I would suggest the following rules as reasonably sure to lead to satisfactory results without making an altogether too lavish provision: In the absence of a definite determination to run filters at some other rate, calculate the drains for the German standard rate of a daily column of 2.40 meters, equal to 2.57 million gallons per acre daily. This will insure satisfactory work at all lower rates, and no difficulty on account of the capacity of the underdrains need be then anticipated if the rate is somewhat exceeded. The area for a certain distance from the main drain depending upon the gravel may be calculated as draining directly into it, provided there are suitable openings, and the rest of the area is supposed to drain to the nearest lateral drain. In case the laterals are round-tile drains I would suggest the following limits to the areas which they should be allowed to drain: Diameter of Drain. To Drain an Area not Corresponding Velocity of Exceeding Water in Drain. 4 inches 290 square feet. 0.30 foot. 6 inches 750 square feet. 0.35 foot. 8 inches 1530 square feet. 0.40 foot. 10 inches 2780 square feet. 0.46 foot. 12 inches 4400 square feet. 0.51 foot. And for larger drains, including the main drains, their cross-sections at any point should be at least 1/6000 of the area drained, giving a velocity of 0.55 foot per second with the rate of filtration mentioned above. The total friction of the underdrains from the most remote points to the outlet will be friction in the gravel, plus friction in the lateral drains, plus the friction in main drain, plus the velocity head. [Illustration: FIG. 4.—PLAN OF ONE OF THE HAMBURG FILTERS, SHOWING FRICTIONAL RESISTANCE OF THE UNDERDRAINS.] I have calculated in this way the friction of one of the Hamburg filters for the rate of 1,600,000 gallons per acre daily at which it is used. The friction was calculated for each section of the drains separately, so that the friction from intermediate points was also known. Kutter’s formula was used throughout with _n_ = 0.013. On the accompanying plan of the filter I have drawn the lines of equal frictional resistance from the junction of the main drain with the last laterals. My information was incomplete in regard to one or two points, so that the calculation may not be strictly accurate, but it is nearly so and will illustrate the principles involved. [Illustration: CONSTRUCTING THE UNDERDRAINAGE SYSTEM OF A FILTER, HAMBURG. [_To face page 42._] ] The extreme friction of the underdrains is 11 millimeters = 0.036 foot.