How Sautmon anp Trovur 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 2 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 wrder- 
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 1t 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; anda 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 imn- 
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 piseiculturists 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 less 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. 
