1874.] On the Sim-spot Period and the Rainfall. 469 



XXIII. On the Sun-spot Period and the Rainfall." 

 By J. A. BBOTJN, F.R.S. 



Having read with much interest Mr. Meldrum's communication to the 

 Royal Society on the apparent simultaneity of excess of rainfall and sun- 

 spot area*, I have waited some confirmation of his conclusions from a 

 more extensive induction. Mr. Hennessey's " Note " in the Proceedings 

 of the Society for April 1874 f induces me to offer the following views 

 and results to the Royal Society. 



It is well known that the amount of rainfall is a very variable quan- 

 tity in some countries and in certain positions, and that when there is 

 a year of drought in one part of the world, there is frequently an excess 

 of rain in another. Any investigation, then, which should be occupied 

 with the average fall of rain over the earth's surface must be long and 

 laborious, unless the variation to be dealt with is large and marked com- 

 pared with others which must be considered purely accidental relatively 

 to the sun's spots. In proof of this I may cite the rainfall at Mussoorie 

 given by Mr. Hennessey:}:, where, as far as the sun-spot area is known, 

 any result favourable to the connexion of the two phenomena depends 

 wholly on the rainfall for 1861, which is upwards of 50 inches in excess 

 of the mean. If this excess be not due to the great spot-area, then a 

 long series of years' observations might be requisite to make the positive 

 and negative errors destroy each other. 



It has been with the intention of determining what may be the effect 

 of a given change of sun-spot area, within a limited district, during a 

 period favourable to the connexion of the two phenomena, that the fol- 

 lowing discussions have been made. We can then say approximately 

 within what limits the excess and deficiency of rainfall lie for the years 

 of greatest and least spot-area, what amount of observations may be re- 

 quired to destroy accidental variations, and whether the result may en- 

 courage more extensive research. 



Mr. Meldrum finds a mean difference of 8-5 inches of rain between the 

 falls for the years of greatest and least spot-area ; but this result is 

 derived to some extent from short series of observations made in different 

 parts of the world, and gives no weight to the rainfall in other years than 

 those considered years of maximum or minimum sun-spots. 



Should there be any connexion betwixt the rainfall and spot-area, we 

 may always in the first instance represent it approximately by an equa- 

 tion of this form, 



AR=/AA, 

 where AR is the excess or deficiency of the rainfall from the mean, A A 



* Proceedings of the Royal Society, vol. xxi. p. 297. 

 f Ibid. vol. xxii. p. 286. 

 $ Hid. vol. xxii. p. 287. 

 Ibid, vol xxi. p :.l()~). 



