1886.] NEW YORK ACADEMY OF SCIENCES. 55 



filtered, exclusive of first cost and interest. Such being the 

 case, I need not go into a statement of the reasons why the few 

 which have been actually brought into use in this country liave 

 been so little successful. The conviction appears to be generally 

 entertained that American ingenuity has discovered a method 

 by which mechanical arrangements have taken the place of the 

 cumbrous English system, and done away with the manual 

 labor required in cleansing. Many contrivances have been 

 brought forward, but they are crude, or have complicated 

 systems of pipes for reversals of the current, or are wasteful 

 in the use of filtered water for cleansing. Recently, however, 

 an extremely simple device has been proposed, which is yield- 

 ing excellent results. As is well known, the efficient part of a 

 filter-bed is the top layer of sand, which need not be more than 

 two feet in thickness. At Poaghkeepsie, on the Hudson Eiver, 

 (notm successful operation) this two feet of sand rests on four feet 

 of gravel and stone which are provided me/'e/y ^0 support the sand 

 and to afford channels for the filtered water to drain away. This 

 gravel and stone are replaced in the mechanical filter used in the 

 American system of water purification, by perforated double pipes. 

 The construction of this filter, called the National Filter, can be 

 readily understood from the accompanying wood-cuts. 



The water enters the filter at A, passes down through the bed 

 of sand, and out through the perforated double pipes at 

 G in the bottom of the filter. These pipes permit the 

 filtered water to flow out freely, but prevent the escape of 

 any of the sand with it. This filter has another device 

 which is different from the English system, and is an entirely 

 novel one. It is intended to thoroughly cleanse the upper sur- 

 face of the filter-bed by means of surface loashing. The dirt 

 and filth are deposited at the surface of the bed, and these are 

 swept off by a reverse current sent through the valve C, and 

 the washing pipe F, and escaping at the top of the filter through 

 the waste-pipe B. In the English filter-bed, the dirt is lodged 

 on the surface, very little being carried below. The operation 

 of cleansing is therefore that of shoveling off as thin a layer at 

 the top as can be taken off by a shovel, washing this top layer 

 in tanks, carting it back, distributing it over the surface, and 

 then starting the operation of filtration over again. In the 

 National Filter, this tedious work of cleansing, which in Eng- 

 land is all carried on by manual labor, is performed by a reverse 

 current. In the course of five minutes' time, the surface is 

 cleansed from the impurities lodged upon it during as many 

 hours of filtering very dirty water. Since the filter is worked 

 under pressure, a greater thickness of the sand is compelled to 

 do duty as the filtering medium, and the longer the time the 



