96 SEASON 1873-74. 



screen, the fry collect and soon remove most of it ; but they leave 

 the flannel alone. Thus the difficulty of a tight joint is overcome ; 

 no one who has not tried has any idea of the difficulty of making 

 a joint tight enough to confine young trout. Stone says, " Then 

 woe to the trout-breeder if his troughs are not perfectly tight ! 

 for if there is a loose joint in the box, or a nail-hole, or aperture, 

 under or about the screen, where water comes in or out, these 

 little creatures will be sure to find it, and one by one will go 

 through it in thousands, even if the crevice is not much larger 

 than would admit a snow-flake." (Domesticated Trout, p. 144.) 

 The safety-screen is sufficiently far above the outlet to be 

 unaffected by the suction caused by the water passing over the 

 ledge ; and as the frame is only an inch in thickness, the space 

 for the water to flow through is nearly the section of the pond 

 instead of a narrow edge ; for example, if the plank pond is 5 feet- 

 wide, and the depth of water on the overflow is 1 inch that is, if 

 the surface of the water in the pond stands 1 inch above the edge 

 of the outlet, there will be 60 square inches of water in section ; 

 now 13 inches back the water stands its own depth above the 

 bottom of the pond, say 13 inches (which is a good workable depth 

 for fry) ; if we allow 1 inch for the thickness of the frame of the 

 safety-screen, then 5 square feet, or 720 square inches, is in the 

 section of water, or just twelve times as much ; and, conversely, 

 the force of the suction will be only one-twelfth. 



OVA-EXTRACTOR. 



The various modes of removing ova for examination, or dead 

 ova to prevent the contamination of the surrounding eggs, were 

 tried one after another. They are all open to the same objection, 

 viz., that the nearest eggs to the one selected are disturbed to a 

 greater or less extent. Perhaps the most objectionable is the 

 French glass syphon, described by Francis Francis, and of which 

 he furnishes a cut on page 80 of his Fish-Culture. It consists of a 

 bent glass tube blown into a bulb at one end, the bulb being 

 finished off like the mouth of a pickle-bottle, and closed by a cork 

 through which a short straight glass tube is inserted, projecting 



