WATER-SUPPLY.

BY EZRA M. HUNT, M. D.

The question of water-supply can never cease to be of prominent importance in all considerations of public health and comfort. The necessity that it shall be abundant and easily accessible is imperative for purposes of cooking, of cleanliness, of provision against fires, of manufactures, and for use as a drink. As a great financial and economical question, it has to do with general material interests, while, in its bearings on personal health, accurate and correct knowledge as to it is of the first importance.

As the original fountain of all water-supply is above the earth, instead of in it, it is a very natural question why we do not gather it as it comes fresh from the clouds, instead of drawing it back again from the ground. There is so much force in the inquiry as to have led many to claim that not sufficient reliance is placed upon rain-water for potable or drinking-water use.

Denton, in his work on sanitary engineering, says: "I hold the opinion that, in fact, there exists no more certain source of a pure and sufficient supply than that of properly collected and properly filtered rain-water, which is, with care, to be secured by all persons alike.

"Rain-water collected from clean surfaces is itself so free from pollution that it requires filtration only to protect the consumer against the accidental defilements of mold, soot, and those minor organic impurities which occasionally collect on roofs."

Even the necessity of filtering depends very much on locality. The purest natural water is rain-water. Whilst, in its descent, it comes through the air, the amount of mineral or organic matter with which it comes in contact is very small, except in cities dense with the smoke from manufactories. It is both theoretically and practically far less than when it is drawn from the ground. It has advantages of oxida

tion and purification fully equal to those occurring in the soil. It is especially free from organic matter. While we do not propose to give it undue prominence over all other sources of supply, yet there must ever be, in parts of this State, much reliance upon it. There will always be places where "rain-water from roofs, or prepared impermeable surfaces, constitute the only source of supply for separate dwellings." We, therefore, briefly outline the mode of its collection and preservation. It is best collected on slate roofs. The leader should always be so arranged as that the first rain can wash off the roof and not discharge into the cistern. Two or three automatic arrangements are used for this purpose. This prevents any fouling from the dust of roofs, the excrements of birds, from leaves, or from the "cellulose or weather-beat" of shingle roofs, if they are relied on. If a leader ends in a hogshead or tank proportioned in size to the roofs, it can receive the first washing, and, when full nearly to the top, an overflow into the permanent cistern will carry off the pure incoming stream, and leave the former to be used for non-drinking purposes. It is best, also, to have the mouth of the leader, as it leaves the roof, protected by a copper gauze, or a galvanized wire covering, so as to prevent any lodgment of leaves, etc.

Whether the cistern shall be near the roof in some upper room, or whether it shall be in the ground, will depend much upon convenience and locality. If near the roof, it should be well built, preferably in a circular form, or, if square and lined, should have such lining as will not furnish lead or copper or too much iron to the water. The overflow should be so arranged as not, when its pipe is empty, to be an open tube to convey foul gases to the water. It, therefore, should not enter into the general soil pipe. The cistern, while constructed so as to be accessible for cleansing and while generally needing a covering, should not be so made as to confine stifled air over the water, but admit of some circulation. As water kept near the roof is apt to become very warm in summer, some devices similar to those used for the preservation of ice are sometimes resorted to for keeping the cistern cooler. Most prefer a cistern in the ground, which then should be deep enough to keep cool in summer and not to freeze in winter, or to be cracked by the action of the frost. As the weight of water is ten pounds to the gallon, the receptacle for any large amount needs to be made strong. Cast-iron or wrought-iron tanks, properly painted or dipped, after the Angus Smith method, are now often used. The circular form is generally the best, as it gives the most strength. The capacity

of the cistern should be ample, as it is best to store the water of long rains rather than that of occasional summer showers. The reason for this is that the first rain-water washes out the impurities in the air and upon roofs. roofs. A tank or cistern holding one thousand gallons would be contained in a space six feet square and four and one-half feet deep, or in one of five feet square and six and one-half feet deep, or in a circular cistern of five feet in diameter and fifteen feet in depth. If we reckon the average rainfall at thirty inches, or seventeen gallons a square foot, and allow a loss of six inches for the first water and short rains, and six inches for evaporation, "there would be left on the average roof of three hundred and sixty square feet, available for storage, five hundred and forty cubic feet of water, or three thousand three hundred and seventy-five gallons in the year, which, for the house, would be an average daily supply of nine gallons." "A tank sixteen feet long and ten feet wide will hold one thousand gallons in every foot of depth." The building of a cistern in the attic needs to be well done in order to prevent leakage. When made in the ground, much will depend on the soil. If a clay, the cement is sometimes applied directly thereto after an accurate circular excavation has been made. The cementing is mostly done on brickwork, laid in the best of mortar. Where bricks are used the coating of cement should not be less than one-half inch in thickness. Where the cement is applied directly to the sides of the excavation it is usual to put on three coats, the whole being not less than one inch in thickness. As cracking of the cement would not only cause leakage of the water, but also its possible contamination from outside sources, the cistern must be made in the very best manner. . The top is usually covered with a stone flag or cast-iron plate, large enough to serve as a man-hole and air-hole. Such cisterns do not need frequent cleansing, but need examination occasionally as to their condition. Some prefer to build two smaller cisterns close to each other, relying upon one for the potable or drinking-water, and the other for the general supply. It is easy to arrange the inflow leader so as to shift it to the drinking water-supply and thus make selection of the time of filling.

