Imagine the startled Sebastian Schmidt, a graduate student from Edinburgh, Scotland. Researching his master's thesis on architectural theory, Schmidt came to Philadelphia over the weekend, looking for gems people don't normally see.

And almost immediately, what does he spot? A brochure that says "Hidden City Philadelphia," about a June-long festival of art installations set inside . . . well, inside the city's gems that people don't normally see.

"That was amazing," he said yesterday, as he wended through town, starting on the 18th floor of the Inquirer tower, just under the building's huge public clock, then moving on to the German Society of Pennsylvania on Spring Garden Street, and to Founders Hall at Girard College.

"I'm interested in sites that tell me more about the city than I find in glossy brochures and main tourist attractions," said Schmidt, 26, pleased to be part of a project so close to his area of study, a research regimen that had already taken him to Chicago, Detroit, Toronto, and Washington, in recent weeks.

Sites you don't find in glossy brochures - a solid description of "Hidden City Philadelphia," a project overseen by Peregrine Arts, a Philadelphia enterprise that, among its several missions, develops arts exhibitions and, here, ties them to specific sites. The idea of "Hidden City" is to install art in spaces you might walk or drive by frequently, but wouldn't necessarily enter unless you had a specific reason.

So, through June 28, the project provides a specific reason - some form of art - to go inside these places, marked by large blue "Hidden City" signs at the entrances. Inside, visitors take fliers with information about the artworks as well as the sites, and view time lines about institutions housed in each building.

In addition to the Inquirer tower, the German Society, and Founders Hall, "Hidden City" invites visitors to wander into the Armory of the First Troop City Cavalry on 23d Street near Market, the Disston Precision facility in the Northeast on State Road, the old Metropolitan Opera House on North Broad Street, and the long-shuttered Royal Theater on South Street. (A full list and schedule are at www.hiddencityphila.org.)

The playful art exhibition by Aleksandra Mir of Inquirer mock-front pages, in the Inquirer-Daily News Building at Broad and Callowhill Streets, became a slight "Hidden City" cause celebre last week when it was removed from the building's first-floor public room after objections from some confused Inquirer staff members who thought it might be a prank.

In the end, it was reinstalled on the 18th floor of the newspaper's distinctive white tower, a much more rarefied venue than the original public room, where it appeared two weekends ago before its dismantling.

Kathryn Goree, the "Hidden City" venue manager at the site, reported a constant string of visitors.

The space offers a sweeping view of three sides of the city, plus a view of colorful tiles, face-to-glaze, on the facade of the North American Building directly across Broad Street.

"I have an interest in buildings," said Anthony Gordon, a Verizon switchman who lives in the Northeast and was taking in the show at the Inquirer building with Grace Bujak.

Like many people visiting "Hidden City," they had an itinerary of buildings they wanted to visit - high on it, Founders Hall, "a building I've always wanted to see," said Gordon.

Inside the regal, balconied and wooden-shelved library within the 700-plus-member German Society of Pennsylvania, at Sixth and Spring Garden Streets, a film installation called Der Sandman, by Stan Douglas, played. "People living in the neighborhood for years and years and years and have never been in here couldn't believe what was here," said artist Tamara Weiss, a volunteer greeter at the exhibition. "It's dramatic when you walk in here, with the lights on the bookshelves."

Some sites, such as the historic Mother Bethel AME Church at Sixth Street near Pine, were busy yesterday with regularly scheduled events - the congregation was making a joyful midday noise in the main sanctuary, where 10 quilts with celestial symbols, by Sanford Biggers, composed the "Hidden City" exhibition there. They hung over the sanctuary balconies.

In South Philadelphia, at Shiloh Baptist Church on Christian Street near 21st, designed by the eminent Victorian-era architect Frank Furness, people took in an exhibit of sculpture and video images, as well as the majestic pipe organ in the room.

The artist, Steven Earl Weber, a resident of Fishtown, walked in with his 4-month-old son strapped into a baby-carrier on his shoulders. "This is great," he said of the site. "I do a lot of work with religion and politics, and I always wanted to do a piece in a church, so this is a huge opportunity."

Upstairs at Shiloh Baptist, in a room with red-and-white brickwork, Iñigo Manglano-Ovalle's thunderstorm sound-installation rumbled. City resident Paul Giannangeli, along with Stephen Wagner and Wagner's visiting mother, Ginny, from North Carolina, went to both installations in the church.

"This is only the second one we've seen," said Giannangeli. "It was the ability to get into these spaces that drew us out initially."

For Schmidt, the grad student from Scotland, the spaces were part of an entire weekend of walking through Philadelphia to find the soul of the city through its look and feel.

"I have to say," he commented after visiting a few "Hidden City" sites, "Philadelphia has been the most overwhelming city to me. It's so rich in textures."

Contact staff writer Howard Shapiro at 215-854-5727 or hshapiro@phillynews.com.