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 CHAPTER XXII. 264. Railroad Signalling 335 265. The Pilot Guard 335 266. The Train-Staff 335 267. First Basis of Railroad Signalling 336 268. Code of American Railway Association 337 268_a_. The Block 338 269. Three Classes of Railroad Signals 338 270. The Banner Signal 338 271. The Semaphore 340 272. Colors for Signalling 340 273. Indications of the Semaphore 341 274. General Character of Block System 342 275. Block Systems in Use 343 276. Locations of Signals 344 277. Home, Distant, and Advance Signals 344 278. Typical Working of Auto-controlled Manual System 345 279. General Results 348 280. Distant Signals 349 281. Function of Advance Signals 349 282. Signalling at a Single-track Crossing 350 283. Signalling at a Double-track Crossing 352 284. Signalling for Double-track Junction and Cross-over 352 285. General Observations 353 286.