The writer, having had occasion in his professional practice to study quite carefully the subject of ground waters, and their percolation or flow through different classes of materials and under varying conditions, is of the opinion that rarely does the cross-section of a stream-channel, filled with sand, gravel and debris, present, even approximately, a homogeneous or uniform mass; and that there are, almost without exception, strata of material much coarser and more porous than the general average. In other words, that it is extremely difficult to arrive at a uniformity coefficient. It is unwise to place much reliance upon an estimated flow where this is the case. The formula may be used with confidence where the layers are artificially made, and where there is no uncertainty regarding the uniform character of the material. In most natural channels there are distinct lines of flow, and under considerable hydrostatic head or pressure these lines of flow would surely enlarge. There is a wide difference between permissible and dangerously excessive percolation through an earth embankment. The local features, economical considerations and magnitude of the risks, all bear upon this question and must be considered for each particular case. It is of interest to compare the estimated cost of the three designs proposed for the Bohio Dam, based upon the same unit prices, as follows: French Engineers’ design $3,500,000 Isthmian Canal Commissioners’ design 8,000,000 Mr. Morison’s design 2,500,000 No comments will be made upon these figures, further than to remark that the successful building of a stable dam, accomplished by the use of an excessive quantity of materials and at a cost beyond reasonable requirements, is mainly instructive as illustrating “how not to do it.” It is creditable to execute substantial works at a reasonable cost, but it reflects no credit upon any one to construct them regardless of expense. Combined Rock-fill and Earth Dam. Fig. 30 shows a section of the Upper Pecos River Dam near Eddy, N. M. This dam is quite fully described by Mr. Jas. D. Schuyler, in his recent book on “Reservoirs for Irrigation, Water-Power and Domestic Water-Supply,” and need not be mentioned in this paper, further than to call attention to the combination of rock-fill and earth which constitutes its particular type of construction. This type of dam is believed to be for many localities a very good one, but up to the present time has only been adopted for dams of moderate height, under 60 ft. The San Leandro Dam, California.