REPORT OF COMMISSIONERS OF INLAND FISHERIES. 95 



animals like crabs, which seek out and crawl into very narrow cracks 

 in the wood. It consists of putting into the box, a large canvas bag 

 as a sort of lining and arranging the filter pump as usual (fig. 16). 

 This apparatus has also proved satisfactory. 



POSSIBILITY OF VARIATION. 



So detailed a description of the apparatus as at present installed 

 and in use might without a further word leave the impression that this 

 apparatus alone fulfills the requirements of this general method of fish 

 culture. On the contrary there is hardly a feature of the whole out- 

 fit that has not been represented, at one time or another during 

 our experiments, by other materials or other forms. The present 

 boxes, for example, have replaced bags of canvas and of scrim and 

 bobbinet, not because the latter failed to give good results, but because 

 they were less durable and otherwise objectionable. Three forms of 

 power transmission have been operated successfully during the devel- 

 opment of the plant. It is obvious that the gasoline engine might 

 under other circumstances properly give place to a different kind of 

 motive power, such as steam or hot-air engines or electric, spring, 

 weight, or water motors. For use in small experiments weight or 

 spring motors, properly governed for speed, have much to recommend 

 them, for individual cars could be independently operated in various 

 localities without the inevitable expense and annoyances of running 

 the engine and the apparatus for power transmission. 



PRECAUTIONS. 



There are, moreover, precautions to be taken in the construction of 

 the cars and other devices. New wood, especially pine, and certain 

 metals, particularly copper and galvanized iron, which are frequently 

 used as screens, are apt to injure, and often prove fatal to young 

 animals even when under other circumstances the circulation through 

 the car would be ample. A very striking instance of the effect of 

 small quantities of copper and zinc-plated screening was furnished in 



