Chapter XX

211. Sanitary Improvement of Public Water-supplies 276
212. Improvement by Sedimentation 277
213. Sedimentation Aided by Chemicals 279
214. Amount of Solid Matter Removed by Sedimentation 279
215. Two Methods of Operating Sedimentation-basins 279
216. Sizes and Construction of Settling-basins 280
217. Two Methods of Filtration 281
218. Conditions Necessary for Reduction of Organic Matter 282
219. Slow Filtration through Sand—Intermittent Filtration 283
220. Removal of Bacteria in the Filter 286
221. Preliminary Treatment—Sizes of Sand Grains 286
222. Most Effective Sizes of Sand Grains 288
223. Air and Water Capacities 288
224. Bacterial Efficiency and Purification—Hygienic Efficiency 290
225. Bacterial Activity near Top of Filter 290
226. Rate of Filtration 291
227. Effective Head on Filter 291
228. Constant Rate of Filtration Necessary 292
229. Scraping of Filters 293
230. Introduction of Water to Intermittent Filters 294
231. Effect of Low Temperature 294
232. Choice of Intermittent or Continuous Filtration 294
233. Size and Arrangement of Slow Sand Filters 295
234. Design of Filter-beds 296
235. Covered Filters 299
236. Clear-water Drain-pipes of Filters 299
237. Arrangement of the Sand at Lawrence and Albany 300
238. Velocity of Flow through Sand 302
239. Frequency of Scraping and Amount Filtered between Scrapings 303
240. Cleaning the Clogged Sand 303
241. Controlling or Regulating Apparatus 305
242. Cost of Slow Sand Filters 307
243. Cost of Operation of Albany Filter 308
244. Operation and Cost of Operation of Lawrence Filter 309
245. Sanitary Results of Operation of Lawrence
and Albany Filters 310
246. Rapid Filtration with Coagulants 311
247. Operation of Coagulants 312
248. Principal Parts of Mechanical Filter-plant—Coagulation
and Subsidence 313
249. Amount of Coagulant—Advantageous Effect of Alum
on Organic Matter 314
250. High Heads and Rates for Rapid Filtration 315
251. Types and General Arrangement of Mechanical Filters 316
252. Cost of Mechanical Filters 318
253. Relative Features of Slow and Rapid Filtration 318

PART IV.
_SOME FEATURES OF RAILROAD ENGINEERING._

CHAPTER XXI.
254. Introductory 320
255. Train Resistances 322
256. Grades 322
257. Curves 324
258. Resistance of Curves and Compensation in Grades 324
259. Transition Curves 325
260. Road-bed, including Ties 327
261. Mountain Locations of Railroad Lines 328
262. The Georgetown Loop 331
263. Tunnel-loop Location, Rhætian Railways, Switzerland 331