and Subterranean Water Storage. 



71 



a careful and minute observation of averages to obtain trust- 

 worthy and decisive conclusions as to progressive change. I 

 will endeavour to put the case as fairly and in as varied a manner 

 as possible, to enable the reader to judge of the value of the 

 conclusion I feel bound to come to. 



Estimating the mean of the period at 25*25 inches, let us first 

 regard as ordinary years all those in which the total rainfall was 

 within 5 inches on either side of this 

 mean. Years when the rainfall was not 

 within these limits may be taken as maxi- 

 mum or minimum years. Within the 

 first twenty-five years of the period under 

 consideration there were eighteen within 

 the assumed limits of ordinary years, 

 nine during which the fall was more, and 

 nine during which it was less than the 

 mean. There were five years of maxima 

 and two of minima. During this period 

 the mean fall was 26*69, or 1*44 inches 

 above the general mean of the fifty years. 

 Within the succeeding period of twenty- 

 five years the mean fall was 23'75 inches, 

 or nearly 3 inches less than in the first 

 twenty-five. Fifteen of the years may be 

 regarded as ordinary, but in these the 

 rainfall was above the mean only four times, and below it eleven 

 times. Only three times during the period did the rainfall 

 amount to a maximum, or exceed 30'25 inches, but as many as 

 seven times it must be regarded as minimum, the fall not 

 amounting to 20"25 inches. 



If we next divide the whole into periods of ten years, we find 

 a result which is best expressed in the above Diagram (HI.) : — 



But it affords a much fairer estimate of the average if we 

 divide the fifty years into nine decennial periods, commencing at 

 intervals of five years. We then obtain the following result : — 



