basin or gallery. Secondary objects may be means to increase yield, the providing of a convenient chamber for the connection of outlet pipe, and in some cases the providing of a settling basin.

Infiltration galleries. Infiltration galleries are horizontal galleries or pipes constructed below the water-table and provided with numerous small openings to admit the water. Their practical application is usually limited to depths suitable for excavation in open cut. They should be located well below the minimum level of the water-table so that ample head will be available, after making allowance for some probable clogging, to produce flow. Graded gravel and sand are usually placed around the openings, to exclude fine material. Masonry and vitrified pipe, being permanent and uninjured by water, are preferable to other materials. Access or inspection manholes and valves for cutting off various sections are desirable adjuncts. In some cases where relatively narrow underground streams occur, underground dams of sheet-piling or masonry walls are constructed immediately below the galleries, to make possible the interception of a greater proportion of the underflow. Galleries have frequently been constructed near surface streams for the purpose of obtaining stream-water filtered through the ground. Many of these have been disappointing, due either to low initial yield, gradual clogging of media or insufficient filtration. Porous substrata are necessary, but do not assure the permeability of the stream bed unless floods keep the bottom scoured clean of fine deposit.

The iron, present in most soils and gravels, is readily soluble in water from which the oxygen has been exhausted by the organic matter originally present in, or taken up, by the water on entering the ground through a muck-covered stream bed. When the water is again aerated by trickling down the surface slope of the cone of depression, this iron in insoluble form is deposited in the sands adjacent to the galleries or wells or in the well screens, and clogging and limitation in yield result.

The yield of galleries is about proportional to the length and to the amount of lowering of water-plane. Depending upon the lowering and upon the character of media the yield per minute per foot of length may vary from a small fraction of a gallon to several gallons. The advantages of galleries over a line of wells are that the yield for equal lowering is somewhat increased, the gallery takes

the place of a collecting pipe, and the water after collection may be pumped with greater economy than by well-pumps. The disadvantages are that the first cost is high, the amount the water plane may be lowered is limited by the elevation of the gallery, the yield when once deteriorated cannot readily be improved and the system lacks flexibility in arrangement and operation.

Dug wells. Dug wells are relatively shallow, masonry-walled wells of diameters varying from 5 to 100 feet, built in excavations made from the surface. A common method is to sink the well by excavating inside of a circular steel shoe, on which the masonry wall is built as the well descends. The main supply is usually derived from the bottom which is left open, although openings in the sides are sometimes added. The bottom should be carried some distance below lowest ground-water level, to provide storage. The yield increases slowly and the cost rapidly with increase in diameter. The top of a dug well should be above the ground surface and surrounded with earth sloping away from the well. A tight cover, preferably of concrete, should be provided to exclude dirt and prevent vegetable growths in the water. The advantages of dug wells over other types are storage capacity, convenience as pump-suction wells and somewhat increased yield. The disadvantages are high first cost, limited practicable depth and limited amount that water level may be lowered.

Driven, drilled and bored wells. An ordinary driven well consists of steel pipe, usually 1 to 4 inches in diameter, to the lower end of which a suitable point and a strainer are attached. It is driven by a hammer or by jetting and is seldom used for depths over 150 feet. Drilled wells are wells sunk in consolidated rock by means of special cutting tools. In the soft formations above the rock the wells are cased with steel pipe or casing. Bored wells are those sunk in unconsolidated formations by suitable boring tools. They are provided with casings set as, or after, the well is bored, and are equipped with screens or strainers set in the water-bearing medium. Drilled and bored wells are adaptable to much greater diameters and depths than driven wells. Selection of one of the three types is usually dictated by character and depth of material to be penetrated and by the desired size of well.

Hydraulics of wells. When a well tapping a non-artesian waterbearing stratum is pumped, the water table is lowered with a "cone

of depression" immediately adjacent to the well and a secondary depression which decreases with increased radius and disappears at the limit of the "circle of influence." If the well is artesian in character the pressure line assumes the same general form, although the water may not be lowered below the top of the water-bearing stratum. In either case the lowering continues and the circle of influence widens until the inflow from the stratum without the circle equals the amount of water pumped. (If the quantity of water naturally flowing in the stratum does not at least equal the pumpage, the lowering and widening effects will continue indefinitely, being limited only by the extent of the stratum.) If the water-bearing stratum is porous the ultimate amount of depression is approximately proportional to the pumpage rate, and this fact is important in estimating effects of changes in conditions. In fissured limestones the draw-down varies more nearly as the square of the pumpage rate. When a number of closely-spaced wells in the same stratum are pumped simultaneously their circles of influence overlap, and their total yield for a given draw-down is much less than the sum of their yields when pumped separately. The amount of this mutual interference depends upon specific data. The effect is illustrated in the following example, which is based upon theoretic analysis by Slichter: Interference of 6-inch wells, radius of circle of influence 600 feet pumped down 10 feet

Specific capacity of wells. The "specific capacity" of a well is the yield in gallons per minute divided by the amount, in feet, that the water-surface is drawn down. A specific capacity of 100 is high, 20 is moderate and 5 is low. Values of 100 or higher may be obtained only in thick strata of coarse sands or in gravels.

