176 SOLAR CHANGES OF TEMPERATURE. 



lines, called enhanced lines, added, while many of the arc lines wane 

 in intensit}". 



It is found that at sun-spot minimum, when the "known" lines are 

 most numerous, the lines are almost invariably those seen most promi- 

 nent in the arc. Passing from the sun-spot minimum toward the 

 maximum the "unknown" lines gradually obtain the predominance. 

 As said before, they ma}^ be possibly "enhanced lines;" that is, 

 lines indicating the action of a much higher temperature on known 

 substances. 



Unfortunately the records of enhanced lines at South Kensington, 

 having been obtained from photographs, are chiefly confined to a 

 region of the spectrum not covered by the visual observations of 

 widened lines in sun-spot spectra. 



We can only point to the evidence acquired in the case of one 

 metal — iron — for which photographs of the enhanced lines in the green 

 and yellow parts of the spectrum have been obtained. 



This evidence quite justifies the above suggestion, for the enhanced 

 lines of iron can be seen revealing themselves as the number of 

 unknown lines increases. 



We are, therefore, quite justified in assuming a very great increase 

 of temperature at the sun-spot maximum when the "unknown" lines 

 appear alone. 



The curves of the "known" and "unknown" lines have been 

 obtained by determining for each quarter of a year the percentage 

 number of known and unknown lines and plotting these percentages 

 as ordinates and the time elements as abscissae. Instead of using the 

 mean curves for all the known elements involved, that for iron is 

 employed, as it is a good representative of "known" elements, and 

 has been best studied. When such curves have been drawn, they 

 cross each other at points where the percentage of unknown lines is 

 increasing, and that of the iron or known lines are diminishing, or vice 

 versa. 



We seem, therefore, to be brought into the presence of three well- 

 marked stages of solar temperature. 



When the curves of known and unknown lines cross each other — 

 that is, when the number of known and unknown lines is about equal — 

 we must assume a mean condition of solar temperature. When the 

 unknown lines reach their maximum, we have indicated to us a + pulse 

 or condition of temperature. When the known lines reach their 

 maxinmm, we have a — pulse or condition of temperature. 



The earliest discussion showed that, generall}" speaking, the unknown- 

 lines curve varied directly and the iron-lines curve varied inversely 

 with the spot-area curve. The curves now obtained for the whole 

 period of twentj^ years not only entirely indorse this conclusion, but 

 enable more minute comparisons to be drawn. 



