XXX. J. M. SMAIL. 



filter was designed by Mr. Hiram F. Mills, Chairman of 

 Committee of Water Supply and Sewerage of the State 

 Board of Health of Massachusetts. It is located on the 

 northern bank of the Merrimack River, immediately east 

 of the pumping station. It is somewhat irregular in outline, 

 having a total length of 750 feet and average width of 140 

 feet and a surface area, including the main carrier of 2*44 

 acres. The filter surface, by transverse carriers, into 25 

 portions or beds, each having an average width of 30 feet, 

 gives an available filtering surface, including lateral 

 carriers, of 2"MJ acres, or without the carriers of about 2*3 

 acres, in constructing the filter, the bed and bank of the 

 river were excavated generally to 7 feet below r usual low 

 water, for a distance of about 150 feet southerly from the 

 old filter gallery, and in the river bank east or down stream 

 a sufficient distance for the total length of the filter. 



Main conduit and underdrains. — The underdrain pipes 

 which extend across the filter from north to south are 

 arranged with centres about 30 feet apart. For a width 

 of 5 feet along the line of drain pipe the excavation was 

 continued to elevation 29 and midway between the lines of 

 underdrains for a width of 5 feet, the depth of the excava- 

 tion was only to elevation 31. The bottom between these 

 two levels for a width of 20 feet was excavated with a slope 

 of 1 : 10. Thus it will be seen that the bottom of the filter 

 when excavated ready to receive the filtering material, 

 was waved, with alternate level ridges, between the lines 

 of underdrains, and flat valleys in which the underdrains 

 themselves are placed. 



The conduit which receives the filtered water is circular 

 in shape, of brickwork 4 inches thick, the bricks are laid 

 without mortar in the end joints, thus allowing spaces a 

 little more than \ inch wide for the entrance of the filtered 

 water. The conduit is 2 feet in diameter for 105 feet from 



