24 Mr. H. M". Witt on the Power possessed by Porous Media 



No. 



ft. 

 2 

 1 

 

 

 3 



in. 

 6 

 

 6 

 8 

 3 



Fine sand 



Coarser sand 



Shells. . . 



Fine gravel . 

 5. Coarse gravel 

 These several layers of filtering mate _ 



fectly flat, but are disposed in waves, as seen in the sectional 

 drawing; and below the convex curve of each undulation 

 placed a porous earthenware pipe, which conducts 

 water into the mains for distribution. 



ials are not placed per- 



the 



IS 



filtered 



Chelsea Waterworks, Thames Bank. Transverse Section of Filter. 

 Downward Filtration. 



a. Water line. b. Top of fine sand. c. Collecting drain (perforated). 



Note. — Depth of water over the sand =4 ft. 6 in. 



The upper layer of sand is renewed about every six months, 

 but the body of the filter has been in use for about twenty 

 years. 



Samples of water were taken and submitted to examination — 



1st. From the reservoir into which the water was at the time 

 being pumped from the middle of the river. 



2nd. From the cistern after subsidence and filtration. 



Experiments of this kind were made on three diff^erent occa- 

 sions, viz. on the 12th of September and the 29th of December, 

 1855 ; and also on the 10th of May 1856 ; and the results are 

 embodied in Tables I., II. and III., each containing four 

 columns, — No. 1 showing the quantities of the several sub- 

 stances originally present, represented in grains in the imperial 

 gallon (70,000 grains) of water ; No. 2, the amounts present 

 after filtration ; No. 3, the actual quantities separated in grains 

 in the gallon of water ; and No. 4, the per-centage ratio which 

 the amounts separated bear to the quantities originally present. 



