The subject of sewage disposal for a single house in the country does not at all present the elaborate problem that is suggested when the disposal of sewage of a city is under discussion. In the first place, the amount of sewage to be dealt with is moderate in quantity; and in the second place the area available on which the sewage may be treated is in almost all cases more than ample for the purpose. Nor is there the complication that arises with city sewage, due to the admixture of manufacturing wastes. The material to be handled is entirely domestic sewage and varies only according to the amount of water used in the house, making the sewage of greater or less strength according as less or more water is used. Sewage from a single house differs only in one respect disadvantageously from city sewage, namely, in the fact that the sewage, not having to pass through a long length of pipe, comes to the place of disposal in what is known as a fresh condition; that is, no organic changes have taken place in the material of which the sewage is composed.

Definition of sewage.

The great bulk of sewage is water, and, in quantity, the amount of sewage to be cared for is about equal to the amount of water consumed in the household, although this will depend somewhat on the habits of the family. If, for example, part of the water-supply is used for an ornamental fountain in the front yard, or if in the summer time a large amount of water is used for sprinkling the lawns, that water is not converted into sewage, and the amount of the latter is thereby diminished; but, ordinarily, it is safe to say that the quantity of water supplied to the house and the quantity of sewage taken away from the house is identical, and since it is much easier to measure the water-supply than the sewage flow, the former is taken as the quantity of sewage to be treated.

In the course of its passage through the house, however, the water has added to it a certain amount of polluting substances, largely derived from the kitchen sink, where dirt from vegetables and particles of vegetable material, together with more or less soap, are carried by the waste water from the sink into the drain. In the bath-room, also, some small amount of organic matter is added to the water, but the proportion of such matter to the total volume of water used is very small, probably not exceeding one tenth of one per cent. This small proportion is nevertheless sufficient to become very objectionable if allowed to decompose, and the problem of sewage disposal for a single house is to drain away the water, leaving behind the solids so disposed that they shall not subsequently cause offense by their putrefaction.

The process of decay is normal for all organic matter and is due to the agency of certain bacteria whose duty it is, providentially, to eliminate from the surface of the earth organic matter which otherwise would remain useless, if not destructive, to man. It is impossible to leave any vegetable or animal matter exposed to the air without this process of decay at once setting in. Apples left in the orchard at the end of the season inevitably are reduced and disappear in a short time. Dead animals, whether large or small, in the same way succumb to the same process of nature, and it has been pointed out that, unless this provision did exist, the accumulation of such organic wastes since the settlement of this country would be so great as to make the country uninhabitable. Fortunately, however, this inevitable process breaks down the structure of all organic material, partly converting fiber and pulp into gas, partly liquefying the material and converting the remainder into inorganic matter which is of vast importance as food for plant life. A cycle is thus formed which may be best illustrated in the case of cows which feed on the herbage of a meadow, the manure from the cows furnishing food for the grass which otherwise would soon exhaust the nutriment of the soil.

Stream pollution.

The first fundamental principle of sewage disposal, therefore, is to distribute the organic matter in the sewage so that these beneficent bacteria may most rapidly and thoroughly accomplish their purpose. During the last fifty years, a great deal of study has been expended on this problem, and while it has not as yet been entirely solved, certain essential features have been well established.

The most important factor promoting the activity of these agents of decay is the presence of air, since in many ways it has been proved that without air their action is impossible. Thus it has been shown that discharging sewage into a stream, whether the stream be a slow and sluggish one or whether it be a mountain stream churned into foam by repeated waterfalls, has little other power to act on organic matter than to hold it for transportation down stream, or to allow it to settle in slower reaches until mud banks have been accumulated which will be washed out again at the first freshet. Experiments have shown that the agencies to which certain diseases are attributed, commonly known as pathogenic bacteria, are frequently, if not always, found in sewage, and that when these bacteria are discharged into streams they may be carried with the stream hundreds of miles and retain all their power for evil, in case the water is used for drinking purposes. No right-minded person to-day will so abuse the rights of his fellow-citizens as deliberately to pour into a stream such unmistakable poison as sewage has proved itself to be. The fact is so well known that it is not worth while pointing out examples. It is enough to say that some of the worst epidemics of typhoid fever which this country has known have been traced to the agency of drinking water, polluted miles away by a relatively small amount of sewage.

