On page 45 we suggested a plan for removing ground-water from a house standing alone. (See Figure 33.) We now give a plan for a house situated in a block, as ordinarily found in this city.

Where houses are built in continuous blocks, as in cities, the arrangement shown on the accompanying plan is appropriate. In this case the pipe under the basement or subcellar-floor should be of unglazed tiles, laid without mortar of any kind, so that the ground-water can get in at any joint, and as it is not a tight drain, but a porous one, for clean water only, it need not be made easy of access, but can be buried. In case a water-closet or other receptacle of foul drainage is needed at this low level, another pipe of iron must be supplied as shown in Figure 33.

Some people connect the ground-drain under the water-seal of the outer trap, but it is not so good a plan as the one shown in the cut, for the following reason: In that case the trap-water sets back into the ground-drain and stands there without circulation, except at such rare intervals as the ground-drain is in active use. This occurs so seldom that the opening where it discharges into the trap is liable to get closed up by fæcal matter and other substances which pass by the orifice, and the ground-drain has generally insufficient fall to enable its contents to crowd through such a pasty plug and clean itself. The longer this plug remains the more compact it becomes. Of course if the ground-drain discharges in the open air, as shown in Figure 33, page 45, this trouble is avoided. During the winter season, and as long as houses are heated by fires, there is always an inward pressure of air at all cracks and orifices near the basement, and the air which is in these ground-drains will at such times press upward through any pores and channels that can be found in the cellar-floor, gradually and surely working its way into the house. The ground-drain is deeply covered and inaccessible, so that if by any chance it becomes defiled by sewage it is next to impossible to clean it. It is, therefore, very important that these ground-drains should be kept quite free from any possible contamination where connecting with the sewage-drain.

Figure 34.

It is generally advisable to put the ground-drains as deep as two or three feet below the cellar floor wherever an outlet can be found at such a level, and this generally precludes the possibility of getting much fall where these ground-drains enter the main house-drain.

In consequence of this condition of things, a very slight obstruction in the house-drain trap, such as may happen from the deposit of a bit of cloth, a broken tea-cup or tumbler in any water-closet above, by a servant who thinks water-closets are made to hide such things, would serve to back up all the sewage of the house into the ground-drain, if care is not taken to prevent such an event.

If arranged as shown in Figure 34, a stoppage in the main trap would cause an overflow in the manhole, which should be inspected frequently and can be cleaned out when occasion requires. But if, as may often happen, it is not possible to get fall enough to prevent such a stoppage backing up the sewage into the ground-drain, some special safeguard must be applied in the shape of a mechanical valve. Such valves are not to be generally recommended in pipes where sewage flows, but this ground-drain is supposed to carry nothing but clean water. Under such conditions a valve may answer a good purpose. Metal valves, and even brass ones, are subject to corrosion, which would seal them fast to their seats if not often lifted.

A good form for this place is a rubber ball-valve arranged as in Bower's trap. But the trap should be specially constructed for this purpose, with a seal of at least twelve inches depth. The clearing-screw should of course be on top instead of at the bottom in this case, and should be examined for removal of sand or silt which the water may possibly bring in, especially during the first flow after the drain is constructed. The space around this trap where the water enters it should be well grouted with hydraulic-cement to prevent any back flow outside the pipe, in case of obstruction of sewage, while the valve in the trap will prevent it on the inside. Such a trap in such a position will hold its water for years, but should be inspected at least twice a year, because it is of great importance to keep it free from sand and supplied with water.

If the connection with the main drain were to be made beyond the trap in the latter, we should lose the additional safeguard of that trap against the admission of sewer-air, which safeguard it is best to retain.