140 THE SUN-SPOT PERIOD. 



to lind relations between sun spots and the appearance of Asiatic 

 cholera, famines, or commercial crises.)" 



So far as concerns climatology almost the only result certainly 

 established thus far, outside the question of temperature, is that 

 derived by Meldrum,^ director of the observatory on the island of 

 Mauritius, who found the mean annual rainfall of the earth slig-htly 

 greater in years of maxima than in those of minima of sun spots. The 

 causes of this variation in rainfall are not yet understood, but I may 

 incidentally remark that the views I have myself advanced in relation 

 to the aurora borealis^ may afford a simple explanation, for I have 

 shown that during years of maximum sun spot frequency the Hertzian 

 waves emanating from the sun induce the formation of the cathode 

 raj's of the aurora Ijorealis more abundantly than in years of minimum 

 sun spots. On the other hand, it is known that the propagation of 

 cathode rays is favorable to the condensation of vapor; thus it follows 

 that water vapor within the atmosphere, other conditions being equal, 

 would condense more abundantly in the form of I'ain during years of 

 maxima of sun spots, as found by Meldrum. 



The idea that the sun spots should have some intiuence on terrestrial 

 temperatures is very old. This view was advanced by Riccioli in 1651, 

 shortly after the discovery of sun spots, but so little was known of the 

 nature and magnitude of their influence down to recent times that in 

 187:^ Wolf was still able to write: "The relation which Herschel sup- 

 posed to exist between sun spots and the mean temperature of the 

 earth is still in question."'' It might seem at first sight strange that 

 while the connection between the sun-spot period and terrestrial mag- 

 netism and aurora was established almost as soon as the question began 

 to be investigated, the exact influence of sun spots on the temperature 

 of the earth, although long suspected, had not been determined even 

 as late as 1872. There are two kinds of causes contributing to this: 



First. While the eleven-^^ear variations of the Aurora Borealis and 

 of the phenomena of terrestrial magnetism are so great as to be readily 

 discernible in the amplitude of the phenomena in question, the effect 

 on temperature is only a fraction of a degree centigrade, as we shall 

 presently show, and thus of an order below that of the accidental and 

 local variations of temperature. 



Second. The researches published on this subject prior to 1872 gave 

 but uncertain and contradictor}^ results, because the authors did not 



«This last idea is not perhaps absurd, for it is certain that if the sun spots really 

 exercise a sensible influence upen terrestrial meteorology, they may indirectly influ- 

 ence harvests, as had been suggested by the great Herschel. But the price of grain 

 depends quite as much, or even more, on political and social circumstances as upon 

 meteorology. 



6 Monthly Notices of the Meteorological Society of Mauritius, December, 1878. 



(^Ch. Nordmann. Eecherches sur le role des ondes hertziennes en Astronomic 

 physiiiue. Rev. Gen. des Sciences, 1*"'' Avril 1902. 



'^llundbuch der Mathematik, Pliysik, GeodJisie nnd Astronomie, Vol. II, p. ;J02. 



