and Suhtcrrancan Water Storage. 



We must now return to Diagram II., wliicli is very im- 

 portant and instructive. It shows, as I have already pointed 

 out, that there is, on the whole, a ten- 

 dency to alternate maxima and minima Diagram _\ .—J/ean Rainfall 

 -.•' • r 11 /? i • • aurinn J<it:e Ferioas of Maq- 



m the ramtalJ, first in successive years, net ic Disturbance, from isi 3 



and next in cycles. These cycles are to 1858 induske. 



variable, ranging from six to twelve ^^ ^ ^ ^ 



years. But it is clear that during the 2 2 2 2 



last half century the vears and cvcles 



of heavy rainfall have been growing 



fewer and smaller, and those of minima 



more frequent, while all the averages are 



lower. Thus there may be said to be 



sixteen periods of different duration, 



measuring from minimum to minimum, 



during the fifty years. The total in the 



years of heaviest rainfall lately has never 



exceeded the totals of half a century ago. 



The total rainfall during the dry years 



lately has been very much below the 



smallest rainfalls of former times, and 



has not been at all compensated bv the increase due to the wet 



years 1852 and 1860, 



Exhibited to the eye, the reduction of the rainfall is recognised 

 in Diagram II. rather by the general tendency of the curve to 

 decline towards the right, than bv any regular difference or group 

 of differences. There is a certain regularity even in the jumps 

 that are to be observed from one year to another, and the rarity 

 of the occasions on which the mean rainfall is the actual fall of 

 the year serves to illustrate the nature of the law governing the 

 distribution. 



There Avould seem to be nothing gained b\' assuming other 

 periods for comparison. I have, however, taken the averages for 

 three and seven years, but the results are of the same general 

 nature, and hardly deserve recording in separate diagrams. The 

 triennial period is too shor.t to give valualDle results. The sep- 

 tennial shows as follows : — 



Means of Rainfall at Greenwich during succeeding periods of Seven and 

 Fourteen Years, from 1815 to 1863 inclusive. 



of 14 Years. 

 . ■ Years. Inches. Inches. 



1815—1821, Mean rainfall 28-7 . 



i 28-3 

 1822—1828 „ 27-9 



t 20-1 

 1829—1835 „ 24-3 i 



