192 SEASON 1875-76. 



the top of the wood dam, and the weight of the water pushes the 

 debris over the zinc on to the apron of the dam ; thus the intake 

 cleans itself. From this it is evident that the height of the over- 

 flow is important, for if it is too near the level of the dam, the 

 water could not rise sufficiently to clean the intake. If, on the 

 other hand, it is too high, the stream in flood would flow into the 

 well and mix with the screened water, not only increasing its 

 quantity, but also loading it with dirt, to clog all the screens in 

 the works. It is when the whole water of the stream is not 

 required that the intake is seen to its greatest advantage. So 

 long as a drop passes the zinc cover and over the apron of the dam, 

 the zinc keeps clean, and the quantity passed by the sluice is prac- 

 tically constant. When the whole stream is taken, the sluice must 

 be set so as to hold the water in the water-trap as high as the low 

 edge of the dam, or there would be too much suction through the 

 zinc to allow of easy clearance when the water rose. This is the 

 secret of the success of this intake. Although any quantity (within 

 wide limits) can be passed, the suction does not increase as a hold- 

 ing force proportionally to the quantity passed, while, after any 

 water flows over the apron, all increase only tends to clean the zinc 

 more perfectly. In winter, in Scotland, the zinc screen is removed, 

 as the stream falls much in frost, and the zinc is apt to freeze 

 temporarily if there is an insufficiency of water, although, when 

 covered with water, it behaves very well. The hake of wooden 

 slat which is substituted acts well, but would allow yearling fish 

 to pass. This is not of much consequence, as fish move so little in 

 winter, especially small fish, which seem to be very susceptible to 

 cold. The water, having passed the sluice into the delivery-box, 

 is divided by the overflow sluice at the end already described. 

 The sluice is set to send the water back on the cut discharging on 

 the 10-inch pipe compartment, and all not required by the 10-inch 

 or its waste passes over into the 12-inch pipe's compartment, 

 and through it to the ponds. The compartment of the 10-inch is 

 covered by a sheet of small-sized perforated zinc (No. 9 is used). 

 All the debris that has passed the intake screen is shot on to this 

 zinc. A little overflow is always allowed for in setting the sluices, 



