24 The Drainage of Fens and Low Lands. 



is carried on by the fore and aft rollers, and the water being 

 breasted up by the fore-roller, causes a thorough mixture of 

 the soil with the water, the earth being converted into sludge. 

 Although soil and sand can easily be removed, a greater 

 effect is obtained with a clay bed from the lighter specific 

 gravity of this material. An illustration and description of 

 this machine will be found in the ^Engineer* of October 28th, 

 1887. 



For removing mud shoals which collected along the Mare 

 Island Straits at San Francisco, revolving buckets were used, 

 the machinery being actuated by the forces of the stream. 

 A floating framework was fitted with an undershot wheel 

 20 feet in diameter. This wheel was driven by the current, and 

 was geared into two drums, which carried double ropes fitted 

 with small buckets. The ropes passed round loose pulleys, 

 which dropped into the mud, carrying the buckets with them, 

 and these on their return discharged their contents into the 

 stream, by which it was carried away. 



Proportion of Size of Drains to Land drained.— 

 No fixed rule can be laid down for the designing of a system 

 of drainage for any particular district, either as to the number 

 of the drains, their arrangement, width, or depth, as these 

 must all be governed by the particular conditions of the land 

 to be drained and the outfall. 



A typical case may, however, be taken, having an area of 

 flat land containing 20,000 acres, discharging into a tidal 

 stream which only allows the sluice doors to be open fourteen 

 hours out of the twenty-four. The rainfall is assumed as 

 l«i„g .hat due to a ,„ Jiet of a„ inch in twenty.four hours, 

 the whole of which is to be discharged off the land. The 

 quantity of water due to this rainfall would be equal to 

 210* 1 1 cubic feet a second; the main drain, running only 

 7 hours- each tide, would require a capacity equal to 359*498, 

 say 360 cubic feet a second. Taking the district as 6\ miles 

 long by S miles wide, this would require one main drain 



