I 1 8 SMALL WATER SUPPLIES. 



by rods as shown. If it is possible to lay on a 

 supply (even very small) at a few feet above the top 

 of the syphon pipe, a cistern may be erected in the 

 house in place of an air-pump, and this connected with 

 an automatic air-valve will serve to keep the syphon 

 always in action. The subject of earthen dams has 

 been discussed, because this chapter would be in- 

 complete without it. At the same time it may be 

 said that for small reservoirs it is not the best form 

 of construction, and that in this connexion, unless in 

 the opinion of the engineer circumstances particularly 

 warrant the foregoing construction, some form of 

 masonry or concrete dam is far superior. The design 

 of such walls demands minute attention, because there 

 are two conditions which have to be satisfied, one 

 (essential), that the wall shall be proof against failure 

 under ordinary circumstances, and the other (very 

 desirable), that this stability and solidity shall be 

 gained by the most economical method of construc- 

 tion ; or in other words, it is the duty of the engineer to 

 dispose of his materials in the most economical manner 

 possible. Walls subject to water pressure on one 

 side are known as retaining walls, and although this 

 term is more often applied to walls, which hold up 

 masses of earth in preference to water, yet the condi- 

 tions of both are essentially the same, and when the 

 case of water thrust is understood the subject of earth 

 pressure is simply a modification. 



Now the water retained by a wall or dam exerts 

 a certain thrust along a horizontal plane on that 

 wall which tends to overturn it. This thrust is 

 known as the overturning moment. The masonry of 

 which the wall is constructed exerts a force down- 



