There are several empirical rules and formulas for calculating the run off from catchment areas and for determining the size of spillways necessary to discharge this flow with safety to the dam. The proper formula to apply in any given case, with the varying coefficients of each, involves a thorough knowledge on the part of the designing engineer of the principles upon which the different factors are based. It is unwise and often hazardous to make use of any important hydraulic formula without knowing the history of its derivation. Experiments are not always properly conducted, and often the deductions therefrom are unreliable. A presentation and discussion of these formulas would require more space than can be given in this study, and the technical reader must therefore consult for himself, as occasion may require, the various authorities cited. Formulas for the discharge or run-off from catchment areas, as determined by Messrs. Craig, Dickens, Ryves and others, are given by most writers on the subject of hydraulics. Reservoir Basin. The next subject of inquiry relates to the reservoir basin. It is necessary that its area and capacity at different depths should be definitely known, and this information can only be obtained by having the basin surveyed and contoured. A map should be made showing contours at intervals of 2 to 10 ft., depending upon the size of the basin and the use to which the reservoir is to be put. Reservoir basins have been classified according to their location as follows: 1. Natural lakes. 2. Natural depressions on main drainage lines. 3. Natural depressions on lateral drainage lines. 4. Arbitrary and artificially constructed basins. Natural lakes may need to be investigated more or less thoroughly to determine the character of their waters, whether saline, alkaline or fresh. It may also be necessary to know their normal depth and capacity, and to make a study of their outlet if they have one. In some instances the storage capacity of a lake may be enormously increased by means of a comparatively low and inexpensive embankment. The area of reservoir basin, mean depth, temperature of the water, exposure of wind and sunshine, losses by seepage and evaporation, all have a bearing upon the available water supply and influence the design of the dam and accessories to the reservoir.