144 



Transaction a. 



A study of this graph shows that the rainfalls from January to May 

 were not enough to balance the water drawn off from the reservoir sup- 

 plying the well ; that the rains in June and July were sufficient to 

 replenish it ; that the almost total absence of rain in August was accom- 

 panied by a still further rise in the level of the water, possibly indicating 

 that the heavy rainfall of the previous months was still percolating to 

 the reservoir ; that the rains of September and October were accom- 

 panied by a slight rise, although they were almost exactly equal to 

 the rainfall of January and February, which were accompanied by a 

 fall in the level of the well ; and that falls took place in November and 

 December. 



These last facts, and also, in part, the rise in August, are probably to 

 be explained by the great amount of evaporation in November, December, 

 January, and February, and its smaller amount in August, September, 

 and October ; that the evaporation might have an effect on the 

 fluctuation of the well did not suggest itself to me early enough for me 

 to install evaporation-gauges. It seems probable that the evaporation in 

 the summer months would exceed the rainfall, and thus assist the lower- 

 ing of the static level of the well ; while in August, September, and October 

 the evaporation would be very slight, and thus all the rainfall would be 

 available for replenishment of the reservoir. The following table by 

 Greaves, taken from Warrington's r ' Physical Properties of the Soil," 

 p. 108, is instructive : — 



Evaporation from a Water Surface near London (Average of Fourteen 



Years). 



Total rain, 25-73 in. ; total evaporation, 20-66 in. 



I think it probable that a graph of the monthly rainfall minus 

 evaporation would approximate the graph of the static level of the 

 well, and I regret that the importance of the evaporation did not occur 

 to me earlier. 



This fluctuation of over 2 ft. during the course of the year is very 

 much greater than that of 10 in. recorded by Captain Hutton, but is much 

 less than one mentioned by Mr. Home, of Leeston, who says that he had 

 there a well which in a dry season was 3 ft. 6 in. below ground-level, and 

 in a very wet season rose to 14 ft. above ground-level. A gravel-pit at 

 Springston about 10 ft. deep is nearly always dry in February, and fre- 

 quently is full to overflowing in August. 



