24 PROCEEDINGS COTTESWOLD CLUB igio 



designed for carrying 75 million gallons a day, but on the syphon-sections 

 only two pipes out of the six provided for have been as yet inserted. The 

 water is of the usual character of moorland waters. It was soon found that 

 a growth appeared inside the iron conveying-pipes, so filter-beds were pro- 

 vided in order that the water should pass through material composed of 

 crushed local rock. The rate of filtration is about 16 inches per hour. It 

 being also found that the water possessed some slight erosive power when 

 allowed to stand in the lead service pipes overnight, i^ grains of finely- 

 powdered chalk to the gallon are now introduced to the filters, by means of 

 an overhead electric railway. This gives the necessary hardening to the water. 



The filter beds were seen on the right just before arriving at the Caban 

 Coch Dam, the model village being on the left down in the valley. 



The Caban Coch Dam is 122 feet in height, 122 feet 6 inches thick 

 at the base, and 566 feet long (measured along its summit line), and can im- 

 pound 8000 million gallons. The tongue of water to the south is that which 

 occupies the lower reaches of the Claerwen Valley, and now covers the site of 

 the " Nantgwillt " — the house occupied by the poet Shelley in 1812, after his 

 marriage with Harriet Westbrook. 



The elegant bridge that crosses the Caban Coch Reservoir, the Careg-ddu 

 Viaduct as it is called, marks a submerged dam. The reason why this 

 submerged dam was constructed was because the point of delivery of the 

 aqueduct at the Birmingham end is 600 feet above O.D., and the compensa- 

 tion water at the Caban Dam is discharged at a level of 720 feet above O.D. 

 This was not high enough to ensure a proper fall, so this submerged dam was 

 made, which rendered it possible to have the bottom level of the aqueduct at 

 770 feet. The top of the dam is 40 feet below the usual level of the water. 

 This top-layer of water (4585 million gallons) is available for compensation- 

 water as well as supply. The submerged dam divides the lower layer of 

 water in the Caban Reservoir into two portions : that below the submerged 

 dam and between it and the Caban Coch Dam being available for compensa- 

 tion-water only ; and that above must always be kept up to its full amount 

 in order to ensure correct charging of the aqueduct. The tower that marks 

 the commencement of the aqueduct is called the Foel Valve Tower. The site 

 of "Cwm Elan," the house where Shelley stayed in July, 181 1, was pointed 

 out on the right bank of the Elan, opposite to and a little up-stream of the 

 Foel Valve Tower, but like that of " Nantgwillt," it is now some 40 feet below 

 the top-water-level of the reservoir. 



Driving along the well-made road, the scenery was greatly admired, the 

 bare Craig Dol-faenog being particularly grand. Then the Pen-y-gareg Dam 

 came into view, the water passing over its summit being wrought into foam 

 by the projecting stone-blocks, and combining the utilitarian with the 

 picturesque, for this battling of the waters on the projecting stones must 

 aerate them well. 



The Pen-y-gareg Dam is 123 feet high, 115 feet 4 inches thick at the 

 base, and has a weir-length of 417 feet 6 inches. The reservoir has a total 

 capacity of 1320 million gallons, and a top-water-level of 124 acres. 



After the little path from the foot of the cascade to one of the lateral 

 towers had been climbed, a fine view was obtained of the artificial lake. 



The next halt was at the Craig Coch Dam, which is 120 feet high, 104 

 feet thick at the base, and has a weir-length of 390 feet,_and impounds some 

 2000 million gallons of water. It is traversed by a viaduct. 



The scenery of the country around this uppermost (Craig Coch) reservoir 

 is very fine, being wild and desolate. Peat-capped tongues of land project 

 into it, and the infalling streams are often dark with the particles of moorland 

 matter. 