In a number of states, laws have been passed which expressly prohibit the discharge of sewage, even from a single house, into a stream of any sort, even though the stream is on the land of the man thus discharging sewage and where it would appear as if he alone might control the uses of that stream. Unfortunately, the machinery of the law does not always operate to detect and punish the breakers of the law, but any law which, as in this case, has so positive a reason for its existence, and violation of which is so certain to bring disaster on persons drinking the water of the stream below the point where the sewage is discharged, any law which appeals for its enforcement so directly to the common sense and right feeling of all intelligent people, seems hardly to need legal machinery for its enforcement. It must depend, as indeed all laws must depend, upon the intelligent support of the community, and surely no law would commend itself more urgently than this one forbidding the pollution of drinking water.

In spite of the fact that the lack of air in the water will prevent bacterial action, there are, nevertheless, many cases where the discharge of sewage into a stream may be permitted as being the best solution of the disposal problem, provided always that the stream is not used and is not likely to be used for drinking water. Such cases occur where the stream is relatively large and where the level of the stream is fairly regular, so that there is no likelihood of the deposit of organic matter on the banks during the falling of the stream level. Examples of this sort might be cited in the vicinity of the Mohawk or Hudson River, or in the vicinity of any of the larger rivers of any populous state, since although the water of the Mohawk is used by the city of Albany for drinking purposes, yet the amount of organic matter which inevitably finds its way into such rivers precludes its use for drinking without filtration. Into the Hudson below Albany there can hardly be any question of the propriety of discharging sewage from a single house.

Again, houses in the vicinity of large bodies of still water may without question be allowed to discharge into those lakes. For example, houses in the vicinity of Lake Ontario or Lake Michigan, or even of much smaller lakes, should not contribute any offensive pollution to the waters of the lake. In New York State, some of the smaller lakes are used as water-supplies for cities, as, for example, Owasco Lake for the city of Auburn and Skaneateles Lake for the city of Syracuse, and, acting under the statutes, special laws have been passed by the State Department of Health, forbidding any discharge of any kind of household wastes into these lakes. The same is done in other states. Here, again, it is a question of the drinking supply which is being considered, and not a question of the possibility of any nuisance being committed.

Treatment of sewage on land.

If no stream suitable for the reception of sewage is available, then the sewage must in some way be treated on land before it passes into the nearest watercourse. For the second fundamental principle about the treatment of sewage is that of all places the action of putrefactive bacteria is most energetic in the surface soil and that it is there that the organic matter of sewage can be most rapidly accomplished. Experiments already referred to have shown not only this, but also that their activity is most noticeable in the surface layers of the soil and that their action continues for scarcely two feet downward, and it is customary to assume that the largest amount of work done is accomplished in the top twelve inches. Further than this, it has been established that in order to persuade the bacteria involved to do their work as promptly as possible, the application of sewage to any particular locality should be made intermittent; that is, that a resting period should be given to the bacteria between successive applications of sewage.

For example, one can recall without difficulty the conditions on the ground at the back of the house where the kitchen sink-drain commonly discharges. At the beginning of summer perhaps a rank growth of grass starts up vigorously in the vicinity, and the path of the surface drain can be traced by the heavy vegetation along the line of the drain. If the slope of the surface away from the house is considerable, no other effect may be noticed through the season, since the surface slope carries away the sewage, spreading it out over the ground so that the soil really has a chance to breathe between successive doses. But if the ground is flat, it will be remembered that before many weeks the sewage ceases to sink into it; the ground becomes "sewage-sick," as they say in England, and a thick, dark-colored pool of sewage gradually forms, which smells abominably. If a piece of hose a dozen feet long had been attached to the end of the drain and each day shifted in position so that no particular spot received the infiltration two days in succession, it is probable that no such pondage of sewage would occur, but that the mere intermittency of the application thereby secured would permit the successful disposal of this sink waste throughout the season.

The same effect is to be noted in some cesspools where, because of the great depth to which they are dug and because no overflow into the surface layers of the soil is provided, the pores of the ground around the cesspool become clogged and choked, and the cesspool becomes filled with a thick, viscous, dark-colored, objectionable-looking, and evil-smelling liquid.

The three principles which will avoid these conditions are, as already stated, plenty of air, presence of bacteria normally found in the surface layers of the soil, and intermittency of application.

