=Sewer Gas.=--This poisonous gas is known chiefly by its effect. It frequently passes the water-traps and enters our sleeping and living rooms, there to do its fatal work. The alternate floods of hot and cold water open the joints of iron pipes, and allow the gas to escape. Leaden gas pipes decay and become perforated, with the same result. Dr. Fergus, in his pamphlet “The Sewage Question,” says: “For some years I have insisted on a careful examination of the soil pipes wherever I have cases of typhoid or diphtheria, and in every case where I could get this carefully carried out I have detected perforated pipes, or have found sewer air getting into the houses in some other way. In many cases the plumbers have declared pipes to be all right, which turned out to be very defective when uncovered.” =Water-Traps.=--These are not so effective in preventing the escape of sewer gas as they are considered to be. Experiments with glass tubes shaped and arranged just as the ordinary water-traps in sinks and closets are arranged have shown that the light gases pass through by the top of the bend, and the heavy gases by the bottom. A rush of wind up the mouth of a sewer, or a heavy dash of rain which fills the sewer and reduces the air space, so increases the pressure of the gas within the sewer and soil pipe that the ordinary water-traps are not able to resist it. Water-traps that are not used for a time become death-traps. The water soon evaporates, and affords an unobstructed channel for the conveyance of foul gases from cess-pool or sewer to the rooms of the house. Houses that are vacated for the summer, and that are without tenants for a time, should be thoroughly cleaned and ventilated, and have all pipes and drains flushed with water before being occupied. =Size, Flow, and Fall of Drain Pipes.=--The efficiency of a drain or sewer depends upon its capacity, its slope or incline, and the velocity of its flow. If the amount of water flowing is proportionate to the size of the conduit, sewers of different sizes give the same velocity at different inclinations. A ten-foot sewer with a fall of two feet per mile, a five-foot sewer with a fall of four feet per mile, a two-foot sewer with a fall of ten feet per mile, and a one-foot sewer with a fall of twenty feet per mile will have the same velocity provided they are filled in proportion to their capacity. The ten-foot sewer will require one hundred times as much sewage as will the one-foot sewer. If it has less, the velocity of its stream will be correspondingly diminished. It is especially important, therefore, that the size of the conduit be adapted to the volume of the stream, as well as to the slope or inclination.