The rate of filtration with mechanical filters should be kept as constant as possible, and can be regulated by devices similar to those described in connection with sand filters. Owing to the smaller areas and capacities, the amounts of water to be handled in the units are smaller, and the regulating devices are thus smaller, and have always been made of metal, either cast iron or copper. None of the devices employed in the above-mentioned experiments has been entirely satisfactory in this respect. The devices employed have been too small, and the water has gone through at too high velocities to allow close adjustment. [Illustration: MECHANICAL FILTERS AT ELMIRA, N. Y. UPPER PLATFORM AND GENERAL ARRANGEMENT OF FILTERS. [_To face page 178._] ] As between the two types of filters, the Jewell filter requires a large loss of head. The water has to be pumped at a sufficient elevation to reach the top of a tank about 18 feet high, while the effluent must be drawn off at the extreme bottom. The Warren filter is much more economical in head, the plants at Pittsburg and Louisville only requiring about 9 feet from the inlet to the outlet. The earlier mechanical filters were usually constructed of wrought iron or steel plates. More recently wooden tanks have been commonly employed, although steel is regarded as preferable. Concrete or masonry tanks have been suggested, but they have not as yet been employed. EFFICIENCY OF MECHANICAL FILTERS. The efficiency of mechanical filters depends entirely upon the use of coagulants. Without coagulants they can only be used to remove very large particles. The efficiency of the filtration depends much more upon the kind, and amount, and method of application of coagulant than upon the arrangement of the filter. In fact, the arrangements of the filter are more directed to the convenience and economy of operation and washing than towards the efficiency of the results. The conditions which control the efficiency of mechanical filters have been discussed in connection with coagulation. With sufficient coagulant the removal of turbidity or mud is complete. Color also can be removed with these filters. The bacterial efficiencies secured with them have been discussed at length in connection with the Pittsburg experiments. With careful coagulation and manipulation it is possible to get 98 per cent bacterial efficiency without difficulty. The results are somewhat irregular, for reasons not as yet fully understood. On some occasions higher bacterial efficiencies are secured with smaller quantities of coagulant, while at other times the efficiencies are less without apparent reason. There seems to be a limit to the bacterial efficiency which can be secured with any amount of sulphate of alumina and rapid filtration, and it is doubtful if a plant could be operated to regularly secure as high a bacterial efficiency as 99 per cent with any amount of sulphate of alumina. PRESSURE FILTERS.