In order to secure the operation of these three principles in the application of sewage onto land, the sewage must be made to pass either over the surface of the land in its natural condition in such a way that the sewage may sink into the soil and be absorbed and at the same time give up its manurial elements to whatever vegetation the soil produces; or, as a modification of this principle, the sewage may be required to pass through an artificial bed of coarse material by which the rate of treatment may be considerably increased. In the latter case, although probably the greater part of the action of the bacteria takes place in the top twelve inches, it is customary to make the beds about three feet thick, chiefly in order to prevent uneven discharge of the sewage through the bed. Finally, wherever, for Æsthetic reasons, it is desirable that the sewage should not be in evidence, either before passing through the natural soil or exposed in an artificial bed, the practice may be resorted to of distributing the sewage through agricultural tile drain laid about twelve inches below the surface. In this way, the sewage is scattered through the top soil, where bacteria are most active, without being apparent, and a front lawn thus treated would not give any indication of its use.

Taking up now in order these three methods of treatment, we may consider some of the details of construction. In spreading the sewage over the lawn or in distributing it on the surface, due regard must be paid to the kind of soil. Clay soils and peaty soils are useless for the purpose of sewage disposal unless as the result of continuous cultivation a few inches of top soil may have accumulated on the clay. This top soil is adapted to sewage purification, provided the quantity applied is not excessive.

Surface application on land.

Two methods of operation may be pointed out. The sewage (and this is the simplest method of disposal possible) may be brought to the upper edge of a small piece of ground, usually sowed to grass, and allowed merely to run out over the surface of the ground. There should be, however, some method of alternating plots of ground, one with another, so that the sewage is turned from one to the other every day. Each plot will then have one day's application of sewage and one day's rest, and this would complete the disposal, were it not for the interference of rain and cold. The winter season practically puts a stop to this method of treatment, and rainy weather reduces the power of the soil to absorb sewage. For these two reasons, it is desirable to have one plot in reserve, or three in all, and the area of each plot should be based on the amount of sewage contributed. For a family of ten persons using twenty-five gallons of water per day the total area provided should be one tenth of one acre, or an area seventy feet square divided into three plots. Figure 67 shows six beds arranged to care for the sewage of a public institution in Massachusetts. As a guide to the amount of land needed, it will be safe to provide at the rate of one acre for each forty persons where the soil is a well-worked loam but underlaid with clay. The effect of this irrigation on the grass will be to induce a heavy, rank growth which must be kept down by repeated cutting or by constant grazing. Both methods are practiced in England, and it may be said in passing that no injury to stock from the feeding of such sewage-grown grass has been recorded. The grass cut from such areas (and the cutting is done every two weeks through the whole summer) is packed into silos and fed to cattle through the winter with advantage. Or, if grazing is resorted to to keep the grass down, the herd is alternated with the sewage from one field to the other, so that the bed which has received sewage one week is used for pasture the next week, and the number of head which can thus be fed is astonishing. In order to secure an even flow of sewage over such grass land as is here contemplated, there must be a gentle slope to the field, and the ditch or drain bringing the sewage to the field should run along its upper side. Openings from the drain, controlled by simple stop planks, are provided at intervals of about ten feet, and no attention is needed further than the opening and closing of these admission gates.

Another method of applying sewage to the surface of the ground is to lead it in channels between narrow beds on which vegetables have grown. These beds are made about eight feet wide with two rows of root crops, such as turnips or beets, set back about two feet from the edge. The beds are made by properly plowing, the channels between the beds being back-furrowed. Here, again, the principle of intermittent application is essential, and the area to be provided is the same as already given for the surface irrigation. Three beds should be provided, as before; but, in general, no provision need be made for carrying off the sewage at the lower end of the beds, since it may be safely assumed that all of the sewage will be absorbed by the soil. Of course, a sandy soil will absorb more water than a clay soil, and if the soil is entirely clay, it is not suitable for such treatment. Sewage passed over the surface of clay soil, however, will, in the course of a few months, so modify the clay as to convert it into a loam, and in this way increase its absorptive power.

When possible, it is desirable to have a plot of plowed ground over which the sewage may pass before reaching the beds, so that the grosser impurities may be left behind and harrowed in or plowed under. If proper regard is paid to intermittent application, no danger from odors need be feared, and the repeated plowing in will increase immensely the fertility of the soil. Nor need one be afraid that all of the manurial elements will be left behind on this plowed ground. About two thirds of the organic matter in sewage is in solution, and this will be carried onto the beds just as if passage over the plowed ground had not occurred.

