4 TOWERS AND TANKS FOR WATER-WORKS. 



open reservoir whose upper or highest elevation shall be some- 

 what in excess of the height to which the water would natur- 

 ally be forced under the stress conditions, otherwise the 

 reservoir will overflow. 



\Yhilst this destructive tendency has been greatly lessened 

 by the use of improved duplex pumping machinery, there is 

 also to be considered in the economy of operation a certain 

 loss of energy due to the force necessary to put in motion the 

 column of water, temporarily suspended at the expiration of 

 each forward stroke of the machinery by the rigid enclosing 

 sides of the pipe-lines. Connections to an open reservoir pro- 

 vide an opportunity for escape and permits an onward move- 

 ment of the liquid column, relieving the " back pressure," 

 and, through its own momentum, effecting a saving in energy 

 necessary to impel it forward. The relief to the pipe system 

 is to the same extent enjoyed by the pumping machinery, re- 

 ducing the strains upon the mechanism and the consequent 

 number and extent of repairs, and, more important still, the 

 liability to accident at some critical moment. Any open res- 

 ervoir or vertical pipe, of whatever diameter and of sufficient 

 height, will afford the desired relief, but it is the usual practice 

 to couple with this desideratum a capacity sufficient for a re- 

 serve supply. 



The accomplishment of these requirements is generally 

 secured for the larger cities by reservoirs of earth and masonry 

 construction for reasons of economy and permanency, and 

 designed to suit topographical conditions and local demands. 



For the same reasons, in all preliminary investigations for 

 the water-supply of the smaller cities and towns, elevated sites 

 suitable for similar construction should be sought and first 

 given careful consideration. 



The subject of the theory, details, and construction of such 

 reservoirs has been discussed by such eminent authorities, and 

 so great a volume of scientific and prolix literature has 1> 



