_Concrete._—The walls of tanks made of concrete depend for their cost upon the cost of the material and the cost of the labor involved. It is usually more economical to use gravel as the basis of the concrete if any is available, and in order that the product may be of good quality it is always best to screen this gravel, separating it into sand and stone. The proper size of screen for this operation should be not greater than one-half-inch mesh. The stone and sand can then be re-combined with the cement in the proportion of one part of cement to two and a half parts of sand to five parts of stone, this mixture making a very strong and impervious combination. The cost of this mixture depends chiefly on the length of haul for the gravel and on the natural grading of the material. If the proportions required for concrete exist naturally in the bank or stream bed from which the gravel is to be obtained, there is little or no waste involved in screening, and the only cost is that of handling the material twice. If, on the other hand, the amount of stone is inadequate, it may be necessary to waste a good deal of the fine sand and enough material has to be shovelled to produce the required amount of coarse media. Assuming that the cost of shovelling the material from the stream bed is fifteen cents a cubic yard, and that the haul is two miles, so that four trips a day are made, then the gravel can be delivered where it is to be converted into concrete at a cost of one dollar for hauling and thirty cents for shovelling, while if the haul is only one mile, so that eight trips a day can be made, the cost will be eighty cents per cubic yard. If any waste of gravel is necessary, these costs will be increased correspondingly. At the site of the proposed plant the sand and gravel must be mixed with the cement and carried to place. It has been found by experience when the mixing is done thoroughly and by hand, and when the resulting concrete can be shovelled directly behind the forms, that the cost of the mixing and placing is about one dollar per cubic yard of concrete. If the concrete has to be wheeled into place this cost will be added to.