WATER FILTRATION. XXXI. 



end of filter gallery ; thence 20 inches for 120 feet, 16 

 inches for 65 feet, 12 inches for 30 feet, continuing with 

 successive lengths of 30 feet each of vitrified pipes having 

 diameters of 10, 8, and 6 inches respectively. The entire 

 conduit was surrounded with 8 inches of gravel, the inner 

 4 inches being of stones about 2 inches in diameter, and 

 this was covered with layers of stones of decreased sizes. 

 From the conduit and old filter gallery, which was pierced 

 at suitable places for the purpose, lateral underdrains of 

 vitrified pipe were laid upon the excavated bottom with a 

 slope of 1 : 100. The pipes were laid in the following order: 

 beginning at the main conduit, one length of 10 inches, two 

 lengths of 8 inches, from 35 to 65 feet of 6 inches, ending 

 with three lengths of 4 inches. The pipes were placed so 

 that the spigot end of one approached, but did not enter, 

 the socket of the next, and the drains throughout were 

 covered with a 4 inch layer of 2 inch gravel. Upon this 

 layer, which averaged about 6 inches in actual depth, there 

 were placed successive courses of gravel of the following 

 sizes and of the approximate thickness (giving an average 

 section) to make a total of 12 inches of unclerdrain material ; 

 2 inches of 1^- inch, 1 inch of f- inches, 1 inch of | inch, and 

 1 incli of -r 6 - inch gravel, with 1 inch of coarse mortar sand. 

 The -yV material was spread out to a width of 17 feet, and 

 the last course was spread out to a width of 20 feet. 

 Beyond the pipes, the largest stones forming the lowest 

 course were laid upon the surface of the excavation and 

 were spread out to a general width of 5 feet, with the next 

 courses each spread out a little beyond the course below. 

 The largest stones used in the underdrains were selected 

 by hand, and if covered with dust or dirt were washed. 

 The stones of the two larger sizes, viz., 2 inch and 1-J inch, 

 were placed by hand at the open joints of the pipes, but 

 the layers of smaller sizes were shovelled into place. 



