FOR PLEASURE AND PROFIT. 



109 



may do so, but it is equally possible that it may not. The 

 perfect pass, then, is one through which the fish can run TVith- 

 ooit the necessity of leaving the water at any part of it. More- 

 over, the water in the pass through which the fish swims must 

 be " dark " water, i.e., it must not be foamy, " white " water. 

 Hence it follows that a succession of falls with " white " water 

 at the bottom of each do not go to form a perfect fish-pass, al- 



WVE* BANK. 



Fig. 22. Plan of a Fish-pass. 



though it is possible that a fish may mount up them by a 

 succession of "leaps." The "step-by-step" principle is the 

 one to be adopted in preference to any other ; but the " steps " 

 should be formed after the manner shown in the next illustra- 

 tion, and Fig. 22 explains how it is done. The drawing shows 

 the position of the dam or other obstruction to be overcome, 

 and it is also a plan of the fish-pass, with boulders so arranged 

 that they are close enough to keep ai sufficiency of water back 

 and so form, a succession of " step-by-step " pools, yet far 



