St. Louis, regulators for proposed filters, 55. St. Petersburg, filters at, 275. Samuelson, 51. Sand, 20. at Albany, 301. analysis of European, 25. analysis of, from leading works, 28. appliances for moving, 68. compactness of, in natural banks, 61. depth of, in filters, 34. depth to be removed from filters, 69. dune, 26. dune, washing of, impossible, 82. effect of grain-size upon frequency of scraping, 32. effect of grain-size upon the efficiency, 30. effective size of, 21, 238. extra scraping before replacing fresh, 71. for filtration, 20, 33. for mechanical filters, 175. friction of water in, 22. grain-size of, 20, 233. in European filters, 24. in Lawrence filters, two sizes of, 100. lime in, 29. method of analysis of, 233. quantity to be removed by scraping, 74. replacing, 71. selection of, 33. size of passages between grains of, 6. sterilized, experiments with, 85. thickness of layer, 34. uniformity coefficient, 21, 238. Sand washing, 26, 76, 304. cost of, 81. water for, 80. Sandstone filters for household use, 183. Schiedam, double filtration at, 273. Scraping filters, 7, 68. Scraping filters, amount of labor required for, 81. depth of sand removed, 33, 66, 69. frequency of, 49, 72, 241. Sedgwick, Prof. W. T., 86. Sediment, removal of, 92, 133. Sediment layer, 6, 31. influence of, upon bacterial purification, 84. thickness of, 33, 66, 69. Sedimentation basins, 8, 133, 293. effect of, 134. Sewage, number of bacteria in, 215. Simpson, James, 83. Soda-ash, application of, 157. Somersworth, N. H., filters at, 253. Storage for raw water, 136. Subsidence, limits to the use of, 142. Sulphate of alumina, action of, upon waters, 144. Surface-waters, use of, unfiltered, 275. Suspended matters, 113, 117. in relation to turbidities, 122. in various waters, 129. The Hague, iron removal at, 192. Tokyo, regulation of rate at, 58. Trenched bottoms for filters, 36, 40, 100. Turbidity, 92, 113. amount which is noticeable, 121. amount in several streams, 124. duration of, 128. in relation to suspended matters, 122. measurement of, 117. power of sand filters to remove, 139. preliminary processes to remove, 133. source of, 123. Typhoid fever in Berlin and Altona, 12, 85, 267. in American cities, 211. Typhoid fever in Hamburg, 271. in Lawrence, 102. in London, 259. in Zürich, 275. Typhoid-fever germs, life of, in water, 216.