682 Mr. James Manpergh [March 18, 



millions of gallons per day : the rod fishers demanded forty millions. 

 They were assisted by the Wye Fishery Board, and, in the back- 

 ground, by the officials of the Board of Trade who administer the 

 Salmon Fisheries Acts ; and ultimately a compromise was come to by 

 which the quantity was fixed at 27 million gallons a day. Since the 

 works have been in course of construction we have had the oppor- 

 tunity of measuring the flow of the river at the spot where the 

 27 millions will have to be discharged, and have found that in very 

 dry weather it falls to something under -ij millions, so that the 

 quantity passing down will, so soon as anv water is taken to 

 Birmingham, be increased at such point six-fold. Of course the 

 capability of so benefiting the river is due to the storing of flood 

 waters in the reservoirs to be constructed. 



Another incidental benefit arising out of this impounding will be 

 the reduction in the volume and violence of destructive floods in the 

 river below. The amount of compensation water in these cases is a 

 fairly well recognised proportion of the water collectable from the 

 watershed area, tljat is to say, where the water is used for trade or 

 manufacturing purposes the proportion is one-third, and where there 

 are only ordinary riparian — including fishing rights — it is about one- 

 fourth. The qujintity of water collectable is as-ccrtained from the area 

 of the gatlicring ground and the rainfall upon it less the evaporation 

 and the volume of water inevitably overflowing from the reservoirs in 

 times of flood. Thus the area we are here dealing with, was deter- 

 mined by accurately marking upon the plans the parting lines or 

 watershed boundaries after careful examination, and in some cases 

 instrumental levelling upon the ground. By measurement from the 

 plans the area was found to be 45,562 acres, the first factor in the 

 calculation, 'i he area is shown by a photograph of a model of the 

 watershed (Diagram No. 2). 



The model was made on a scale of 6 inches to a mile, that is, 

 880 feet to an inch horizontal, and 300 feet vertical, and upon it the 

 reservoirs are represented as made and filled with water. The rain- 

 fall might have been a much more difficult thing to determine than 

 the area, hut that very fortunately the Lord of the Manor, Mr. Kobert 

 Lewis Lloydj and his fither before him had kept a rain-gauge regu- 

 larly from the year 1871 onwards, at the family mansion of Nant- 

 gwillt, in the lower part of the Elan Valley, and at a spot on the 

 watershed area to be appropriated. 



So soon as it seemed jDrobable tliat the matter would be proceeded 

 with, several other rain-gauges were erected at several pomts upon 

 the shed, with the assistitnce of Mr. Symons, the last, and a most 

 worthy gold medallist of the Society of Arts. Then, by a comparison 

 of these with the long-term gauge at Nant-gwillt and others in the 

 surrounding country, it was decided that the mean annual fall of a 

 long ser'es of years upon the watershed might be taken at about 

 C8 inches, and the average of three consecutive dry years at 55 inches 

 — this latter being the figure always used in these estimations — as 



