ON THE RAINFALL OF THE BRITISH ISLES. 101 



Of course it was not to be expected that the resxilts of a single year should 

 agree exactly with the mean of two other years, still less when the size of 

 gauge used was different, and the locality so opposite as the inland district 

 of Calne and the rock-bound Yorkshire coast. We therefore look upon it as 

 satisfactory that in only four months out of eleven do the ratios at Calne and 

 Hawsker differ more than 3 per cent. In April, June, and IS^ovemher they 

 are identical. The Calne results are thus strongly confirmed ; and it may be 

 considered as certain that pit-gauges always exceed those at one foot, 

 although the precise amount of excess remains to be determined. 



In our last Eeport we expressed the hope that we should this year be able 

 to state the residt of the discussion of all the rainfall registers which were 

 absolutely continuous from January 1, 1860, to December 31, 18G9. We 

 have the pleasure of doing so in two respects, viz. (1) with reference to their 

 bearing on the question of the existence or otherwise of secular variation of 

 rainfall in the British Isles, and (2) as data indicative of the distribution of 

 rain over the country. 



The secular variation of rainfall, or the relative dryness and wetness of 

 different years and groups of years, is one of the most important and difficult 

 branches of rainfall work. It has been treated in our Reports for 1865, and 

 very fully in that lor 1866. In the latter we gave the calculations in detail, 

 from which the values shown on the accompanying diagram were obtained. 

 Referring to that Report for full explanation, we have onljr now t-o mention 

 that the subsequent years 1866 to 1869 have been computed in the same 

 manner and added to the diagram (fig. 1). We may also remark that various 

 observations collected since its publication have confirmed the general accuracy 

 of the curve quite as much as could have been anticipated. On the present 

 occasion we do not intend to discuss the relative rainfall of different years, but 

 the relation of the fall during the ten years 1860-69 to previous decades. 

 For this purpose we have grouped the yearly values in decennial periods, 

 similar to those adopted in our 1867 Report, whence we obtain the following 

 result : — 



Table II. — Ratio of Rainfall in each ten years since 1730 to the Mean of 



sixty Years, 1810-69. 



Having previously pointed out the peculiarities of the earlier portion of 

 the curve, it is only necessary on the present occasion to caU attention to the 

 last forty years, whence it will be seen that, according to this mode of inves- 

 tigation (which is principally based on Enghsh returns), three out of the four 

 decades had a rainfall nearly identical, and the other (1850-59) considerably 

 below them, the deficiency being nearly 7 per cent. 



This result is based on a combination of records, as fully explained in our 

 1866 Report. We proceed to examine how far it is corroborated by individual 

 stations, but are at once confronted by the paucity of stations of which per- 

 fectly continuous records for even half a century exist. We therefore con- 

 fine ourselves to the forty years, from 1830 to 1869, for which period we 



