64 FISH-CULTURAL ASSOCIATION. 
passing to consecutive positions of rest under the conditions 
above stated. How this idea has been realized in practical con- 
structions, will be understood by references to the figures and 
descriptions. 
If we take a hemispherical bowl (fig. 1) and holding a marble 
at A, upon the edge of the bowl, we release it, it will fall under 
the influence of gravity through 4' to A’, coming to rest at 4’, 
some distance below the edge of the bowl The vertical distance 
between the positions 4 and A’, measures the force of accelera- 
tion that has been counteracted by friction by traveling the con- 
strained path A, A’, A’. 
If now, we take a number of similar bowls and cut them off 
to the line 4 4’, and arrange them as in fig. 3, and start a marble 
at D’, it will pass from D' to C’, reaching C' with no greater 
velocity than that acquired in passing from A to A®. If, how- 
ever, the marble was allowed to roll unobstructed from 4 to A’ 
down the incline plane D, C, (fig. 2) it -will have acquired a 
velocity equal to 8/ Dé, approximately, 
We see, then, in this case how ‘it is possible to deliversa 
molecule from a given position to a definite lower position, 
without the increase of velocity that would arise if the moiecule 
fell freely under the action of gravity or rolled down a smooth 
incline. If it be possible to compel every molecule of water 
descending through a fish-way to submit to the conditions above 
indicated, then the problem how to control the velocity of a de- 
scending current would be solved. Now to apply this to liquids, 
we arrange a series of "bent tubes, shown in fig 4. By suitable 
arrangements we keep the longer branch of the higher tube of 
the series full of water. The water escaping from each tube 
will rise against gravity until it comes to rest; then falls into 
the longer branch of the adjacent tube in the series, and after 
passing through the entire series be finally discharged from the 
shorter branch of the lowest bent tube, with no greater velocity 
than it acquired in passing through the first member of the 
series. 
Construct a series of these tubes with branches brought close 
together, cut away obliquely the upper end of the longer branch 
