SOLAR CHANGES OF TEMPERATURE. 177 



The "widened line" curves are quite different from those furnished 

 by the sun spots. Ascents and descents are both equally sharp, 

 chanafes are sudden, and the curves are relative!}^ flat at top and bot- 

 tom. The crossings are sharplv marked. 



During the period since 1879 three such crossings have occurred, 

 indic-ating the presence of mean solar temperature conditions, in the 

 years 1881, 1886-87,' and 18!»2. It was expected that another crossing 

 with the known lines on the rise would haA^e occurred in 1897, indicat- 

 ing thereby the arrival of another mean condition of solar tempera- 

 ture, but as yet no such crossing has taken place. * * * 



It becomes, therefore, of the tirst importance to correlate the times 

 of mean solar temperature, and of the + and — heat pulses, with the 

 solar weather cycle, in order to arrive at the temperature history of 

 the sun during the period which now concerns us. * * * 



CONNECTION OF THE SPOTS WITH PROMINENCES. 



In 1869, when a sun-spot maximum was approaching, the promi- 

 nences wore classiiied b}' one of us into eruptive and nebulous; the 

 former showing many metallic lines, the latter the hydrogen and 

 helium lines chiefly. * * * 



The eruptive prominences, unlike the nebulous ones, were not 

 obserA'cd in all heliographic latitudes; but, according to the extended 

 obser^-ations of Tacchini and Ricco, had their maxima in the same lati- 

 tude as the spots. * * * This is corroborated by what Professor 

 Respighi many years ago stated : "In correspondence Avith the maxi- 

 mum of spots, not only does the numl)er of the large protuberances 

 increase, but more than this — their distribution over the solar surface 

 is radically moditied.'" * * * 



In photographs near sun-spot maximum the concentration of the 

 prominences in zones parallel to the Equator is perfecth^ obvious at a 

 glance. Eruptive or metallic prominences are thus seen to cover a 

 much larger area than the spots, so that we have the maximum of solar 

 activity indicated, not only by the increased absorption phenomena 

 indicated by the greater number of the spots, Init by the much greater 

 radiation i)honomena of the metallic prominences; and there seems 

 little dou])t that in the future the measure of the change in the amount 

 of solar energy will be determined by the amount and locus of the 

 prominence area. 



Spots are, therefore, indications of excess of heat, and not of its 

 defect, as was suggested when the term " screen" was used for them. 

 We know now that the spots at maximum are really full of highly 



^According to the observations the mean was reached in December, 1886, (jr Jan- 

 uary, 1887. 



SM 1900 12 



