1876.] Rainfall in connexion with Sun-spots. 381 



cent, are favourable. Of the fifteen years in which the depths were 

 greatest 60 per cent, were within two years of maxima sun-spot years, 

 and 61*5 per cent, of the years in which the depths were least were 

 within two years of minima sun-spot years. The latter percentages, 

 although favourable, are less so than those for the rainfall, owing to 

 obvious causes. 



(8.) All the above results have been obtained by strictly adhering to 

 the adopted years of maximum and minimum sun-spot, and by taking 

 the absolutely wettest and driest years, however little the rainfall in 

 them might differ from that of other wet and dry years in their neigh- 

 bourhood. It is clear that this was subjecting the hypothesis to a severe 

 test ; for there is some doubt as to the positions of the years of maxi- 

 mum and minimum sun-spot, especially in the earlier periods of observa- 

 tion ; and, assuming a connexion between rainfall and sun-spots, it might 

 well happen that the absolutely wettest and driest years would not 

 always, or even often, fall within two years of the epochs of maximum 

 and minimum sun-spot. The results, therefore, may be considered 

 highly favourable ; but they become still more so if, in a few cases, the 

 epochs of maxima and minima be altered to the extent of one year, and 

 if, for the absolutely wettest and driest years, almost equally wet and 

 dry years be occasionally substituted. A slight alteration of this kind 

 would, for instance, raise the percentage of favourable maxima and 

 minima periods on the continent of Europe from. 75 to 100, and make 

 all the wettest and driest years favourable. Similar results would be 

 obtained for the other districts of observation. 



(9.) The ratios of the maximum and minimum rainfall to the mean 

 rainfall (especially the latter ratio) are nearly the same for each part of 

 the globe ; and this points to the sun as the main agent in producing 

 rainfall variation. The ratios of the maximum and minimum river-depths 

 to the mean depth are almost the same as those for the rainfall. 



(10.) The years of maximum and minimum sun-spot are characterized 

 by great rainfall and river oscillations, a circumstance which also points 

 to the sun as the disturbing cause *. 



(11.) As the mean duration of the sun-spot period is 11*1 years, 

 while the interval from the minimum to the maximum is about 3*7 years, 

 and from the maximum to the minimum about 7*4 years*, the durations 

 of the rainfall and river cycles, and the intervals between their epochs of 

 minima and maxima and maxima and minima, should approach those for 

 the sun-spot cycle, supposing that the three cycles are connected by a 

 common cause. Now this is found to be generally the case. The 

 intervals between the consecutive wettest and between the consecutive 

 driest years for Great Britain are 11-3 and 10*8 years respectively, 

 while the interval from the driest to the wettest years is 3*9, and from 

 the wettest to the driest years 6*8 years. For the continent of Europe 

 * The greatest river-fluctuations occur in minima years. 



