WATER. 67 



overflow. When forming a lake, the operator should bear in mind 

 the practicability of introducing either a hydraulic ram or a turbine, 

 where the useful work it can do may be needed. For example, 

 a stream that fills a 3- in. pipe with a fall of 6 ft. will force the 

 delivery of 2,000 gallons of water daily to a point 100 ft. above the 

 water-level. 



A lake may be made in two levels, that are separated by a weir, 

 which should be constructed at some comparatively narrow place, 

 over which a bridge can be thrown ; or a cascade can be made at 

 such point ; or the bridge may be placed where a short river joins 

 the broader lake area. 



In commencing the excavation for a lake, all the soil should 

 first be removed from the area and be stored in a convenient posi- 

 tion for use ; or be used at once, in position, to form plantations 

 connected with the lake. Such soil, so utilised, has not only its 

 own inherent value, but will give a luxuriant vegetation more 

 quickly than poorer soil, or soil of lesser depth ; an advantage, in 

 the special circumstances, often of particular usefulness. 



Storage of the water, and maintenance of the proper level, are 

 important considerations. The average evaporation from a water 

 surface amounts to about }i in. per day, or about 30 in. in the 

 year. It is greater from shallow than from deep water, and 

 naturally more in summer than in winter ; it is greater from running 

 than from still water. 



The mean total annual rainfall in the London district is about 

 26 in. Heavy rain, falling for 24 hours, yields about i in. to the 

 depth ; but occasionally rain falls that gives 3 in. of depth in i hour. 

 On an average, about three-fifths of the rainfall is available for storage. 

 It will thus be seen that an extraneous water supply is desirable for 

 adjustment of the normal level. A method of approximately ascer.- 

 taining the quantity of water available from a flowing stream is 



