Contrary to popular myth, plumber Thomas Crapper did not invent the flush toilet. One of his contemporaries, though, did create the first toilet that prevented sewer gases from entering the home. Englishman Joseph Adamson's 1853 design — the siphon flush — eventually made obsolete both the chamber pot and the outhouse. Adamson's invention, like all modern toilets, relies on the tendency of a moving liquid to continue flowing, even in defiance of gravity: The tank is kept full, and during a flush, the water rushes into the bowl, creating a surge over the weir (or dam). The flow stops when the bowl is empty, and the tank refills in preparation for the next flush. Originally, tanks were placed high above the bowl to get water moving forcefully enough to clear the weir, but by 1915, narrower, smoother porcelain passageways allowed quieter, 5- to 7-gallon tanks to be mounted on the backs of bowls. The next giant leap in toilet technology came in 1994, when federal law restricted tanks to 1.6 gallons per flush, but to those who used the first generation of low-flow toilets, this leap seemed more of a stumble. "They often needed two flushes," says This Old House plumbing and heating consultant Richard Trethewey. Manufacturers largely fixed that problem by further modifying the passageways to move a reduced amount of water more vigorously into the bowl.