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



charcoal^ for we bad learnt that the resiilts thus obtained did not 

 admit of comparison with those of filtration through sand at 

 Chelsea, on account of the great difference in the original com- 

 position of the water; therefore, in order to establish at the 

 same time the absolute effect of charcoal as a filtering medium 

 upon the purer water at Kingston, and also to compare it with 

 that of sand, two experiments were made simultaneously with 

 the same water, the one of filtration through charcoal alone, and 

 the other through sand alone, both under precisely similar cir- 

 cumstances. In both these experiments the filtration was not 

 effected laterally as in the foregoing case, but downwards in the 

 usual way through beds of the same size of the respective filter- 

 ing media contained in wooden troughs, the water being ad- 

 mitted at the top and passing off from below. 



The sand-filter had an area of 4 square feet, and consisted of 

 the following materials : — 



ft. in. 



Fine sand 19 



Shells 1^ 



Qravel 11 



Coarse gravel 9 



2 9 

 The rate of filtration being half a gallon per minute. 



Table V. — Results of Sand Filtration. 



Apart from its special interest, as compared with the following 

 experiment made simultaneously through charcoal, the following 



