148 THE RAINFALL OF GUERNSEY. 



form a dry group. These years were preceded by a group the 

 mean of which was below the average and it has also been 

 followed by dry years. 



The records show us that there have been groupings of 

 this type all through the 76 years, but the important thing we 

 have to notice is that the wet and dry groups follow each 

 other and with a certain amount of regularity. The last, with 

 an average of over 37 in., was one of 14 years' duration, all the 

 years of which were wet years. That was followed by a long 

 set of groups with some wet years, but with means below the 

 average. Between the wet groups are the dry groups occu- 

 pying a series covering 25 years. Another thing we notice is 

 that in dry groups there are wet years and in wet groups there 

 are dry years, but these are not frequent enough to alter the 

 character of the group. 



There is a lesson to be learned, indeed two. One is that a 

 long period is required to enable us to acquire a knowledge of 

 the sequence of these groups. The second lesson we have 

 learned is that the oscillations are of such a nature that we can 

 be sure that a dry group will certainly follow a wet one. 



There are a few practical considerations which claim our 

 attention. The only one I can spare time for to-night is the 

 effect of the dry and wet groups of years on the water supply 

 of the island. I take it that the end and chief object of the 

 Society to obtain an exact knowledge of these subjects is that 

 such knowledge may ultimately be of service to the Island, and 

 that therefore any application that I can make on the subject 

 of the water supply derived from the long period of rainfall 

 observations are suitable for discussion. The grouping of 

 rainfall into definite wet and dry groups may be made to yield 

 a very strong argument for the adoption of some method of 

 securing the excess of the wet groups to tide the island over 

 the dry. 



The last dry group was a long one and the effect was so 

 serious, the deficient rainfall being co-incident with an increase 

 of greenhouse culture, that there was a very serious shortage. 

 That shortage was the cause of a movement to obtain a better 

 supply, and the States took a great amount of trouble in 

 obtaining detailed statistics of rainfall and its influence on 

 streams and wells. 



It is not necessary to remind the meeting of the fact that 

 decisions of great importance were arrived at, and had the dry 

 years continued the plans made would no doubt have been 

 carried out ; but as I have shown the dry group of years was 

 followed by a wet one, the extreme pressure was relieved and 



