122 SMALL WATER SUPPLIES. 



wall will stand more according to the amount of 

 reinforcement. 



Let W = weight of wall in Ibs., tons, etc., per foot 

 run. 



A = the area of base of wall i ft. wide. 



Z = B D 2 where B = the width and D the thickness 

 of the wall (B is usually i 'o as i ft. wide of the wall 

 is taken), while a fourth quantity is required = M 

 which is equal to W x d. 



Then a familiar equation 



?* . <33> 



will when worked out give the compression on the 

 outer joints when the sum of the two quantities is 

 taken and the tension on the inner ones when the 

 difference is taken, the sign denoting that either 

 the sum or the difference is to be taken. The differ- 

 ence should for bed joints in masonry prove a nega- 

 tive quantity. The question will receive further 

 consideration. 



In the meantime, it may be said that in the case of 

 a river in which the water is intended to be stored 

 up for the purpose of providing water-power rather 

 than a supply for domestic purposes, a different form 

 of construction is employed for holding up the water 

 for moderate depths. It is rather what is known as 

 a weir than a dam, and is of a much greater thick- 

 ness than is necessary to merely withstand the 

 thrust of water, and is not subject to calculation but 

 is intended rather to be capable of passing large 

 quantities of water when required over its crest. 

 The foundation should be firm, and if necessary a 

 row of piles should be driven down at the foot of 



