136 SEWAGE AND ITS PURIFICATION 



very impure effluent ; the same occurs from the production of 

 cracks in clay by drying, or from the fissures so common in 

 chalk formations. At Beverley, in Yorkshire, the top layer of 

 clay had become extensively cracked in the summer, allowing 

 raw irrigation sewage to reach the chalk beneath, whence it 

 travelled through fissures about half a mile to a deep well that 

 was a portion of the water-supply of Beverley.^ The reports to 

 the Royal Commission on Sewage^ give examples of under- 

 drained farms with shallow surface soil over clay, where " the 

 sewage partly finds its way into the drains through cracks "; in 

 one case the efiluent contained pieces of straw up to J inch 

 long, which had reached it through these cracks, and the 

 effluents were generally of very b ad q uality. 



Stretford Sewage Farm (Mersey and Irwell watershed) reports 

 that the drains should be laid in parallel lines, not in herring- 

 bone fashion. Trouble has been encountered from worms and 

 rats causing holes, which allow sewage to run straight to the 

 drains. 



''In dealing with an infected worm area, we pump the very 

 strongest sewage we can from the bottom of the tank, and on some 

 occasions, previous to dosing the land with strong sewage, have 

 sunk down to the drains and temporarily blocked them up, and have 

 thus been so far successful in killing them that the land has again 

 continued to produce as good an effluent as before. The worms do 

 not die in their holes, but come to the surface. They have caused 

 an imperfect effluent even when the drains have been as much as 

 4|- feet in depth." 



Dr. Houston has counted the number of bacteria and spores 

 present in twenty-one different soils. Among them he finds : 



Organisms per 

 Gramme of Soil. 



1. Sandy soil near the sea... ... ... ... 8,000 



2. Suburban garden soil, not recently manured ... 518,000 



3. Dark garden soil, manured six months previous ... 795,000 



4. Light-coloured soil, not recently manured or 



disturbed... ... ... ... ... 1,051,000 



5. Black loamy soil, occasionally having farmyard 



manure ... ... ... ... ... 1,084,000 



6. Rich heavy clay, periodically manured... ... 2,531,000 



7. No. 3 above, after recent manuring ... ... 3,308,000 



8. Garden soil treated with human faeces and urine 



for six months previous ... ... ... 26,780,000 



9. Sewage field, from a trench along which sewage 



had been running a short time before ... 115,000,000 



^ See p. 9, also a report to the War Office by Davies and Tyndale in 1902. 

 2 Vol. iv., part ii., 1904, pp. 243-249. 



