Observations on Rainfall. 97 



above Nantwich House. Wandle-road increased 6-57 per 

 cent, for every 100 feet of elevation above a gauge fixed at 

 Mitcham, having an elevation of 50 feet above sea level. 



In the summer months, not unfrequently more rain falls on 

 the low grounds than at a higher elevation. This is due to 

 thunder storms, which, as a rule, follow the valley lines. For 

 example, 3*82 inches of rain fell last July in Mitcham, as 

 against 2*8 inches at Wandle-road. It is only by a long series 

 of observations that local discrepancies disappear and true 

 averages are arrived at. 



The average rainfall in Croydon during the past twenty 

 years has been 27-25 inches, and as a rule it will be found that 

 if we deduct one-third from the average fall it will represent 

 the year of minimum fall, and if we add one-third to the 

 average it will represent the year of maximum fall. It should, 

 however, be observed that during the past 20 years we have 

 had no very dry years, or such as occurred in the decade 

 immediately preceding this period, so that the average of the 

 last twenty years exceeds the true average. In the decade 

 from 1861 to 1870 the average rainfall in Croydon was 25-83 

 inches, while in the decade from 1871 to 1880 the average 

 rainfall was 28-66 inches, showing that the last decade has 

 been considerably wetter than the one preceding it. 



It is to the increased quantity of rain falling upon the high 

 lands of the North Downs that we are and shall continue to 

 be dependent for our supply of water. Although rain increases 

 with the elevation of the ground, yet it is a singular fact that 

 observations made upon the collection of rainfall above the 

 surface of the ground at any particular point show that the 

 rainfall diminishes with altitude above the ground. So long 

 since as 1766, Dr. Heberden placed a rain gauge on the roof 

 of Westminster Abbey, which showed that less rain was 

 collected at that elevation than on the ground. A series of 

 observations on the diminution of rainfall above the ground 

 were made at York, and the results recorded in " The Tran- 

 sactions of the British Association." These observations have 

 been very fully reported upon by the Rainfall Committee of the 

 Association, of which committee Mr. G. J. Symons, F.R.S., 

 was the indefatigable secretary. The three years' observations 

 at York, in the years 1832-5, gave the following results : — 



Height of gauge Quantity Collected Ratio of 



above the ground. in 1832-5. decrease. 



Feet. Inches. Inches. 



o „ 2 ... ... 65*44 ••• ••• 100 



43 .. 8 52-17 79-7 



212 „ 10 38-97 59-6 



