Want to visit a public park in China? It'll cost you.

Many parks in China charge admission, usually a fraction of a yuan. That's small change if you happen to be a tourist, as I was two years ago. But if you're a service worker looking for a green spot to spend your lunch break, or to carve out some solitude during your daily routine, the fees can add up.

In America, we have a tradition of treating public space differently. Urban parks are the physical embodiment of our democratic system, maybe the only place in our increasingly lopsided society where rich and poor can come together as equals. Anyone can walk into an urban park, sit on a bench, and enjoy the sunshine, gratis. At least, that's how it is supposed to work.

But should you want to stroll through Franklin Square one fine evening this spring, it'll cost you, too. And it won't be small change.

Franklin Square - a gift to the city from William Penn - is now cordoned off with a chain-link fence and an opaque black curtain. The enclosure, which looks like something you would find at a top-secret construction site, was installed last week so the park's nonprofit manager, Historic Philadelphia Inc., could lease the public square to a private company for a nightly Chinese lantern festival. For six more weeks, until June 12, only paying customers will be allowed to use William Penn's square after 6 p.m. An adult ticket will run you $17.

Of course, as Historic Philadelphia argues, you can still wander the seven-acre park free during the day. That misses the larger issue. Besides being an eyesore, the fence trumpets a blaring message of exclusion. It wouldn't be tolerated for five minutes at Rittenhouse Square. I can understand why the lantern company, a subsidiary of a Chinese corporation, wanted the fence: By closing the square off to the city at night, it makes the space feel like The Tempest island and turns the glowing displays into a magical experience. Unfortunately, the atmosphere inside during the day is more like a high-security military base.

What's shocking is that the Department of Parks and Recreation, which leases the square to Historic Philadelphia to manage on the public's behalf, not only signed off on the deal, it thinks the event is a good thing. As Commissioner Kathryn Ott Lovell told my colleague Stephan Salisbury, the lantern display "will be a huge draw for people and an economic driver, not just for the park but for the surrounding businesses."

Sorry, parks exist to provide city-dwellers with a green respite, not do yeoman's work for the economy. It's bad enough that our transit systems and cultural institutions are expected to pay their way. The decision to monetize Franklin Square in the evenings takes the philosophy of privatism to a disturbing new level.

A bit of history: Parks became revenue-generators for cities only recently, after a 50-year roller coaster of decline. In the 1970s, when American cities were desperately battling the cancer of blight and flight, our urban parks fell into ruin. The crisis was epitomized by images of overflowing trash cans and crumbling beaux arts balustrades veined with graffiti. Cities began to climb out of the abyss only when they started taking proper care of their parks again.

Because it wasn't easy for cash-strapped cities to find the money for repairs, they discovered an arrangement known as a public-private partnership. The hugely popular municipal strategy is what enabled Historic Philadelphia to take over Franklin Square and the Center City District to manage the plaza in front of City Hall.

It's easy to see why such deals are so appealing. The private manager does the heavy lifting of raising money for renovations and white-glove maintenance. The public gets to enjoy a clean, well-run park. They make taxpayers feel they're getting something for nothing. Until they discover the hidden costs.

Take a look at Historic Philadelphia's board. It's a Who's Who of corporate and political bigwigs, without a single ordinary resident - despite Franklin Square's proximity to the park-deprived Chinatown neighborhood. Same goes for the Schuylkill River Development Corp., another private park manager. Though it does have a community representative on its board, it's dominated by developers. That may explain why it didn't raise a fuss when Children's Hospital decided to build a garage next to the Schuylkill Banks trail.

There is no doubt Historic Philadelphia has done a great job rescuing Franklin Square, long the most problematic of Philadelphia's historic squares. Stranded between the Vine Street Expressway and the Ben Franklin Bridge ramps, it had no natural constituency of residents to tend its paths. Jane Jacobs, the patron saint of all things urban, famously wrote it off as "possibly America's biggest skid row park."

Historic Philadelphia now raises $850,000 a year to keep Franklin Square in tip-top shape, with lush gardens, a splendid fountain, and popular playground. The group earns most of its revenue from concessions, including a burger shack, party room, and miniature golf.

When the lantern festival was proposed, Historic Philadelphia saw a chance to generate a lot of revenue fast toward the annual operating budget. It hopes to gross $220,000. "We would have loved to make the event free," said Cari Feiler Bender, Historic Philadelphia's spokeswoman. "But it takes a heck of a lot of sponsorship. Believe me, we tried. What should we do?"

How about finding another revenue-generating event that doesn't shut out the public?

It's true that other parks often close for private events, like the annual fund-raising galas at Rittenhouse and Washington Squares. What makes the lantern festival different is the duration. "That's a tremendous additional closure," said Peter Harnik of the Trust for Public Land, a national advocacy group. "It's a very slippery slope.

It's not a far step from leasing parkland to private companies to selling that land. Philadelphia has come close to the precipice several times, most recently with a velodrome in FDR Park. We know public-private partnerships aren't going away any time soon. The least a city can do is make sure the emphasis is on the public part.