How SALMON AND TROUT LEAP. 409 



spring, giving the impetus. This spring must be made, of 

 course, by the assistance of every fin that can aid it, but chief- 

 ly by a strong stroke of the tail. Unfortunately, however, 

 the majority of mill-dams are so spread out across rivers 

 that the water runs over them in the thinnest possible sheet, 

 and the soundness of the dam requires a foundation on the 

 lower face. This foundation is assisted and protected by a 

 wooden sheathing called the apron, and this is placed as near 

 the surface of the water as possible, and extends down stream 

 for fifteen or twenty feet below the dam, so that the under- 

 standing of the dam may not be undermined ; and thus it 

 constantly occurs that while the pool below the dam is of 

 great depth and capacity, yet it only forms a sort of reservoir 

 for the fish, which the owner of the dam catches at his lei- 

 sure, the fish being unable to approach the dam even so as to 

 swim or pass over it ; and a dam of this sort, if only three or 

 four feet high, would be as impassable to salmon as if it were 

 four times that height. A salmon will scull up a pretty swift 

 stream that does not perhaps cover his back, so long as his 

 tail and pectoral fins, which are the propelling power, are im- 

 mersed,* provided in such waters he is not called upon to 

 make a perpendicular jump. This he can not do without a 

 run to start him. In considerable depths, for a short space, 

 a salmon can force his way through extremely rapid and 

 heavy waters, but there are limits to this capability ; and the 

 difficulty which pisciculturists labor under is the ascertaining 

 what weight or rapidity of water a salmon can stem. Some 

 salmon, of course, can stem a stronger torrent than others, but 

 the problem must be taken as applicable to the weakest fish, 

 not the strongest, inasmuch as the object is chiefly to per- 

 mit the passage of female fish very heavily laden with ova. 

 A female fish, full of eggs, carries something like a fourth of 

 its own entire weight in that commodity, and unless such fish 



* The tail is the most important organ in this proceeding, the fins being 

 used chiefly for balancing and steering the fish, though they all aid propul- 

 sion on unusual occasions calling for great and sudden effort. 



