature is still largely a matter of conjecture. The 

 coronal spectrum was long ago noticed to be com- 



