METEOROLOGY OF THE SUN AND EARTH. 75 

 METEOROLOGY OF THE SUN AND EAETH.' 



By Peof. BALFOUK STEWART, F. E. S. 



SINCE the last meeting of the British Association, Science has had 

 to mourn the loss of one of its pioneers, in the death of the vet- 

 eran astronomer, Schwabe, of Dessau, at a good old age, not before he 

 had faithfully and honorably finished his work. In truth, this work 

 was of such a nature that the worker could not be expected long to 

 survive its completion. 



It is now nearly fifty years since he first began to produce daily 

 sketches of the spots that appeared upon the sun's surface. Every 

 day on which the sun was visible (and such days are more frequent in 

 Germany than in this country), with hardly any intermission for 

 forty years, this laborious and venerable observer made his sketch of 

 the solar disk. At length this unexampled perseverance met with its 

 reward in the discovery of the periodicity of sun-spots, a phenomenon 

 which very speedily attracted the attention of the scientific world. 



It is not easy to overrate the importance of the step gained when 

 a periodicity was found to rule tliese solar outbreaks. A priori we 

 should not have expected such a phenomenon. If the old astronomers 

 were perplexed by the discovery of sun-spots, their successors must 

 have been equally perplexed when they ascertained their periodicity. 

 For wliile all are ready to acknowledge periodicity as one of the natural 

 conditions of terrestrial phenomena, yet every one is inclined to ask 

 what there can be to cause it in the behavior of the sun himself. Mani- 

 festly it can only have two possible causes. It must either be the 

 outcome of some strangely hidden jDcriodical cause residing in the sun 

 himself, or must be produced by external bodies, siich as planets, act- 

 ing somehow in their varied positions on the atmosphei'e of the sun. 

 But whether the cause be an internal or external one, in either case 

 we are completely ignorant of its nature. 



We can easily enough imagine a cause operating from the sun him- 

 self and his relations with a surrounding medium to jjroduce great 

 disturbances on his surface, but we cannot easily imagine why dis- 

 turbances so caused should have a periodicity. On the other hand we 

 can easily enough attach periodicity to any efiect caused by the plan- 

 ets, but we cannot well see why bodies comparatively so insignificant 

 should contribute to such very violent outbreaks as we now know sun- 

 spots to be. 



If we look within we are at a loss to account for the periodicity of 

 solar disturbances, and if we look without we are equally at a loss to 

 account for their magnitude. But, since that within the sun is hidden 



' Opening Address in Section A, at the Bristol Meeting of the British Association. 



