Something is missing. Something obvious. Usually, it's so obvious you can't avoid it - in Center City, or leaching out into the neighborhoods and suburbs.
But even though it's gone, few seem to have noticed.
"I totally didn't," said Steve Red, an advertising executive whose offices are just a few feet beneath the 17-ton object that has abruptly vanished.
Ring any bells?
No bell, in fact, is ringing, and that's the point. The bell in question is exactly where it's always been, if quiet of late. More than 450 feet above Broad and Chestnut Streets, Founder's Bell sits bestilled, depriving the area of its hourly ablutions of the chaotic city soundscape.
In residence for more than eight decades, the bell is not swung, but rather is struck by a hammer guided by a mechanism whose pins and gears eventually wore down. That part was fixed, but now the timer controlling how many times the hammer swings - 12 times at noon, once at 1 - isn't working. Repairs are in process, says its keeper, David C. McFarland of KTR Management Services, manager of One South Broad Street, the building beneath the bell.
"It's been out a couple of months. Hopefully we can get it up and running soon," said McFarland, though he was unsure of when that might be.
Edith Wharton wrote of "that deep clangour of bells which periodically covers Rome with a roof of silver." London's great poet-reporter John Betjeman observed "the roaring flood of a twelve-voiced peal from Paul's." For Mahler, the domain of bells was - well, let's not bring the afterlife into it.
But in Philadelphia, the deep D that usually sounds each hour is more like a slow subliminal pulse, assuring listeners - consciously or not - that life is proceeding as it always has, as it should.
Like its more famous if mute and earthbound brother eight blocks to the east, Founder's Bell has had a more itinerant life than one might expect from so large an object. Forged in England and commissioned by Rodman Wanamaker as a tribute to his father, department store pioneer John Wanamaker, it was first placed atop Wanamakers at 13th and Chestnut Street in 1926, and moved at some point to its present spot, built as an annex to the store.
The PNB letters just above the bell went up in the 1950s after the building was bought by the now-subsumed Philadelphia National Bank.
McFarland says the bell's tolling hours are more limited than they used to be, restricted to between 7 a.m. and 10 p.m. "We can't ring it during evening hours since Philadelphia has become more of a 24-hour city with all the conversions to residential."
Not that he gets complaints about its peal.
"We get two kinds of calls: one from people wanting to know why it's not ringing, and the other from someone who says you're two minutes off. It's somewhat of an art to get it as close as we do."
To denizens of Broad and Chestnut on Monday, nothing seemed amiss. Several newsstand operators, coffee shop clerks, smokers huddled in the cold, four doormen at the Ritz-Carlton Philadelphia - not one noticed the bell had fallen silent.
"I thought it was still ringing," said Keith Hedrick, a security guard in the Widener Building.
You could think of it as John Cage's little prank from beyond the grave. Maybe in this centennial season of his birth, the man who taught us to listen beyond concert music for the accidental music of life is making us listen a little harder.
Time was, time was a more precious commodity. People depended on public clocks and bells to align the daily routine. Now, when it returns, Founder's Bell will resume its ever-lessening utilitarian role.
"I mean, in this day and age, most people are checking the time on their cellphones," says McFarland. "But for the sake of the city, we probably should get it fixed. It's still nice to have something that rings."