to this end, both public and private funds, in a way that will not contribute, ultimately, toward a harmonious system and consequently to a great extent wastefully.

In view of these facts and the statistics of the census bureau quoted, it becomes apparent that the separate system of sewerage must be widely extended.

CHAPTER IV.

PLANS.

In designing a system of sewers for a town, there are several things to be taken into consideration before deciding upon the plan to be adopted. The principal points to be considered are: the size of the town, its situation with reference to the disposal of sewage, the compactness of its buildings, its topography, its water supply, the character of the soil, the sanitary habits of its citizens; and its financial condition.

The amount of sewage in any town will depend upon the number of its inhabitants, their habits, and the abundance and convenience of the water supply. In a town without a public water supply the amount of sewage per head will be much less than where water is abundant. With the introduction of water works comes the multiplication of water closets, and a rapid increase in the use of water for baths and various household purposes, and the amount of sewage will rapidly increase. The volume of sewage to be provided for, in any case, may safely be taken to be equal to the volume of water used.

Sewage Disposal.-The disposal of sewage is a problem of the highest importance. When sufficient fall can be obtained the sewage is usually carried by gravity to the nearest stream, or large body of water. The effect of sewage pollution on streams and lakes is a question which is rapidly growing in importance as our population grows more dense, and more towns are sewered.

In the older countries of Europe the pollution of water courses by sewage has forced itself upon the attention of government officials, and stringent laws have been passed to protect the purity of streams. In this country the time is not far distant when the pollution of streams and lakes by sewage will

need to be forbidden by law or in many localities pure drinking water in any considerable quantities will not be obtainable.

The State of Massachusetts has already taken steps in this direction through "An act to protect the purity of inland waters." Canada has also done the same.

In many cases there is no available out-fall for the sewage, and the question of its disposal comes up at once with the inception of sewer projects.

The methods for purifying sewage by chemical processes are many and various. The object of these processes is to so purify the sewage that the water may be turned into the streams. The residuum is used as a fertilizer and for various other purposes but is practically valueless in its crude state. Wherever the right kind of land is available the sewage may be used for irrigating crops, and this has been successfully done in many cases.

Sometimes a town site is so flat that sufficient fall cannot be obtained to carry off the sewage. In this case the sewage may

be pumped, or raised by Shone's System.

Storm Water.-In towns where the houses are at considerable distances apart, and no very large proportion of the surface is paved, the storm water will usually be easily disposed of without providing any underground channels for it. But in large cities compactly built, where the greater part of the surface is paved, and where the water would need to run in the streets for long distances to reach an out-fall, provision must be made for the storm water. This may be done by enlarging part of the sewers so as to carry the surface water as well as the sewage, or by constructing special conduits for the surface water. These special conduits may, in most cases be very much shorter than the sewers, as the storm water can be delivered into any natural water course within the town, while the sewage must be carried entirely out of the town. In every town there will problems arise peculiar to the circumstances in each case; and the details of the plan best adapted to any given requirements must be worked out to suit the conditions of that special case.

The Preliminary Survey.-Before any definite plans can be determined upon, a careful topographical survey must be made. A study of a reliable map of a town, with the heights of the street corners and points at changes of slope noted on it— or, better still, with the contour lines drawn on it-will enable the engineer to determine approximately what grades are available for the sewers; the best lines for the mains; and will enable him to so design the laterals as to lead the sewage by the most direct route to the out-fall.

These approximate calculations can then be tested by final computations made from the diagram in Chapter VI. If there are any defects in the assumptions of inclination made, it will become apparent from the diagram when the sizes are determined, and proper corrections can then be made.

It must not be forgotten in determining the grades to be adopted, that a continuous rise along the crown of the sewer is required for the upward passage of air-currents, as well as a continuous descent along the invert for the downward flow of sewage. To accomplish this, it is necessary when the sewers flowing into a man-hole are smaller than the outflowing sewers to which they are tributary, to raise the crowns of the former slightly above that of the latter. This frequently occasions a considerable loss of grade. For example, if a man-hole having an eighteen inch outlet has a twelve inch and a ten inch sewer tributary, and to insure a free delivery at all times we raise their crowns an inch above the crown of the outlet, the invert of the twelve inch pipe will be raised above the grade line seven inches and that of the ten inch pipe nine inches. This is illustrated in the drawing of a man-hole, Plate V. Another reason for raising the inverts of inflowing pipes at man-holes, is, that obstructions are more likely to occur at man-holes than when the sewer has its full circular section, and the increased descent thus secured tends to prevent deposits.

A preliminary survey for sewers should include such measurements as will enable the engineer to make a map of the town and profiles of the streets. The lengths and directions of the

street lines should be carefully measured, and levels should be taken at every one hundred feet and at every change of slope of the surface. A datum should be selected and its distance below some well known fixed point in the town be given. Bench marks for levels, and reference points for line should be established at every street intersection.

The establishment of bench marks should be the first step in taking the levels, and it should be done independently of the surface levels, as extreme accuracy in the final location of the grade line in construction is necessary in order that portions of the system which may be constructed separately may be properly joined. After the territory is covered with a proper system of bench marks, which have been carefully checked by cross lines and found to be correct, the surface levels can be taken very rapidly and with less care, as any error is not carried but is eliminated at the succeeding bench mark.

The transit and level notes of the preliminary survey should be carefully preserved, after they have served their purpose in the preliminary work, as they will serve as a check on the succeeding work.

From these notes a map can be made and the contour lines drawn on it (as in the maps shown in front of book), or, the height of a sufficient number of stations marked on it.

The survey should also include outlying portions of the territory which may belong to the same natural drainage basin, and for the storm water of which it may be necessary to provide special conduits. Rough profiles of the streets can now be made, and the grade lines of the sewers laid down on the profiles. The method of determining the proper grades is fully described in the following pages, as also the method of determining the loss. of elevation on curves and in the case of a smaller sewer being tributary to a larger one.

Having located the out-fall of the sewers and established its height, it will be best, in determining the grades, to work from the out-fall along the mains to the laterals, as this will show the height at which each junction must be, and what fall is available.