BEST FORM FOR DAMS. 213 



is no better material for a dam than earth which contains 

 a large proportion of clay, with enough sand intimately 

 mixed in the mass to make it easily worked, and closely 

 compacted. But dams may be made of rock and timber, 

 as well as of earth, the former materials being selected, 

 when the work needs to be of the most substantial 

 character, to enable it to resist the wearing action of 

 strong and heavy currents of water which would tear 

 away an earth work in a short time. Where this contin 

 gency is likely to occur, only timber or rock should be 

 used, and the manner of construction should be left to 

 the direction of a practical engineer. For a work of 

 timber, cribbing filled in and backed with rock, and 

 planked thoroughly well, will be found very substantial 

 and satisfactory. There are many different kinds and 

 forms of cribs with which the hydraulic engineer is 

 familiar, of which those may be selected that will meet 

 the particular features of the cases requiring them, and 

 which for want of space cannot be referred to here. A 

 few will be described further on, of those only which 

 may be found useful to the irrigator who desires to per 

 form his own engineering, and in cases where professional 

 assistance may not be required. 



Upon the form of the dam will depend, in a very great 

 measure, its strength and stability, for it is evident that 

 the form has much to do with its power of resisting the 

 enormous pressure bearing upon it, and which is always 

 exerted either to overthrow it or to push it from its 

 foundation. Further than this, the form of a dam should 

 be such as will best resist the wearing and abrading action 

 of the water. A typical form of a perfect dam is shown 

 at fig. 101. The reasons why such a form is best adapted 

 for its purpose may be briefly stated as follows : 



It is evident that a structure, intended to sustain a 

 pressure of a body of water, can fail only in two ways, if 

 its solidity is preserved from disintegration by the wear- 



