Older Filters 52 Apparatus For Regulating the Rate and Loss of Head 55 Apparatus For Regulating the Rate Directly 57 Apparatus For Regulating the Height of Water Upon Filters 59 Limit to the Loss of Head 60 V. CLEANING FILTERS 68 Frequency of Scraping 72 Quantity of Sand to Be Removed 74 Wasting the Effluents After Scraping 74 Sand-washing 76 VI. THEORY AND EFFICIENCY OF FILTRATION 83 Bacterial Examination of Waters 93 VII. INTERMITTENT FILTRATION 97 The Lawrence Filter 100 Chemnitz Water-Works 107 Application of Intermittent Filtration 111 VIII. TURBIDITY AND COLOR, AND THE EFFECT OF MUD UPON SAND FILTERS 113 The Measurement of Color 114 Amount of Color in American Waters 115 Removal of Color 117 Measurement of Turbidity 117 Relation of Platinum-wire Turbidities to Suspended Matters 122 Source of Turbidity 123 The Amounts of Suspended Matters in Water 129 Preliminary Processes to remove Mud 133 Effect of Mud upon Sand Filters 137 Effect of Turbidity Upon the Length of Period 137 Power of Sand Filters to Produce Clear Effluents from Muddy Water 139 Effect of Mud Upon Bacterial Efficiency of Filters 141 Limits to the Use of Subsidence for the Preliminary Treatment of Muddy Waters 142 IX. COAGULATION OF WATERS 144 Substances used for Coagulation 145 Coagulants Which Have Been Used 150 Amount of Coagulant required to remove Turbidity 150 Amount of Coagulant required to remove Color 153 Successive Applications of Coagulant 154 The Amount of Coagulant which Various Waters will