ON D 5A 5A A 4 x 7~7~1 1 = 2 , 1 5 6~4~ 2 X 13 X 34A --A 11 X 7 X 99 X 11 X 49-4,648 Hemet = L(l +~+~a + z~ Y Y2 Y 3 +W,+. Y4 . . where y = -. y E = 4/28 .) . . . . . (7) 28 P that the hydrostatic uplift ranges from a maximum at the upstream face to zero at the B. Allowance for hydrostatic pressure, as for a pervious dam. If it be assumed downstream face, the effect is almost the same as taking the weight of the dam in water. This assumption is correct for a purely triangular dam and is a reasonable approximation in this case. Therefore it is necessary only to write ( 8 -1) instead of 8 in formula (7). which gives C = & ( l + Y + Y + -+ -+ 1/2(8 -1) 7 49.5 2 431 y3 4,648 y4. . .) DiscussionThe Authors introduced the Paper with aid of a film. Mr C. A. Risbridger observed that the present very valuable Paper, dealing with the development of the Claerwen gathering ground, when added to the classic Paper presented in 1912 by the Mansergh brothers,l dealing with the development of the Elan gathering ground, would almost complete the thrilling story of the development of those gathering grounds. He said " almost " because the story might be rounded off in due course by information based upon the very careful records of rainfall, run-off, losses, and so on, which had been maintained on those gathering grounds during the past 50 years. Although the present Paper dealt mainly with constructional matters, it would probably not be out of place to make very brief reference to the total storage which now existed in terms of the yield expected to be obtainable from the gathering ground. The number of days was in fact slightly more than 200, on the basis of a yield of 104 million gallons per day. That figure of 200 might be compared with a figure of 180, which Mansergh had intended to provide in his original project, and with 150 days, which would be obtained by applying the old Hawksley formula of y = 500 divided by the cube root of the average annual yield of the three driest consecutive years. It was a great tribute to Mansergh that with the information provided by only one long-standing rain-gauge on the gathering ground 60 years ago, supplemented by two or three very short-term gauges, he had See footnote 1, p. 249. Downloaded by [] on [12/09/16].