In cesspits and old drains the soil and putrid matter have been for years allowed to accumulate, and the poison from such matter, when distributed into the open air through gratings in the new sewers or into houses, is, when inhaled into the system, the cause of these zymotic outbreaks. In tracing these old drains and in preventing stagnant gases from remaining in any portion of the drain, the engineer or architect cannot pay too much attention, as confined gases when charged with poisons from putrid matter are the principal factors in producing disease. Many persons place a well-constructed trap at the inlet, and another some distance along the drain, say at the end of a building or grounds, without any ventilation between the two traps. In fact this used to be a common occurrence; but it should never be done. If the drain should be a 6-inch one, and the traps 50 feet apart, the amount of gas between the two traps would average 9 cubic feet, and this gas would in the ordinary working of the drain remain for years, getting more poisonous the longer it remained undisturbed. The owner of the house, knowing that he had a good trapped drain connected to sewers, would feel himself safe, and naturally think his house healthy. Far better for him if the house were drained into a ventilated cesspit, as when the gases in the drain became released, which may occur by the siphoning of the traps at the house-connection, the danger would be equal to the emptying of a disused cesspit, and carrying the contents through the house. The more a person tests the working of gas in sewers or drains the more he will find that branch drains from their construction supply the poisons which render the gases in the sewers themselves so noxious. In 1880, whilst engaged in tracing the course of an outbreak of typhoid, I made a series of experiments with a view to trace the source from which the disease emanated, and every experiment proved that the origin of the disease lay in the gases which were in contact with putrid sewage matter, existing in old drains and cesspits attached to the sewers as well as in the gases which were confined between traps. The distribution of the disease was due to the imperfect construction of the sewers, drains, and sanitary fittings. The most successful experiment, and the one from which the greatest result was obtained, and which I have ever since most successfully used, was in determining the state, size, and condition of the drains underground, and also that of the house or buildings, by measuring by compression the gas contained in the sewer or sanitary fittings. The principal cause of its distribution was the compression of the gas between the water-traps, the siphoning of house-traps leaving at times a free passage for the gas to enter the house.