Philadelphia has a great tradition of placing sculptures in important public spaces, most of them constructed of long-lasting materials like metal and stone. But the newest work to take its place in the city's outdoor art collection is conjured out of nothing more than mist and colored light. Fleeting as a heartbeat, it pulses to the unseen rhythms of the city, then vanishes.

Conceived nearly a decade ago, the ephemeral and unusual artwork by Janet Echelman finally made its debut Wednesday night before more than 100 people at Dilworth Park, in front of City Hall. The piece was supposed to be part of the original plaza renovation, which was completed in 2014 — Philadelphia's answer to the beloved Cloud Gate and the Crown Fountain in Chicago's Millennium Park. But a lack of funds kept the park's manager, the Center City District, from activating the sculpture.

Even though the project, called Pulse, is still short $2.9 million, the Center City District decided to go live this fall with a simplified version of Echelman's design, said its president, Paul Levy. After a few taps on a keyboard, the first puffs of colored vapor shot from the plaza's stone surface.

As planned, the mist rolled along a track that precisely followed the route of the SEPTA trolleys located directly below the plaza. The response from onlookers was an audible chorus of oohs and aahs as the fog cloud blurred views of City Hall's marble facade. They hardly noticed that two other mist tracks were dormant, the ones following the Broad Street Subway and the Market-Frankford Line.

In the original design, three lines of mist were supposed to rise simultaneously, each reflecting their designated SEPTA colors: green, orange, and blue. "Rather than try to do the whole thing at once, we decided to start with just the trolleys," Levy said.

The plumes don't merely follow the path of the underground transit line, their appearance is generated by the movement of the trains themselves. Every time an underground trolley pulls out of the City Hall portal, it triggers a fog machine below the plaza that releases the vapor clouds. Once the trolley leaves the station, the mist dissipates and Echelman's sculpture effectively disappears. Until the next trolley arrives.

Pulse is part Las Vegas light show, part high art. Because watching clouds of mist roll across the plaza is so entertaining, the piece is likely to become a popular attraction, much like the digital screen in the Comcast tower lobby. After the speeches were done, children and adults stood in the tracks posing for selfies with the steam puffs, which because of Wednesday night's temperature and humidity had staying power, lasting minutes before dissipating.

But the transitory artwork also succeeds in linking several strands of Philadelphia history in a meaningful way.

The site where City Hall now stands was originally the location of Philadelphia's first waterworks, and the mist evokes the memory of that technological marvel. As the city developed, the area next to City Hall became the place where Philadelphia main train lines terminated. Until the Broad Street Station was demolished in the 1950s, massive, steam-belching, coal locomotives pulled right up to City Hall's doorstep to disgorge their passengers, emitting big puffs of vapor not unlike those generated by Pulse.

Now powered by electricity, those trains run invisibly underground. The point of Pulse, Echelman says, is "to reveal the unseen circulation system of the city."

Like the trains themselves, the vast network of pipes and diffusers needed to operate Pulse are hidden below ground. Although there are other sculptures that employ water and mist, Echelman believes her piece is the first to tie their movements to a transit system. That makes Pulse an abstract visualization and celebration of a transit system that we often take for granted.

Because fund-raising remains sluggish, Levy said, it is not clear when the Center City District will be able to activate the two other lines. When all three are in motion, Echelman says, the green, blue, and orange mists will mix and blur, like the colors in a Mark Rothko painting. (When the skating rink is open at Dilworth, Pulse will not operate.)

For now, the city will have to make do with the green line. Considering the Eagles' Super Bowl success, and its importance to the city's psyche, that isn't a bad place to start.