Factors affecting yield of wells. The potential yield of a well is determined principally by the capacity of the water-bearing medium to transmit water to the sides of the well, assuming of course that yield is not limited by depletion of natural supply in the stratum.

This transmitting capacity depends upon the character and thickness of the stratum and upon the amount that the water level may be drawn down.

The direct effect of varying the diameter is relatively small. The indirect effect of increasing diameter may be large, as in cases when the yield is limited by the size of the pump that may be placed in the well, or by insufficient strainer area. In deep, artesian wells the restricting of friction losses due to upward flow in the well may partly dictate diameter. In sand strata, high velocities clog the strainer openings and rearrange and compact the particles near the strainer, progressively decreasing the yield. This action is much less marked when velocities are moderate.

Gravel-packed wells. The yield and specific capacity of wells in unconsolidated sands may often be increased, and clogging troubles greatly reduced, by removing the sand from around the strainer and substituting selected, graded gravel. A coarser strainer is then used and the effective diameter of the well becomes that of the pocket of gravel. In fine sands the improvement is often much more than would appear likely from mere consideration of relative diameters, due, no doubt, to the fact that the velocity of water leaving the sands is reduced below the velocity at which finer particles are picked up. In some cases the gravel deposit extends out several feet from the strainer.

The general method has been used on wells of all depths up to several hundred feet. The procedure varies, but generally involves the pumping out of sand through an inner casing and the simultaneous feeding downward of gravel between the inner and an outer casing. The method is most applicable to shallow wells in fine sands. In coarse sands it effects little improvement and in deep wells it is costly.

Strainers. In unconsolidated materials some form of strainer is required to exclude the particles of sand and gravel from the well and to allow water to enter freely. In coarse gravel relatively large perforations in the lower part of the casing may suffice. More often the materials are finer and special strainers are necessary. These may be of brass wire mesh, but are usually cylinders of brass tubing containing a multitude of narrow, horizontal or vertical slots of any desired width. The openings are usually flared toward the inside of the strainer, to diminish clogging tendency. The outer width of openings varies from 0.004 inch upward according to the fineness

of the water-bearing medium. The net total area of strainer openings should be such that velocities through the opening will not exceed 2 or 3 inches per second.

In shallow wells of large diameter, when the water-bearing medium is gravel, or when gravel-packing is used, strainers are sometimes made of porous concrete. These consist of uniform gravel containing enough cement to bind the particles without filling the voids. Their advantage is freedom from corrosion.

Safeguards against pollution. Surface washing and the groundwater for a few feet below the surface are actual or potential sources of pollution and should be carefully excluded from wells. They may enter by direct overflow at the well top, through leaky casings or by working down to the water-bearing stratum in the space sometimes left outside the casing. Casings should be of considerable thickness to resist corrosion, should have tight joints, and should either be carried somewhat above the ground level or terminate in tight concrete pits whose sides extend above ground level. If wells are in a stream valley, flood levels should be investigated and casings. carried above them. Casings of drilled or bored wells should extend down to the top of the water-bearing formation and the annular space outside the casing should be sealed with grout, puddle or fine sand, the means varying with local conditions. Gravity pipe lines used for the collection of water from wells should be tight and preferably made of cast iron. Leaky vitrified collecting pipes caused the notable typhoid epidemic at Salem, Ohio.

Ground vs. surface water. Compared with unfiltered surface water supplies, the relative advantage of ground water supplies as a class are that the natural water has low bacterial content, little color, uniformly low temperature and is clear and generally free from industrial wastes. The relative disadvantages are that (1) the water is likely to be hard or high in other objectionable dissolved minerals or gases, (2) the quantity available in most localities is limited, (3) works for obtaining ground water are subject to comparatively rapid depreciations, (4) methods of pumping from wells are relatively inefficient and (5) more or less speculation is involved in predicting such factors as the ultimate supply capacity of a stratum, the yield per well, the pumping lift and the amount of maintenance work required. The comparison is general and in a specific case it is suggested that the points mentioned be considered and proved or disproved separately.