ON THE SUPPLY OF WATER TO TOWNS. 6? 



gravitation. Many smaller towns, particularly in the limestone, chalk, and 

 oolite districts, also derive their supplies from springs, but the supplies thus af- 

 forded are in general comparatively insignificant to those obtained in other ways. 



The quantity of spring-water yielded by any given district varies materially, 

 not only according to the amount of rain which falls, but also according to its 

 geological character. Sand, gravel, chalk, limestone and other absorbent 

 rocks, yield springs in the greatest abundance ; next to these, the more 

 loosely stratified rocks, such as the coal-measures, the millstone grit, and the 

 old red sandstone; least of all the closely-bedded slate rocks and the 

 primitive formations. 



Chalk and sand absorb nearly all the rain which falls upon the surface. 

 There are few large rivers or streams in these formations, for little water 

 runs away in floods, that which is absorbed escaping again at the points of 

 greatest depression, or along the edges of some impervious stratum on which 

 the measures may rest. Thus chalk springs are generally found at the foot 

 of the chalk hills, either at the lowest level of the ground, or where the lower 

 beds of this formation, above the greensand, are comparatively impermeable. 

 The springs of the upper greensand issue along the upper edge of the gault, 

 an impervious bed of clay on which it rests ; and the springs of the lower 

 greensand, where they again rest on the Wealden or Kimmeridge clays. 

 The water absorbed by the lower oolite is thrown out by the lias clay, and 

 the carboniferous limestone-water passes either through clefts or fissures in 

 the rock to some convenient outlet ; or having penetrated to the bottom of the 

 limestone bed, is thrown out by the thick beds of shale which lie beneath. 



The sands of the new red sandstone formation also absorb most of the 

 water which falls upon them, as do also the local beds of sand and gravel 

 found interspersed amongst the clays of the diluvium. 



From all these sources, produced by absorbent measures, large quantities 

 of spring-water may undoubtedly be procured, often continuing with little 

 daily variation, and frequently so situated as to be easily available for the 

 supply of towns. Many single springs yield several hundred thousand 

 gallons a-day; some amount to upwards of 1,000,000, and there are a few 

 which far exceed this quantity, forming at once rivers of considerable 

 volume — such are the source of the Aire at Malham Cove in Yorkshire, the 

 Syreford Spring and Seven Wells near Cheltenham, the Hogg's Mill River 

 near Ewell in Surrey, the spring at Holywell in Wales, and many others. 



But the most abundant quantity of spring-water yielded by any extended 

 district is probably that which is found in the greensand formation in Surrey. 

 Here this formation rises into hills of considerable elevation, Hindhead and 

 Leith Hills being nearly 1000 feet above the level of the sea, forming arid 

 Wastes or sandy deserts almost destitute of vegetation, which are eminently 

 absorbent of water. The water thus absorbed issues in springs of the 

 greatest purity, forming collectively, in the dryest seasons, a volume of 

 water at Guildford from a comparatively limited tract of country, exceeding 

 40,000,000 gallons of water a-day, 33,000,000 of which are the produce of 

 the greensands, not exceeding on the average 2^° of hardness. One stream, 

 the Potsford Brook, which rises in the Leith Hills and falls into the Albury 

 Brook a little above Guildford, is under four miles in length, and yet gra- 

 dually and almost imperceptibly increases to a daily volume, as measured in 

 extreme drought, of nearly 5,000,000 gallons of pure spring-water. After 

 running one mile, it contains 800,000 gallons a-day, in the second it is 

 augmented to 1,400,000, and at the end of the third mile to 4,400,000. 

 The gross quantity of soft spring-water which might be conveniently collected 

 in this district at an elevation of about 120 feet above the Thames at 



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