SOME NECESSARY CONSIDERATIONS. 211 



the dams requires to be arranged so as to raise the level 

 of the water above them to a sufficient hight, and yet 

 leave an open passage with a current that may not be in- 

 surmountable to vessels or boats navigating the stream. 



When dams of the ordinary construction are required, 

 it may be necessary to consider, before the work is begun, 

 the principles upon which their stability is founded. 

 This is more especially necessary, when the work is of. 

 any considerable magnitude, and where a failure may in- 

 volve the loss of the money spent, and much direct and 

 indirect damage besides. The chief points for consider- 

 ation in this regard are, the position of the dam in the 

 stream ; the material of which it is to be made ; the form 

 most consistent with permanence and stability ; and the 

 manner of its construction. 



The position of the dam has reference only to its 

 power of resisting the pressure of the water behind it. 

 No increase of the flow of water into the canal, or lateral, 

 ca,n be gained bj placing a dam in a diagonal position 

 across the stream, instead of at right angles to the banks, 

 as has been stated by some who have written upon this 

 subject in the public journals. As an instance of the in- 

 correct and misleading notions thus spread abroad, by un- 

 informed writers, might be cited the following from an 

 article on "Practical Irrigation in Colorado," published 

 in the Eeport of the Department of Agriculture for 1871. 

 The writer says, (( it has been contended that the stagna- 

 tion of water extends to a sensible hight, above the 

 horizontal line of the regurgitation from the dam or 

 sluice, or any other fixed obstacle. This is accounted for 

 ly the compression or closer adhesion of the particles of the 

 water." Again he says, "if you confine the water, and 

 divert it from its natural course, you may compress it into 

 a smaller space ; but the same quantity will be found be- 

 low the compression, as is found above it!" Now, it 

 ought to be known that water is practically incompressible, 



