An experienced engineer gives a velocity of three feet per second as the least that should be allowed for the outlet drain of a house. To secure this flow a four-inch drain should have a minimum inclination of one inch in ninety-two; a six-inch drain, one in one hundred and thirty-seven; a nine-inch drain, one in two hundred and six; and to attain the above velocity of three feet per second at these inclinations they must run not less than half full. The great purpose of all modern sewage systems is to carry off all waste matters before they have time to decompose. =Joints of Drain Pipes.=--These should be made so smooth within as not to impede the flow of sewage, nor become obstructed by catching thread, strings, hair, and other floating substances. They should be so tight as absolutely to prevent any leakage of either fluid or gaseous matter, and render impossible the entrance of the small filaments of the roots of trees growing along their course. They should be supported on solid pillars of brick or stone, and not spiked to cellar walls where a slight settling will force the joints and cause a leak. They should be so firmly supported at every point that after the joints have been cemented no possible change of direction or slope of pipe can occur. Any such change is sure to work disaster. =Sewer Ventilation.=--No sewer is safe that does not have a free current of air passing through it. Motion and aeration are the safeguards against infection. Sewers should be constructed so as to secure a constant flow, with no sharp angles or short turns to impede its progress, and with frequent vents leading to the surface of the street. Thus diluted, the sewer gas becomes harmless, the pressure in the conduits is relieved, and the danger of the gas forcing its way through the water-traps into the living and sleeping rooms of our homes is avoided. =Location of Closet.=--The water closet should be so placed as to have an exterior window, by means of which it may be fully ventilated. Under no circumstances should the closet open directly into the bedroom. When entered from the hallway or landing, the conditions may be improved by cutting off half of the space as a vestibule or outer apartment, thus preventing any foul odors from reaching the sleeping rooms. For reasons of convenience as well as of health the bath-room and lavatory should be separate from the water closet. =Disposal of Garbage.=--In cities, the public authorities collect and dispose of the solid waste of the kitchen. In the country, and wherever chickens, cows, or pigs are kept, these waste substances may be utilized. Some private families burn them. Where this cannot be done they should be removed from the dwelling far enough to prevent their decomposition from giving rise to any unpleasant or unwholesome odors. No compost heap should be maintained within one hundred yards of a dwelling.