8 10 THE POPULAR SCIENCE MONTHLY. 



tubs, containing lime-Avater, a substance which is known to be very 

 efficient in securing the precipitation of organic matters. The tubs are 

 jDrovided with dashers, which keep the lime-water constantly in sus- 

 pension, and with gauged faucets, which permit it to be introduced 

 regularly in the proportion of two hundred to two hundred and fifty 

 grammes of lime to the cubic metre of water into the current of foul 

 water which is flowing at the foot of the tubs. A few eddies produced 

 by means of artificial obstacles placed in the conduit secure immedi- 

 ately a complete mixture of the lime with the foul water. As soon as 

 the lime-water is introduced, the waste waters almost wholly lose their 

 oifensive odor, and cease to oflFer the slightest danger from noxious 

 exhalations. 



The water having been thus prepared in its passage through the 

 conduit, is distributed into ten basins of decantation, each about sixty- 

 five feet long, twenty feet wide, and five and a half feet deep, which 

 are arranged side by side. Each of the basins has a capacity for the 

 decantation of a thousand cubic metres of liquid a day. The water is 

 constantly entering at one end in each, and flowing out over the top 

 at the other end. The rate of flow is almost imperceptible, being 

 hardly a millimetre a second, and precipitation takes place as com- 

 pletely as if the water were quite still ; consequently, the water goes 

 out fully clarified. Thus a course of about sixty-five feet in length, 

 which, at the rate of 'OOl of a metre a second, represents a delay of 

 nearly six hours in the basin, suffices to clear the water of all matters 

 in suspension. A talus of mud is gradually formed in the bottom of 

 the tank, which at the end of a week becomes flush with the surface 

 of the water at the entrance-end, and just covers the bottom at the end 

 of the outlet. The basin has now produced all the effect of which it 

 is capable. If any more water is allowed to go through it, it will con- 

 tain mud in suspension, for it is still in the act of precipitating it when 

 it goes out. The operation must be stopped here. We close the feed- 

 ing-gate of the tank, draw off the clear water that is left in it by 

 means of a decanting-tube, and lay bare the talus of mud. 



The bottom of the basin is slightly inclined in a contrary direction 

 to the course of the water, and is provided with a large valve at the 

 lower end. On opening this valve, the mud, which is still in a very 

 liquid condition, is passed into a lower basin the drainage-basin of 

 the same capacity as the former one, and so disposed that its upper 

 surface is a little below the bottom of the same. The first tank may 

 be put in operation again immediately after the mud has been drawn 

 from it. 



The side-Avalls of the drainage-basin are of tight masonry, but the 

 bottom is made as permeable as possible. For this purpose a floor of 

 scoria is prepared and provided with a series of pipes which lead the 

 water out into a collector. The arrangement is admirably adapted 

 to the purpose for which it is designed, and quite obviates the diffi- 