FILTERS.

As the subject of filtration comes up in connection with rain-water as also with other waters, we shall here say much that is applicable to

all forms of artificial filtering. Its design is often three-fold. First, the removal of all foreign particles in suspension. The retention of dissolved matters which are in solution or too minute for the first process of straining. The aeration of the water, or a process of oxidation, by which actual change is wrought upon organic matter in the water. To this might be added sedimentation, which is merely the settling of particles which, being of a higher specific gravity, or greater weight than the water itself, settle to the bottom without any real filtration.

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The first process is purely that of mechanical separation or straining, which, by furnishing some fine porous substance, separates many the finer particles which would not settle soon or at all to the bottom by sedimentation. In large reservoirs settling basins are often used for this form of sedimentation. Even if filters have to be used, the opportunity given for the settling of the coarser particles makes the subsequent filtration more effectual.

It is seldom necessary to use such coarser methods for cisterns if the roofs and leaders are properly cared for, although the settling of such particles to the bottom of the cistern is a reason why no pipes of outflow should be as low as the bottom of the cistern, and also a reason for occasional thorough cleansing, or at least yearly.

The second method of action of a filter has been called that of adhesion. Prescott illustrates it thus: "A solution of organic coloring matters, though so perfectly free from suspended solids as to show no particles under the microscope, when passed through certain porous substances, leaves the coloring matter behind. The capillary attraction of the porous surfaces for the dissolved solids takes them out of solution. Dissolved gases are, to some extent, withdrawn from solution in the same way." The process seems to depend upon the fact that particles of sand, charcoal, or other substance, are so close to each other that no rills of water can flow between them, but only drops or minute capillary currents of the water. It is then brought directly in contact with the adhesive or absorptive surfaces of the material used, and so the very finest particles and the dissolved solids are retained.

If there is too much weight of water it is pressed too rapidly through the filter and so is not allowed time for this slower action. It is a rule in filter-beds not to have over two feet of water.

The third process, that of oxidation, although regarded by most as distinct from this, is closely associated with it. The water thus passed

minutely through adjacent surfaces, itself gets greater capacity for air admixture or adhesion, while the minute particles of sand or charcoal also allow much air between their surfaces. Thus the suspended or dissolved particles are especially exposed to air and undergo that process of oxidation by which organic material is destroyed. "The oxygen condensed by adhesion in the pores is extra active." By the minuteness of the particles of the filtering material and by compelling the water and the air to be jostled about in minute currents amid multitudes of solid inorganic particles, you get that motion which is always favorable to oxidation, and therefore to the removal of all changeable or decayable matter from the water. So not only is this removed but when removed the air and water aid in its destruction, and so help to preserve the filtering-bed in action. Yet, as often more air than is supplied by the water or by the aid in the filter material is needed, as a rule filter-beds should not be so constantly covered as to prevent access of the atmospheric air, for it is one of the conditions of a good filter that it should expose the organic matter which it catches to the largest possible amount of atmospheric air both during the act of its catching it and at intervals between, by the free access of air thereto. Sand, although valuable as a mechanical separater, is too porous to exclude all suspended matter, and if used alone, too fine by reason of its compactness, and does not favor a capillary flow either of the water or the air. Animal charcoal, and especially granulated animal charcoal, being closer in texture on the surface and more open in its particles, has a more valuable porosity, except that it is more difficult to make the water pass actually through its substance."Due care of a filter requires that all suspended matter, i. e. floating particles, should be removed before the water reaches the filterer." Charcoal probably owes more of its value as a filterer to the minuteness of its pores or interstices through which the water in its minute circulation is brought into contact with air, than to any other property. Those who construct filters need to bear in mind such facts so as to adjust mechanical arrangements and the use of materials.

Filters also have a certain value in the more general aeration of the water. Thus, water which has been boiled and has a deadened taste, although free from organic matter, by subsequent filtration through aerated material has this overcome. The importance of occasional access of air to filters is such that Denton says that "all filtration in