OF NOVEMBER, 1894, IN" HERTS. 



143 



to show their numerical relation to the area of the chief river- 

 basins. The area of Hertfordshire is 633 square miles, but it will 

 be convenient and sufBciently accurate for my purpose to consider 

 it to be 630. This area may be approximately divided between 

 the different river-basins as follows :— Great Ouse (Nos. 1-3), in 

 the noi'th of the county, 75 square miles; Thame (No. 4), in the 

 north-west, 16; Colne (Nos. 6-10), in the west (including a small 

 portion of the Brent), 210; and Lea (Nos. 11-18), in the east, 

 330. These areas are in the following ratios: 5, 1, 14, 22. The 

 number of rainfall stations in the four river-basins is as follows : 

 Ouse, 5 ; Thame, 1 ; Colne, 12 ; Lea, 17. There are therefore 

 required, in order to get an exact representation — one rainfall 

 station to every fifteen square miles — only two more stations in 

 the basin of the Colne, and five more in that of the Lea. If the 

 relative areas of the basins are considered, it will be seen that 

 a result sufficiently accurate for any practical purpose would be 

 attained if there were two more rainfall stations in the basin 

 of the Lea. 



The mean rainfall in each of the river-basins for the period 

 under discussion is given in the following table : — 



Table II. — Rainfall in the RivER-BAsrisrs, 23rd October to 

 14th November, 1894. 



From this table we may get a very near approximation to the 

 actual mean rainfall in the county by multiplying the rainfall in 

 each river-basin by its area, adding together the products, and 

 dividing by the area of the county. The result of this operation 

 for the four days 11th to 14th Nov. gives a mean for the county 

 of 2*94 inches, and for the twenty -three days 23rd Oct. to 14th 

 Nov., a mean of 6-34 inches. 



An inch of rain in depth gives a weight of water of 64,636 tons 

 per square mile (= nearly 101 tons per acre). Therefore, in the 

 four days 11th to 14th Nov. there fell within the limits of the 

 coimty in the river-basin of the Ouse nearly 15 million tons of 

 water ; in that of the Thame nearly 3^ million tons ; in that of 

 the Colne over 48 million tons ; and in that of the Lea nearly 

 53|^ million tons ; giving a total for the whole of Hertfordshire 

 of nearly 120 million tons. If the mean rainfall as shown in the 

 first table were taken, the result would be 124^ million tons, 

 which is greater than the quantity arrived at by taking the results 



