474 



On Filtering the Waters of Tanks. 



[No. 6. 



3. The slates of all kinds. 



4. Porous sand-stones. 



5. Jumma or glass of brick kilns. 



6. Small shells and shelly sand. 



7. Koah or pounded brick. 



limestone or pisolite. 



Coarse kunkur gravel of the 

 ironstone kunkurs or pisiform 

 iron ore. 



Ironstones and iron earths of 

 all kinds from Basalt and 

 earthy iron ores to the late- 

 rites of Midnapore and Cut- 

 tack, and the yellow ferrugi- 

 nous earths. 



4. Chalk and massive kunkur. 



5. Limestones, as that from Syl- 



het. 



6. Black hornblende sand from 



Saugor Island, or other parts. 



7. White clays of various kinds. 



8. Soorkey. 



9. Charcoal and coke. 



10. Bone charcoal. 



We desire to obtain first pure looking water, not forgetting that 

 water may appear to be pure and yet hold unwholesome matters in 

 solution. Filtering at one operation, both upwards and downwards is 

 perfectly easy, and will in most cases give clear water at once. This 

 is managed as follows : — See Plate. 



A. is a wall enclosing any space with openings of any kind at the 

 top only. 



B. a second wall with small arched openings b below. 



C. is a third wall with openings only at the top again discharging 

 into the reservoir D. for the clean water. 



Now if the spaces between A. and B. and B. and C. be filled with 

 any good filtering materials from the first list, as fine washed sea 

 sand from Saugor or the coarse Muggra bally used by our masons, 

 the water which enters through A. (being the surface water which is 

 always the clearest)* will filter through it downwards beneath B. and 



* Some of the holes at A. &c. are always supposed to be below the level of 

 the Tank, when at its lowest level in the dry season. 



