25: 



NA TURE 



\Jan. 14, 1886 



radiation at times of maximum than at times of minimum 

 sun spot frequency ? 



In reply to tliis question, we must acknowledge that of 

 direct evidence derived from the actinometer we have 

 hardly any. Certain preliminary observations made by 

 Mr. Hennessey at the headquarters of the Trigonometrical 

 Survey in India may perhaps induce us to imagine 

 that the sun may be most powerful on occasions of 

 maximum sun spot frequency, but this is far from 

 conclusive. We have, however, very strong indirect 

 evidence in favour of this conclusion. This is derived 

 partly from the facts of terrestrial magnetism and 

 partly from those of terrestrial meteorology, that from the 

 former being the stronger of the two. It is well known 

 that the sun produces changes in the magnetism, of the 

 earth. It gives rise, for instance, to the solar-diurnal 

 variation of the needle which is a systematic change ; and 

 it likewise produces magnetic storms, these words being 



employed to denote changes of a peculiarly abrupt and 

 irregular kind. Now undoubtedly these both imply an 

 energetic action of some kind on the part of the sun, and 

 we have strong grounds for supposing that this energetic 

 action is connected with the radiating power of our 

 luminary. But both of these solar actions upon the 

 magnetism of the earth are decidedly stronger in times of 

 maximum than in times of minimum sun spot frequency. 



Such electrical phenomena as the aurora borealis and 

 the currents which take place in the crust of the earth 

 are likewise peculiarly developed on the same occasions. 



In Fig. 21 we have a diagram representing the con- 

 nection between sun spot maxima, the maxima of declina- 

 tion range, and the frequency of the aurora borealis. 



When we come to meteorology, the evidence before us 

 is not so conclusive, although here also what we have 

 tends, I think, in the same direction. Mr. Meldrum has 

 shown that there are most cyclones in the Indian Ocean 



Solar Spots, IVIa6netic Declination. and Auroral DisplaYSi 



about times of maximum sun spots, and M. Poey has 

 proved the same thing with respect to West Indian 

 hurricanes. On the whole, too, and in the majority of 

 stations, the rainfall is greatest on the same occasions. 



Again, let us take the barometric pressure of the air. 

 Here a little reflection will convince us that the peculiar 

 variations in the distribution of such pressure are really 

 caused by the sun. For instajice, we know that Western 

 Siberia during the winter season has a pressure decidedly 

 above the average, and we should therefore imagine that 

 in years when the sun is peculiarly powerful the winter 

 pressure in Siberia would prove to be particularly high. 

 Now this is just the state of things which Mr. Blanford 

 has found to correspond with years of maximum sun spot 

 frequency, and thus the evidence is in favour of these 

 being also years of maximum solar power. The Indian 

 meteorologists have derived similar conclusions from the 



observations made in India. In fine, we may, with the 

 balance of probability in our favour, adopt the conclusion 

 deduced by Mr. Baxendell at a comparatively early period, 

 who found that the forces which produce the movements 

 of the atmosphere were apparently more energetic in years 

 near maximum than in years near minimum sun spot 

 frequency. 



Let me now proceed to indicate the nature of the in- 

 formation which the spectroscope gives us regarding the 

 planets and comets of our system. Since the moon and 

 the various planets are illuminated by the sun, their 

 spectra will necessarily be built upon that of the sun. 

 As a matter of fact, this is found to be the case ; but we 

 ought to bear in mind that the solar rays that reach us 

 from a planet must have penetrated some distance into 

 the atmosphere of that planet. In doing so they will 

 most probably have suffered spectral absorption, the 



