CHAS. E. FOWLER. 283 



Slow or so-called natural filtration through sand has 

 been practiced in Europe for half a century. About 

 thirty years ago the city of St. Louis was considering 

 the question of purifying its water supply and employed 

 the late James P. Kirkwood, civil engineer, to investigate 

 the filtration systems of Europe with a view to their 

 application to the needs of that city. As the result of 

 his investigation he submitted a report that has become 

 standard authority for the condition of filtration at that 

 date. St. Louis did not adopt that method of purifi- 

 cation. 



A few years later, however, the water supply of the 

 city of Poughkeepsie was projected and Mr. Kirkwood 

 employed as consulting engineer. According to his plans, 

 and under his supervision, the filtration system of this city 

 was constructed upon the general principles and arrange- 

 ment of the slow sand-filtration systems of Europe. 



Our city thus became the first in this country to adopt 

 a systematic purification of its water supply by sand 

 filtration, the mechanical methods having been invented 

 several years later. 



In this method of filtration the standard rate at the 

 present time is about three inches vertical per hour — 

 less than one-sixtieth part of the rate adopted in the 

 mechanical filters. 



Consequently the area of sand surface to filter a given 

 quantity per day must be proportionally greater. There- 

 fore basins of masonary or of earth embankments are 

 constructed to contain tbe filtering materials. Sand is 

 the filtering medium, and as in the mechanical filters, 

 means must be provided to retain the sand while the 

 filtered water passes freely out. This is sometimes ac- 

 complished by placing upon the bottom of the basin a 

 layer of coarse, broken stones, then a layer of smaller 

 stones, then successive layers of coarse and fine gravel 

 until the upper layer of gravel is fine enough to support 



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