CHAS. E. FOWLER. 279 



fore treatment 4.3, after, average of two, 1,5. I was 

 urged to enquire of Dr. Cyrus Edson, late of the Board 

 of Health of New York City, as to its efficiency and did 

 so. He replied saying that he had caused many 

 analyses, chemical and bilogical, to be made of water 

 treated by this process, and although he had not pre- 

 served them he could assure me the results were most 

 satisfactory, particularly from a bacterialogical stand- 

 point, the water so treated being rendered absolutely 

 sterile. 



At the present time there seems to be but one practi- 

 cally efficient means of purifying water on the large 

 scale, as practiced in this country and Europe, namely, 

 filtration through sand. 



Two methods of sand filtration are in use, to wit : 

 rapid, commonly known as mechanical, filtration, and 

 slow or natural filtration. 



The first or rapid method is largely in use in this 

 country but, as yet, seems to have found little favor 

 abroad. In this method the sand is placed in tanks of 

 iron or wood to a depth of from four to six feet. Perfo- 

 rated pipes, or other appliances, are arranged on the bot- 

 tom of the tank so as to retain the sand and yet permit 

 a free exit of the filtered water. They are usually 

 operated under a greater or less pressure or head of 

 water, the top of the tanks being closed. The water is 

 forced through the sand at a rapid rate, usually about 

 seventeen feet, vertically, per hour and filtering about 

 3,000 gallons per square foot in twenty-four hours, de- 

 pending of course upon the condition of the applied 

 water. 



With this high rate the accumulation of silt and other 

 impurities is very rapid and the sand soon (in twenty to 

 thirty hours) becomes so clogged that the required quan- 

 tity of water will not pass through. It is necessary, 

 therefore, that some means be provided by which the 



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