Artificial sewage beds.

In order to secure a higher rate of discharge of sewage through the soil it is best to arrange an artificial bed which shall be made of coarse, sandy material which will allow a rate of at least 10 times that already given. The best material out of which to make such an artificial bed is a coarse sand; that is, a sand whose particles will not pass through a sieve which has 60 meshes to the inch and which would pass through a sieve of 10 meshes to the inch. Such an ideal sand will purify sewage at the rate of 50,000 gallons per acre per day, or an acre will take care of the sewage of at least 1000 persons. This means that it is necessary to provide about 50 square feet for each person in the family, or a family of 10 persons could have all the sewage taken care of on an area 25 feet square. The same principle of intermittency of application, however, must be observed by dividing the bed into three parts, so that the sewage may be alternated from one bed to another. Practice has indicated that it is better to shift from bed to bed about once a week and to deliver the sewage onto each bed intermittently; that is, to discharge a bucketful at a time with short intervals between, rather than to allow a small stream to flow continuously onto a bed. Such a bed should be about 3 feet deep, as already stated, and preferably should have light concrete side walls and bottom, as shown in the sketch (Fig. 68). Ordinarily, the surface of the sand will be level, and the dose of sewage applied to the bed will cover it a fraction of an inch deep, and in the course of an hour or so will disappear into the sand and reappear in the underdrains as clear water.

In cold weather a thin sheet of sewage spread out over the surface of the sand would freeze before penetrating the bed; therefore, in the winter time, it is usual to furrow the beds; that is, dig furrows across the beds 2 or 3 inches wide at the bottom and about 10 inches deep, so that in the bottom of these furrows the sewage may be, partly at least, protected against frost. It has been found that, if sewage is discharged intermittently,—that is, in bucketfuls into such furrows,—the beds open and allow the filtration of the sewage. To be sure, the purification effected in cold weather is not quite that accomplished in warm weather, but the results are sufficiently satisfactory, and no nuisance ensues.

Subsurface tile disposal.

The other method of distributing sewage over land is by means of draintile placed in shallow trenches, so that the sewage may leach out into the soil through the open joints of the pipe. These draintiles receive the sewage intermittently, and by the constant rush of water are presumably filled throughout their length. The sewage then gradually works out of the joints into the surrounding soil, and the pipes are empty and ready to receive another dose when next delivered.

Two essential points must be considered in the successful operation of such a plant: the grade of the tile and the length of the tile.

The grade of the tile must be properly adjusted to the porosity of the soil; that is, in open, porous, and gravelly soils a grade must be steeper than in loamy and dense soils. The reason is manifest. In a gravel soil, the sewage is at first rapidly absorbed, so that as the sewage goes down the pipe line the first joints take up the water and deliver it to the soil, where it disappears, and probably no flow reaches the end of the line at all. This means that the soil surrounding the first joints does the work which the entire pipe line was intended to do and thus becomes overworked. When overworked, the soil always refuses to do anything, so that when the succeeding joints take up the sewage and in their turn become overworked, the line is useless. If, on the other hand, the grade had been steep enough to carry the sewage down the pipe line gradually so as to secure a uniform distribution, then the same or approximately the same amount of sewage would be taken out of the pipe at each joint, securing a long life for the system. In loamy soil, on the contrary, there is not the same absorption at the joints, and so on a steep grade there is the tendency for all the sewage to follow down the pipe line to the lower end and there escape to clog the soil and thus spoil the system. As a general average, it may be said that the proper grade for such a subsurface distribution pipe line in a fairly good sandy loam should be 5 inches in 100 feet; less than this as the loam becomes clay and more as the loam becomes gravel.

