XXXII. J. M. SMAIL. 



Filter Sand. — Immediately over the centre lines of the 

 underdrains, that is over those portions excavated to the 

 greatest depth and extending 5 feet on either side, there 

 was placed a body of sand with a maximum depth of 5 feet 

 in the centre. Over the ridges of the excavations for the 

 filter, midway between the lines of underdrains and for a 

 distance of 10 feet on either side, a body of sand was placed, 

 having a minimum depth over the flat portion of the ridge 

 of 3 feet ; the effective size of the sand over the underdrains 

 was approximately G'2.~> millimetres, and of the remainder 

 about '30 millimetres. 



It will be seen from this species of construction, that the 

 lowest portions of the filter surface are those directly over 

 the ridges of the excavation and half-way between the 

 underdrains; the general elevation of the surface of these 

 parts being 3 feet below the usual low water elevation of 

 t lie river, and 1 foot below the sand surface over the uuder- 

 drains. For a width of 5 feet over the centre line of both 

 ridges and underdrains the surface of the sand is flat, and 

 between these level areas the sand has a slope of 1 in 10. 

 This form of surface is such that water can be readily in- 

 troduced upon the filter to refil without producing currents 

 sufficiently rapid to cause appreciable disturbance. The 

 sand was deposited in two layers, the first being well com- 

 pacted before the second was put in place. The filter is 

 filled by gravitation from the river, but pumps are provided 

 in ease the water in river falls below gravitation level. 

 The working level is generally kept at 2 feet over the 

 lateral distributing carriers and 1 foot over the crown of 

 the head when at grade. 



The system is an economical one for a comparatively 

 small community. The principal advautage appeared to 

 be that vent pipes for exit of air, as is usual in European 

 filter beds, were not necessary, the wavy formation of beds 



