Progress and Problems of Solar Physics. 137 



Ample confirmation of this periodicity has been obtained 

 since that time, but irregular variat:ions have been 

 noticed and the period from maximum to maximum has 

 varied from seven and one-half to sixteen years; during 

 the recent cycle fully twelve years intervened between 

 minima; and the maximum, which ought to nearly reach 

 the summit of the curve during the year 1904, seems yet 

 some distance in the future, as only comparatively small 

 and few spots have darkened the disc of late. Aurorae 

 and terrestrial magnetism seem to obey the same law of 

 the solar periodicity and are subject to like variations. 

 They are so intimately connected with solar outbursts 

 that often individual large spots are coincident with 

 world-wide auroral, magnetic and electrical manifesta- 

 tions. It is probable that this periodicity is due to the 

 conditions within the sun itself, perhaps owing to the 

 cooling process going on beneath the photosphere; the 

 present activity in the study of variable stars may 

 throw some light on this difficult problem. 



Another important question yet remaining unsolved 

 is the constancy of the solar radiation, but investigations 

 now in progress may solve the difficulty. Regarding the 

 effective temperature of the sun. Young in his latest 

 views on the constitution of the sun places it as not far 

 from 6,000 degrees.^"^ 



Many interesting and ingenious theories have been 

 advanced to account for the maintenance of the sun's 

 radiation: the Helmholtz contraction theory, whereby 

 the sun's globe in cooling contracts, and heat being 

 mechanically evolved as a consequence, was promul- 

 gated in 1853, and while probably true may not be the 

 only source of energy. The newly discovered element 

 radium and other radio-active substances may throw 

 much light on this problem. 



A curious hypothesis of solar physics was published 

 by Schmidt, of Stuttgart, in 1891, ^p^ who argued from 

 mathematical and other reasonings that the sun was 

 entirely gaseous and the photosphere and appendages 

 merely optical phenomena due to circular refraction. 

 Enough has been said to indicate some of the great 

 problems which still confront astronomers and physic- 



