1911] on Water Supply. 105 



running- down hill on the one side and up hill on the other ; for 

 Lyons, in France, was supplied long ago by means of lead pipes from 

 12 to I'S inches in diameter, 9 miles in length, and worked under a 

 head of 200 feet. It is true that the favourite method of engineers 

 before the nineteenth century, when cast-iron pipes came into use, 

 was to cross valleys by bridge aqueducts, and this, apparently on the 

 ground that the materials of their pipes (either trunks of elm-trees 

 hollowed out from which our word " trunk main " has survived to 

 us, or lead) did not lend themselves to the conveyance of large 

 volumes of water as the ordinary aqueduct did. The favourite 

 system of supply in this country to-day is undoubtedly by means of a 

 gravitation system, either from natural lakes like Loch Katrine or 

 Thirlmere, or from artificial lakes — or, as they are called, " impounding 

 reservoirs," These reservoirs, by means of a dam formerly formed of 

 earth, with a core of impervious puddled clay, but now more frequently 

 of masonry, catch and impound the water which falls upon the gather- 

 ing ground A. Of course it is desirable to avoid a gathering ground 

 which consists of cultivated land or upon which there is any consider- 

 able population. The best gathering ground is one which is composed 

 of impervious rocks — for porous strata steal too much water — and 

 which is covered only with mountain pasture or moorland. In many 

 cases no objection is made to the existence of sheep upon the gather- 

 ing ground ; but Liverpool, in the case of its Eivington works, has 

 purchased the whole of the gathering ground, and, after destroying 

 and pulling down many of the farms and buildings, has kept a great 

 part of the gathering ground free from sheep and let it to a sporting 

 tenant. The largest gathering ground in this country dealt with 

 by water works is the Birmingham gathering ground, which will 

 collect the water from 44,000 acres, while the Thirlmere scheme 

 of the Manchester Corporation has only a contributory area of 

 11,000 acres, and the Yyrnwy works of Liverpool Corporation only 

 22,000 acres. 



Hundreds of, dams have been built in this country to collect the 

 waters from gathering grounds in connection with water supply to 

 towns, or water supply to canals and waterways. The wall which 

 impounds the water at Yyrnwy is 85 feet high. The Manchester 

 Corporation, which had to deal with a natural reservoir, constructed 

 a dam only 50 feet in height, but the masonry embankment at Caban 

 Coch, the reservoir of the Birmingham Corporation, is 122 feet high 

 above the bed of the river ; and some of the other dams of that great 

 scheme, when complete, will be 128, 120, 101, and 98. These walls, 

 of course, have behind them immense quantities of water varying 

 from 8000 million gallons, in the case of Birmingham, down to very 

 small number of gallons which run down a mill goit. The tensile 

 strain or tear of such a mass of water as that at Caban Coch is 

 enormous, and in that case the work is strong enough to bear such a 

 strain up to 12 tons per square foot. The Bouzay dam, near Epinal 