The other essential point for the successful operation of this method of distribution is to provide a proper length of pipe for the number of persons contributing sewage. The soil itself will absorb about the same amount as when the sewage is spread over the surface, so that a family of ten persons would require, as before, an area about 70 feet square. The pipe lines may be laid in different sections, provided the different lines of pipe are not nearer together than 10 feet. On an area 70 feet square there would be, therefore, 7 lines of pipe each 70 feet long, or 490 lineal feet of pipe in all, or 49 feet per person. The writer generally allows 40 feet in well-cultivated soil as a reasonable length of pipe for each person in the family. If the soil is sandy, this may be reduced one half, but need not be increased under any conditions, since a soil requiring a greater length of pipe than 40 feet per person would be so dense as to be unfit for use. To properly arrange the lines of pipe on a sloping ground requires careful study of the inclination of the ground and of the relation of direction of lines of pipe to slope. Usually the slope of the ground is greater than the 5 inches per 100 feet just referred to, but by laying out the lines of pipe across the slope instead of with it any grade desired may be obtained. Nor is it necessary that these lines of draintile be run in straight lines; they may very properly follow the curving slope, the proper grade being always carefully maintained.

Common agricultural tiles three inches in diameter and costing about two cents per running foot are suitable material for these distribution lines. The sewage enters these distribution lines from a larger pipe, usually six inches in diameter, and a difficult adjustment is presented that each branch tile line shall receive its own proportionate share of the sewage. If only one line of tile is provided, say 200 feet long for 5 members in the family, then all the sewage goes into that line with no question of distribution arising, but if a number of short parallel lines must be used, as shown in the sketch (Fig. 69), the difficulty of subdividing the sewage properly among the different branch lines becomes very great. For that reason the writer prefers to use not more than two lines, with the possibility of delivering the sewage alternately in the one and the other. In this way, the bed not receiving sewage is resting, while the other bed is acting, and also the outlet for the sewage is always definitely known. And particularly in the case of these subsurface tile, the necessity for the intermittent dosing is apparent, since with small, constant trickling discharges the difficulty of distribution through the long length of tile is gradually increased, and usually saturation of the soil occurs from joint to joint, as already described. Therefore it becomes most necessary, in this case, for the best results on the soil not merely to alternate the beds receiving sewage, but also to effect the intermittent discharge onto the beds or through the pipes although the sewage itself may flow very uniformly in volume.

Automatic syphon.

This intermittent discharge is accomplished by constructing on the pipe line from the house and before it reaches the beds an "automatic syphon," as it is called, the operation of which may be described as follows: As the sewage enters the tank containing the syphon and rises outside the syphon-bell, air is compressed between the water surface inside the bell and the water left inside the syphon-leg. With greater and greater height of water outside, this compression inside becomes greater and forces the water in the syphon-leg lower and lower. Finally, the water sinks so low as to allow the compressed air to escape suddenly around this bend, instantly relieving the compression, and the water outside rushing in to fill up the space occupied by the air starts the syphon (see Fig. 70).

This syphon, in size suitable for a single house, costs about $12 delivered, and will always be available to secure an intermittent dosing of the bed or pipe line. Usually the chamber in which this syphon is placed holds about one hour's flow, so that it may be estimated that this syphon will discharge on the bed every sixty minutes. The exact interval of time is not essential nor, perhaps, important, although it may be noted that the coarser the material,—that is, the nearer uniform all the sand particles are to the largest size passing the ten-mesh size,—the smaller must be the dose applied, but the more frequently must the application be made. This has been very thoroughly studied in Massachusetts, and the views of experts on this subject may be found in the report of that Board.

Such an intermittent discharge may be made and often is made by a hand valve leading out from this chamber in institutions or in private houses where some one constantly is available for the purpose. Thus it becomes the duty of the man in charge every hour or perhaps three times a day to pull the valve and allow the sewage to discharge (see Fig. 71). An overflow pipe should always be provided, so that if he forgets to pull the valve, the sewage will still find its way into the system rather than out on the ground.

Sedimentation.

As a matter of economy of operation, it has been found desirable to take out from the sewage before the treatment already described as much of the solid matter as may be reasonably done, and for this purpose sedimentation is made use of. Most of the solids in sewage are slightly heavier than water, so that if they be allowed to stand in the water for a short length of time, they will settle to the bottom of the tank and allow the liquid above to pass on, considerably clarified. It has been found worth while to do this, since all three processes described are interfered with if the solids taken out by sedimentation are allowed to be deposited either upon the surface of the ground, giving rise to odors as well as to objectionable appearances, or onto the surface of the sand beds, which they clog up, or in the three-inch tile drain, which may be filled in a short time.

