The third epoch is marked by the experiments at Louisville, Pittsburg and Cincinnati, which have greatly increased our knowledge of the treatment of waters containing enormous quantities of suspended matter, and have reduced to something like order the previously existing confused mass of data regarding coagulation and rapid filtration. The first edition of this book represented the earlier epochs before the opening of the third. In the five years since it was written, progress in the art of water purification has been rapid and substantial. No apology is needed for the very complete revision required to treat these newly investigated subjects as fully as were other matters in the earlier editions. In the present edition the first seven chapters remain with but few additions. Experience has strengthened the propositions contained in them. New data might have been added, but in few cases would the conclusions have been altered. The remaining chapters of the book have been entirely rewritten and enlarged to represent the added information now available, so that the present edition is nearly twice as large as the earlier ones. In the appendices, also, much matter has been added relating to works in operation, particularly to those in America. NEW YORK January, 1900. CONTENTS. PAGE Chapter I. INTRODUCTION. 1 II. CONTINUOUS FILTERS AND THEIR CONSTRUCTION 5 Sedimentation-basins 8 Size of Filter-beds 10 Form of Filter-beds 11 Covers for Filters 12 III. FILTERING-MATERIALS 20 Sand 20 Sands Used in European Filters 24 Effect of Size of Grain Upon Efficiency of Filtration 30 Effect of Grain Size Upon Frequency of Scraping 32 Selection of Sand 33 Thickness of the Sand Layer 34 Underdraining 35 Gravel Layers 35 Underdrains 39 Depth of Water on Filters 45 IV. RATE OF FILTRATION AND LOSS OF HEAD 47 Effect of Rate Upon Cost of Filtration 48 Effect of Rate Upon Efficiency of Filtration 50 The Loss of Head 52 Regulation of the Rate and Loss of Head in the