There is a great work of art in the center of Dilworth Park. Unfortunately, you can't see it right now.

Called Pulse, the site-specific piece by Janet Echelman was commissioned in 2009, just as a major reconstruction was being planned for the City Hall plaza. Her idea was to use colored light and mist to trace the path of the subways as they rumbled under the park's surface. Ephemeral and magical, the piece was meant to be the capstone of the plaza's $55 million overhaul, a civic gesture that could be as captivating as the famous sculptures of Chicago's Millennium Park.

But nearly 18 months after Dilworth reopened under the auspices of a private manager, the Center City District, Pulse is only half-finished. A network of water pipes was installed under the plaza - at a cost of $1 million - but the CCD has been unable to raise enough money to complete Echelman's artwork. In an email, the organization's president, Paul Levy, said he remained committed to realizing Echelman's vision.

Fund-raising is never easy in Philadelphia, which seems to lack the legions of deep-pocketed, civic-minded donors that cities like New York and Chicago produce in abundance. Yet the effort to build Pulse is now going into its sixth year.

Why does the art have to come last?

There is no doubt that Dilworth's improvements have already enlivened the city's civic heart, transforming a once-unloved plaza into something like a communal living room. Philadelphians gather there to lunch in warm weather, ice skate in winter, and enjoy movies and concerts in the evenings. But for all its success, something is missing: a recognizable marker that would elevate the all-purpose plaza into a memorable place.

We've seen how dramatically that can happen at Millennium Park, where sculptures like Cloud Gate (aka The Bean) and Crown Fountain have become tourist destinations. Built, like Dilworth, on a platform in the center of downtown, the park might have been just another segment of Chicago's lakefront strip had it not been for the distinctive artworks. They have become beloved attractions in their own right, drawing people into their orbit.

You don't have to go all the way to Chicago to witness that effect. Philadelphia was the first city in America to institute a One-Percent-for-Art program, requiring certain government-supported projects to set aside money for public art. As a result, you can now practically navigate through Center City using public art as a compass. Robert Indiana's LOVE sculpture is such a powerful marker that no one thinks to call the park that houses it by its real name, JFK Plaza.

Echelman's Pulse could be a similarly defining element at Dilworth. "There is nothing like this anywhere," says Penny Balkin Bach, head of the Association for Public Art. Most of Echelman's pieces are made from fabric and float overhead, like colorful flying jellyfish. Pulse would be even lighter and more transitory, but at ground level. Echelman evocatively describes its curtain of mist as "a living X-ray of the city's circulatory system."

Like LOVE, Pulse was supposed to be the product of the One-Percent program because the CCD received nearly $6 million from the city for Dilworth's redesign. Even so, the CCD did not plan to include public art in the project until the Art Commission made it a condition of its approval. The business group manages the park under a 20-year lease with the city.

So how much money would it take to finish the artwork?

That's a hard question to answer because the CCD asserts the information is private. "We do not have to answer that question," the CCD's communications director, Linda K. Harris, told me. In an email, Levy added, "we are actively presenting the project to different donors. When we have a success story to tell, we will be eager to tell it."

He did, however, refer me to the CCD website for the Pulse. At the bottom of a long description, it says naming rights are available for $4 million.

But the price of naming rights and the cost of completing Pulse are not necessarily the same thing, according to several specialists in public art. Earlier CCD estimates, those specialists say, put Pulse's remaining price tag around $2 million. The CCD may have asked for the $4 million naming gift to create an endowment for maintaining the Pulse's complex technology.

Levy has an impressive track record when it comes to fund-raising. He managed to bring in an astonishing $55 million to renovate Dilworth. It's just too bad the lavish budget was not enough to finish the job. But the CCD is already part way there: Since 2013, it has collected more than $500,000 in grants from the Knight Foundation and ArtPlace America to construct Pulse.

Sadly, Dilworth is not Philadelphia's only recent government-funded project where public art is viewed as nonessential. You won't see a single painting or sculpture gracing the corridors of Family Court, completed in 2014, or the Broad Street wing of the Convention Center, finished in 2011, despite their stature as important civic buildings. Because they were built and funded by the state Department of General Services, they were exempt from the city's One-Percent requirement.

The good news is that Family Court and the Convention Center have finally changed course. Both have gone through an extensive artist selection process and will soon begin installing pieces in their lobbies and corridors. That will be especially welcome at Family Court, where a four-story wall set aside for art is visible through the glass of its Arch Street facade.

That leaves Dilworth. Though the Art Commission took a brave stand in compelling the CCD to make public art part of the new park, arts advocates complain it does not have an effective system for enforcing its decisions.

In the short term, at least Dilworth will be enhanced by the presence of real art. Thanks to the planned reconstruction of LOVE Park, the city intends to move its namesake sculpture there Tuesday, where it will remain until early 2017, when it will be sent to a conservationist for cleaning. Maybe that brief experience with art will lead the way to a more permanent marker at Dilworth.