THEY’RE EVERYWHERE, so common that we scarcely notice them. They’re ignored, uncelebrated, the butt of jokes about peripatetic dogs, yet they play a vital role in public safety. In their 200-year history, fire hydrants have saved countless lives and billions of dollars.

The ancestor of the fire hydrant is the even more humble fireplug, a term often still used. Fireplugs date back from at least the 1600s, when firefighters would drill holes in wooden street mains to provide water for bucket brigades. Afterward, they stopped each hole with a wooden plug and marked the location in case the plug was needed again. After fire destroyed three-quarters of London in 1666, the city installed new mains with predrilled holes and plugs that rose above ground level.

The evolution of the fire-plug into the hydrant is sparsely documented. In the 1700s, valves began to replace the simple wood stoppers, and firefighters began carrying portable standpipes —vertical outlets—which were inserted into the plugs. This basic configuration is still in wide use in Britain and other European nations.

The word hydrant seems to appear for the first time in connection with Philadelphia’s ambitious municipal water system, which began construction in 1799. The Philadelphia waterworks, which employed steam pumps to draw water from the Schuylkill River, supplied a system of hydrants, which at least initially were used both for firefighting and as a source of sanitary water for the city’s poor. The hydrant consisted of a cast-iron standpipe enclosed in a polygonal wooden housing. When hoses began to replace the city’s bucket brigades after 1803, the hydrants were outfitted with suitable nozzles.

In the winter months, these essential weapons in the battle against fire were under constant assault by ice. On the coldest nights, watchmen inspected the hydrants hourly, releasing small amounts of water to prevent freezing. Water won’t freeze as long as it stays inside mains buried below the frost line. An obvious but mechanically difficult solution was to keep water out of the hydrants until it was needed. The predecessors of today’s “dry barrel” compression hydrants appeared as early as 1812., according to Thomas Ingalsbe, a hydrant salesman and the owner of a Web site for hydrant aficionados, www.firehydrant.org . In dry-barrel hydrants, water enters through a valve located below the frost line and then drains from the barrel when the hydrant is not in use.

Ingalsbe, who became interested in fire-hydrant history while attempting to scrounge parts for clients with very old hydrants, says that it wasn’t until the 1850s that dry-barrel hydrants became truly practical. The basic configuration of most American hydrants has changed little since. A stem nut, usually on the top of the hydrant, opens a valve to admit water into the hydrant. Closing the main valve automatically opens a drain valve, allowing the hydrant to empty. In areas where freezing is not an issue, many municipalities use the “wet barrel” or “California” hydrant, which is less expensive and simpler in construction and can be activated more quickly because it is filled with pressurized water at all times. The practice of using a separate system of high-pressure water mains just for firefighting began in Rochester, New York, in 1874 and has spread to most major cities.

In the 1930s, manufacturers began to produce “traffic hydrants,” designed to break off on impact without damaging the main valve. (Contrary to what we see in the movies, knocking over a hydrant does not usually produce a geyser.) There have been countless other refinements in materials and manufacturing techniques, but the design of fire hydrants has been remarkably consistent over the years. Perhaps the most visible change is the addition of a larger nozzle after four-inch hoses became standard firefighting equipment. Still, hydrants from the Civil War do not look strikingly antiquated today.

Throughout the 1800s, different cities ordered hydrants built to different specifications. Fire companies called in to help out in neighboring municipalities often found their hoses were incompatible with the local nozzles. Even today there is not a uniform standard for hydrant nozzles. Because of their longevity, the expense, and the sheer number of hydrants—more than 70,000 in New York City alone —retooling them to a single standard is often not an option. Fire companies today often carry hose adapters for different types of hydrant nozzles.

The hydrant does not appear to be headed for obsolescence any time soon. Perhaps, one day, embedded microchips will assist in its monitoring and testing. Already, special foams let firefighters douse flames with less water (and hence less damage to property), but this will only lessen, not eliminate, dependence on hydrants. Ingalsbe says, “My guess is that the fire hydrant of today will look very similar to the fire hydrant of 2101.” And even if some technological wonder renders hydrants obsolete, they’ll likely be with us quite a while before people get around to removing them.