The house drain b should join the sewer above the line d, as shown, and should be provided with a light, hinged flap valve to prevent water from the sewer backing up in the house drain.

SEWER PIPE.

Sewer Pipe is made salt-glazed and slip-glazed. Salt-glazed pipe is made of a dense clay that vitrifies when fired, and unites with salt thrown into the kiln to form an impervious surface. Slip-glazed pipe is made from a porous clay that does not vitrify when fired; hence, it is covered with a coat of vitreous clay before firing to fill the pores and give it an impervious surface. The slip glazing does not unite with the body of the pipe, however, but forms a thin film over the surface that is easily chipped or peeled off.

Salt-glazed pipe is the kind most commonly used for sewer construction. The weights and dimensions of various sizes of sewer pipes can be found in the tables of sewerpipe proportions given.

DOUBLE-STRENGTH SEWER PIPE.

Roof leaders proportioned as follows have been found, in practice, to give satisfactory results:

For small roofs, 1 sq. in. in sectional area of the leader for each 150 sq. ft. of roof surface. For medium-sized roofs, 1 sq. in. in sectional area of the leader for each 200 sq. ft. of roof surface. For large roofs, 1 sq. in. in sectional area of the leader for each 250 sq. ft. of roof surface.

In cold climates, roof leaders that cannot be exposed to the sun should be located inside buildings. Then, water from the melting snow cannot freeze in the leaders and burst them.

WATERPROOFING WALLS.

Damp cellar walls are due either to water soaking through from the outside, or to wet bottoms, from which water rises in the walls by capillary action. The decay of vegetable matter contained in dirty water generates gases injurious to health, so that the prevention of dampness in walls is a point of great importance. Provision must be made to convey the water away from the walls, as well as to make the latter damp-proof. No earth should be placed against the wall, a 12" or 18" space next to it being filled with broken stone or

gravel, with, if practicable, an openjointed tile drain laid at the bottom (see illustration). The outside of the walls and footings should be plastered thickly with 1-to-1 cement mortar; or, preferably, with asphalt and coal tar, mixed in the proportion of 9 to 1, and applied, while hot, about in. thick to the dry walls. The asphalt course should also extend through the walls, as shown at the under side of the concrete floor, which should be 3 or 4 in. thick, and laid on a 6" or 8" bed of broken stone. In place of concrete, a very durable coposition made of 60 parts of hot asphalt, 10 of coal tar, and 30 of

The ascent of moisture in walls may be prevented by inserting on the footing course two courses of roofing slate or very hard brick, laid with broken joints in cement mortar, or a few layers of tarred felt may be used.

SEWAGE EJECTION.

Conditions. In tall office buildings, hotels, underground railway stations, etc., plumbing fixtures are sometimes located below the level of the street sewer. Under such

conditions the sewage from the floors below the sewer level is collected in a sump, from which it is automatically discharged to the sewer by mechanical means.

The use of pumps for sewage ejectment from buildings is not much favored by sanitary engineers at present, the method of removal by means of compressed air being preferred. When pumps are to be used, however, centrifugal pumps may be selected.

Compressed-Air Ejection.-There are several designs of automatic ejectors for removing sewage by means of compressed air, all of which operate on the same general principle. For instance, they must all have a perfectly air-tight receiving or sump tank, which should be located in a water-tight chamber built of iron or brick. The sump tank must be located at such a level that all drainage from the subsewer system will flow into it by gravity. The inlet pipe must be provided with a check-valve to close when the pressure of air is applied to the tank and open freely by the flow of sewage when the pressure is released. The tank must have a vent pipe that operates automatically, to permit the escape of air after the tank has been emptied, but which closes automatically when the tank is full and the compressed air is applied. There should be an apparatus to automatically open an air valve and admit compressed air when the tank is full and to automatically close the air valve when the tank is empty. A check-valve should be in the discharge pipe from the tank to keep sewage in the discharge pipe from returning to the tank. An air compressor is needed to provide compressed air; also, a tank to receive and store it, and an air pipe from the air tank to the automatic valve on the sump tank. An electric or water motor is required to operate the air compressor automatically when the pressure in the air tank is lowered. The ejectors should be in batteries of two, each being of sufficient size to discharge all the sewage from the drainage system. The motors, air compressors, and air tanks should be located in the engine room or other convenient place, where they are open to inspection and repairs at all times.