134 ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1915. 



STUDY OF THE SUN. 



Is the periodic increase in the number of sun spots definitely con- 

 nected with the flux of heat which we receive from the sun? Tlie 

 question has been answered in various senses, and it must still be con- 

 sidered as under litigation. The discordance of the statistics, when 

 they are not coordinated in point of time, may result from a general 

 variation in the transparency of the earth's atmosphere. For in- 

 stance, the greater or less diffusion of volcanic dust suffices to explain 

 this discordance. And it seems quite certain that the eruption of 

 Mount Katmai (Alaska) in 1912, as well as that of Krakatoa in 1883, 

 have had effects of this nature. At any rate the passage of this 

 atmospheric disturbance does not occur simultaneously in widely 

 separated countries and the parallelism of the solar-constant values 

 found by the methods of Dr. Abbot in California and in Algeria, 

 Africa, prove that very perceptible variations can be imputed to the 

 sun. These variations up to the present appear rather irregular than 

 periodic. 



Fabry and Buisson have found that the solar spectrum is cut off at 

 the violet end by an absorption band due to ozone. The presence of 

 a layer of ozone, formed in the upper part of our atmosphere by the 

 action of the ultra-violet light of the sun, is not improbable. It 

 would in that region somewhat alter the laws of absorption and 

 (slightly) alter the value of the solar constant. 



The micrometric examination of the numerous plates taken at 

 the Observatory of Zo-Se (China) under the direction of P. Cheva- 

 lier, shows that the sun underwent, from 1905 to 1909, a measurable 

 and somewhat variable elongation along the polar diameter. It is 

 not the first time that such a change has been suspected, but it is as- 

 serted now, it seems to us, with an imposing train of evidence. The 

 mean photographic diameter surpasses by 0.6" that which is gener- 

 ally admitted on the authority of Auwers. An indication pointing in 

 the same direction, results from the discussion by Simonin of the 

 plates of the eclipse of April 17, 1912. 



The documents resulting from the last solar eclipse still furnish 

 material for interesting publications. Father Cortie gives the de- 

 scription of several limited bundles of rays, each one issuing from a 

 spotted region of the sun and showing marked effects upon terrestrial 

 magnetism. In the American photographs of the flash spectrum 

 taken at Daroca in 1905, Mitchell found the whole counterpart of the 

 Fraunhofer spectrum. The only differences occur in the relative in- 

 tensities of the lines. Neither Mitchell nor Evershed are disposed 

 to consider the presence of radium as established in the sun's cliromo- 

 sphere. 



The powerful spectroscopes continue to give numerous results rela- 

 tive to the velocities which rule at the various levels in the sun. But 



