MIDDLETHIRD HATCHING-HOUSE. 109 



invention. It consists of a frame batten, with a bottom of floor- 

 ing and a top of perforated zinc x---- ' ' 9 -- 



stretched on a moveable frame 



(Fig. 49). A pipe is inserted in 



the centre of one end of the box 



(Fig. 50) to carry off the cleaned FJG - so-s 



water, while all leaves and de"bris are swept off the zinc by the 



force of the water. 



Care must be taken to place it at an angle to the direction of 

 the stream, and the lower edge must be given a slight fall. The 

 principle is, that water passes over the zinc with sufficient force to 

 keep it clean ; the swirl formed by the hang of the screen and the 

 angle of its inclination greatly assists this. When the quantity is 

 insufficient to fill the pipe, it all passes through the screen, and the 

 debris, leaves, etc., collect, from the lower edge of the zinc 

 upwards, till they almost cut off the water, which then begins to 

 flow over the top, and soon cleans the screen again. In floods, 

 when much more water is passing the screen than the pipe can 

 take, the surge back from the full pipe clears the surface from any 

 debris held by the suction of the water. The only danger, if the 

 screen be placed in an exposed situation, is cat ice, which might 

 freeze to the zinc ; but in practice it is found that although a 

 bridge of ice forms across the screen the water passes freely under. 

 The great principle is to have a little waste always running over, 

 and a perfectly even flow through the pipe is assured. But the 

 screen will also act in the most satisfactory manner under all 

 circumstances. 



A small sluice (Fig. 51) is formed behind the box to regulate 

 the flow of water through the pipe. This should 

 be set so as to keep the water level with the lower 

 edge of the screen-box, so that no rise of the stream 

 may increase unduly the supply. The leaf-screen 

 has since been slightly modified to enable it to be 

 placed in the water-course itself, thus avoiding the FIG. 51- scale ^. 

 expense of building a dam. I have now had one at work at 

 Craigend for eight years, and although the zinc has been many 



