170 AMERICAN FISH CULTURE. 



dam, a coffer dam having been first erected above to keep 

 off the water. 



" In this opening, a new subdam was erected, so that its 

 comb or highest elevation would about equal the level of 

 the water below the principal dam when the fish are run- 

 ning (a little over three feet say). The lower slope of this 

 subdam was placed at an inclination of one in fifteen, and 

 the sides of the aperture in the main dam were dentated 

 or framed in a series of offsets, so as to promote the forma- 

 tion of eddies in the current passing over the subdam. 



"When the fish are running then, in the spring, the 

 water in the aperture will be under the influence of gra- 

 vity in opposite directions. The lower water will try to 

 attain its level, the top of the subdam and the upper water 

 rushing through the aperture will meet and certainly drive 

 it back, but with a force considerably impeded by the 

 cushion, so to speak, of lower water. 



" The fish will be nosing along the foot of the main dam, 

 as is their wont, and finding its passage open, agitated 

 though it be by these contending currents, they will 

 endeavor to pass up, and let us hope they will succeed. 

 But should they fail in the first few trials, there are the 

 recesses at the sides where the eddies are sure to be formed, 

 and where they may gather strength for a renewal of the 

 trial. I am informed, by persons in the neighborhood of 

 Columbia, who have seen this aperture of ours with the 

 water running through it, that there are many passages in 

 the Conewago rapids below, which are much more difficult 

 of ascent than this is; and which, of course, the fish must 



