THE SUPPLY OF WATER TO SALMON PASSES, “55 
working or not, may be utilised to enable the fish to sur- 
mount the original obstruction. 
The supply of water is, after all, the “ bone of contention ” 
between dams and passes. The miller appreciates the 
truth of the old saying that— 
e... You never will 
With water once passed by propel the mill ;” 
and he accordingly objects to any device that may possibly 
reduce the supply of water to the mill-wheel in dry weather. 
For the efficient working of a fish-pass, of whatever kind, a 
supply of water through it is essential. The easiest way to 
secure this is of course to make the entrance to the pass a 
trifle lower than the crest of the dam, but this would pro 
tanto diminish the effective power of the wheel whenever 
the level of the water fell below the crest of the dam. To 
compensate for this it would be necessary to slightly 
increase the height of the weir; the effect of which would 
be twofold. It would first of all ensure the full volume of 
water, that would otherwise fall down the whole width of 
the dam, passing down the pass, and it would, in the second 
place, increase the power of the mill by the additional 
weight of water that would be dammed up, as well as by 
the increased length of time during which the river would 
be held back for the use of the mill. In this way the 
requirements of the pass could be calculated to a nicety, 
and they could be served without injury to the necessities 
of the mill. But, even without cutting into the crest of the 
dam, it would in many cases be advantageous to gain the 
same end by slightly raising the level of the dam along its 
entire length, with the exception of that part where the 
mouth of the pass is situated; in this way, although the 
pass would be less efficacious than one built with its upper 
