292 SCIENCE PROGRESS 



faint lines that are affected by widening, indicating that a sun- 

 spot does not rise very high above the photosphere, if we regard 

 the umbra as an elevation, and the penumbra as a shallow 

 depression. 



Again, a vapour of a metal, for instance that of iron, will be 

 represented by lines of various intensities in the solar spectrum, 

 ranging from very faint lines to some of the most prominent. 

 Hence it follows that some molecules giving certain radiations 

 of iron, as a typical vapour, are confined to low levels, while 

 others extend to higher levels. Now, Dr. St. John finds, and 

 his results have been corroborated by still more recent observa- 

 tions of Mr. Evershed, that the radial velocities of the vapours 

 in the penumbrae of sun-spots decrease with an increase in the 

 intensities of the spectral lines. In other words, low-lying 

 vapours move outwards more rapidly than higher-lying vapours. 

 For instance, in the case of iron there is a gradual decrease 

 in velocity from ros miles per second for lines of intensity i, 

 to o*35 miles per second for lines of intensity 8. Taking the 

 whole series of lines of iron, from intensity oo, or very faint 

 lines, to very strong lines of intensity 40, and referring the 

 other elements to the same scale, a sort of sounding-rod is 

 constructed to gauge the depths of sun-spots. One interesting 

 result is that we see to a greater depth into the sun in the red 

 end of the spectrum than in the violet. 



Again, after a certain level in a sun-spot has been reached, 

 at a depth represented by iron lines of intensity 10, that is from 

 eclipse results about 2,000 to 2,500 miles above the photosphere, 

 the gases and vapours, chiefly sodium, calcium, and hydrogen, 

 flow inwards. This means that there is in the upper levels a sort 

 of indraught. Hence we may conclude, from the evidence, 

 " that the outward velocities in the reversing layer at the outer 

 boundaries of the penumbra of sun-spots may be referred, with 

 great probability, to a low-lying vortex." The quotation is 

 from the Annual Report of the Director of the Mount Wilson 

 Solar Observatory (Carnegie Institution of Washington) for 

 the year 191 3. But we must confess that we fail to see any 

 evidence for a whirling or vortex-motion in the observations 

 summarised above, so far as they concern the radial movements 

 in sun-spots. Undoubtedly there is evidence of a whirling or 

 vortex motion in the flocculi above sun-spots, that is in the 

 hydrogen flocculi. 



