2. The possible sources whence the required quantity of water can be obtained, including the legal as well as physical limitations to its con tinued use.
3. The quantity of the water obtainable from each source, as regards its chemical constitu ents in order to judge of its economical as well as its sanitary value. It sometimes happens that a water otherwise acceptable is so hard or contains in solution such quantities of various chemicals that the amount of soap required for washing with it is greatly in excess of that re quired by another water more difficult to pro cure, so much so that the cost to individual consumers for its constant use will be greater than the cost to the community of the introduc tion of the other supply.
4. The character of the water biologically; the number and nature of the organisms which are found in it. This is most important from the sanitary point of view. The identification of certain organisms as pathogenic or disease producing marks a decided advance in sanitary science and is of great interest to the hydraulic engineer. It is found that the removal of the injurious organisms from water can be effected by filtration, and it now is considered to be the engineer's function to design or construct works for filtration through sand or other material which experiment proves to be adapted to the purpose.
5. The quantity of water obtainable from each source which appears generally suitable, the area of the watershed, the amount of an nual rainfall upon it, the distribution of the rainfall through the year, the geological and topographical features of the surface, the wind movement and the range of temperature of the air, with especial reference to the probable evaporation losses, must all be taken into con sideration. Whenever the minimum rate of daily run-off from the watershed exceeds the maximum daily consumption to be provided for, reservoirs must be constructed to retain the stream-water in times of excessive flow, and deliver the excess gradually as required.
6. The location of reservoirs requires a thorough acquaintance with the topography of the district, and their construction demands a high order of both theoretic and practical ac quaintance with the action of water on various materials and with the form and method of their use. The construction of dams of earth,
concrete and masonry has been the subject of careful study within the last few years, and the general principles have been 'fairly well es tablished. There still remains a number of un settled problems due largely to the progress of the mechanical arts and the introduction of new modes of construction, particularly of con crete and steel.
7. The means of conveying the water from the source to the point where it is to be used must be studied. The conditions of this prob lem are unlike those which are encountered in rivers, or in canals for either irrigation or navigation. A nearly uniform quantity of water must be continuously carried a long dis tance at as great a velocity as is consistent with safety and economy. It must be protected from loss by evaporation and by leakage of the channels, and kept free from pollution on its route. These conditions are best fulfilled by an enclosed conduit or pipe of masonry or metal, with a smooth interior surface. The relations between different values of n, which can be obtained by using different classes of material and of construction, and values of c, obtained by using different grades and alignments, have been for many years the subject of carefully conducted experiments by scientists. It is the function of the hydraulic engineer to apply to the special case he has in hand the use of the materials which are available and the mode of construction practicable in the case, in ac cordance with the latest results of scientific re search. Up to the present time the best form and material for large conduits seem to be ma sonry or concrete, of horseshoe form, with smoothly plastered interior surface; or steel plate circular pipes, with as few irregularities caused by rivet-heads as possible, sometimes lined with cement mortar and encased in con crete.