It has been further found by experience that if these sedimentation tanks are made large, really larger than necessary for sedimentation, in some way a large proportion of the matter accumulating in the tank will disappear, so that the amount of sediment to be taken out of the tank is not as large as might be expected. In fact it is usual for such tanks to run one or two years without cleaning, although the amount of solids shown by chemical analysis to have been removed from the sewage would fill the tank twice over.

It has been found that a tank, in order to do successful work in separating solids and in eliminating as much as possible of the sediment, needs to be of a capacity to equal about one day's flow of the sewage, and this is a good basis for computation. Here, again, the fact that the sewage from a single house is considerably fresher than the sewage from a city must be remembered, since, while many cities build tanks holding only one third or one fourth of their daily flow with good results, in the case of a single house this is not possible, and the tanks, if built at all, ought to hold at least the full day's flow. Ten persons, at 25 gallons each, furnish 250 gallons per day or 33 cubic feet. The tank, then, must be large enough to hold this volume, and suitable proportions generally require that the tank be at least 5 times as long as wide. A certain allowance must always be made for deposit in the bottom and for the accumulation of scum on the top, so that an extra foot or more of depth is desirable. The tank, then, to furnish the required 33 feet, might be made 3 feet wide, 3 feet deep, and 5 feet long, and probably in no case would a tank much smaller than this be used.

There are two or three details of tank construction which may be suggested, although almost any kind of tank will answer the purpose. It is desirable in order that the surface scum may not be disturbed, and in order that the inflowing sewage may distribute itself as uniformly as possible across the tank, to attach an elbow to the entering pipe so that the sewage enters about halfway between the top and bottom of the tank (see Fig. 72). Similarly, at the outlet or weir an elbow should be provided because it is not desirable to allow the floating matter of the surface to be carried onto the bed, and a pipe taking off liquid, open halfway between top and bottom, will carry away but little of either the surface scum or bottom sediment. Such a tank must be built of concrete or masonry or timber, although the latter is not to be recommended because of its short life. The walls of an ordinary tank may be built 6 inches thick at the top and 12 inches to 18 inches thick at the bottom, the latter being necessary if the depth is over 8 feet. The tank should have 6 inches of concrete on the bottom, and the roof may be made of flagstone or of concrete slabs in which some wire mesh has been buried.

It is not necessary to ventilate this tank, although it is desirable to have perhaps a foot of air-space between the water level and the roof of the tank. During the first few months of its operation such a tank is very likely to smell badly, and, if ventilators are provided, the presence of the tank will be well known by the odors sent off. After the tank has been in operation two or three months these odors gradually disappear, due presumably to the fact that the surface of the water in the tank has become coated with a thick blanket through which odors cannot penetrate. On the other hand, there have been a few cases recorded where the production of gas in a septic tank was so great that an explosion occurred, tearing off the roof and otherwise doing considerable damage.

The full plant, therefore, will consist of the settling tank, receiving the raw sewage from the house and discharging it into a small tank holding about one hour's flow and containing the automatic syphon apparatus for intermittent discharge. This dosing tank must provide for one hour's flow at the maximum rate of flow, and should hold about one fourth of the total daily flow. Then the ground area, either natural or artificial, which receives the intermittent discharge from the dosing tank, completes the installation (see Fig. 73).

Underdrains.

The question of installing underdrains will arise only in cases where the ground water, always to be found below the surface somewhere, comes up so high as to affect the disposal of sewage. Usually no underdrains will be needed unless the ground water gets up to within three feet of the surface, and, in a number of cases, underdrains have been laid under a sewage filter at considerable expense, only to find when the filter was in operation that they were never in use. In clay soils the underdrain is not necessary. In fact, it may be noticed that the underdrain is not for the purpose of taking care of the sewage, but rather of draining off the soil-water and preventing its interference with the action of soil on sewage. This principle will indicate where underdrains are necessary and where not.

When used, underdrains should be laid from three to four feet below the surface in parallel lines about fifteen feet apart and on grades of not less than one foot in one hundred. It is always better to have the underdrains too large than too small, and drains less than three inches in diameter should not be used, and they should increase in size to four inches and then to six inches as the separate drains are brought together. The writer has seen a six-inch underdrain running full of ground water collected within a distance of a hundred feet, but this was in gravel soil through which the water passed very freely. No exact rules can be given for the size of the underdrains, but it will be noticed that, since water passes through clay soil slowly and through gravel soil rapidly, larger pipes must be used where the soil is coarse.