144 



SEASON 1874-75. 



on the contrary, the rain continues, the stream rises rapidly in the 

 narrow neck, and dams back over the stream, completely stopping 



the eddy, and very much 

 increasing the suction of 

 --' the screen, so as to pre- 

 vent the now-increased 

 force of the current 

 sweeping off the debris, 

 and so allowing any 

 dirty water to pass into 

 the dam. As the spate 

 increases it becomes 

 fouler, but the dam is 

 safe ; the only effect is 

 to fill the overflow. 

 Now, this being led 

 round the dam, and the 

 soil on the top being 

 gravel, some water fil- 

 ters through, cleansed 

 by the filtration, and a 

 sufficient supply passes 

 the overflow to feed the 

 rearing-boxes below. As 

 the spate falls the water 

 clears, and on more 

 de"bris coming down, the 

 friction of the current 

 soon cleans the inlet 

 screen, and all goes 

 smoothly (Fig. 101). 



In autumn, when the 

 leaves fall, and in spring, 

 when the trees bud, the 

 wind frequently covers 

 KIO. ioi-scaie &,. the surface of the dam, 



