WATEE 5 



from the ground, and also from the leaves of plants. 

 In very wet seasons percolation as well as evaporation 

 occurs. 



The rainfall in England averages 30 inches a year, 

 after deducting loss from evaporation, the amount collected 

 from the roof space— calculated at 60 square feet per 

 head — amounts to 748 gallons a year, or two gallons per 

 person daily. 



An inch of rain delivers a little over four and a half 

 gallons on every square yard of land, or 101 tons per square 

 acre. In its passage through the air it becomes highly 

 aerated, and absorbs such impurities as may be in the air, 

 especially those found in the air of cities or the vicinity 

 of manufacturing towns. Ammoniacal salts, nitric and 

 nitrous acids, sulphurous acid, and the products of coal 

 combustion, may all be found in greater or less amounts 

 depending on the locality. The rain simply washes the 

 air, and depending upon the amount of impurity in the 

 latter is the impurity or otherwise of the rain water. It 

 also carries down with it numerous micro-organisms, 

 mainly micrococci ; these are much more abundant in 

 the first rain that falls, particularly after a prolonged 

 drought. 



Collection of Water. — For the supply of large cities it has 

 been found that the best source of water is obtained either 

 from natural or artificial lakes, which collect the upland 

 surface water from hills. Such water is very pure, contains 

 but a small amount of mineral matter, and is practically free 

 from contamination. The scheme of bringing the drinking 

 supply to Glasgow from Loch Katrine 34 miles away, 

 to Manchester from a distance of 90 miles, and other 

 cities from sources at a considerable distance, is a bold 

 one, and has been attended by the best results. The pure 

 soft water of Loch Katrine has greatly raised the standard 

 of health in Glasgow, while at the same time it is said to 

 save £36,000 a year in soap. 



Water collected in large basins or lakes is liable to a 

 vegetable growth, which though it colours and even 



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