One of the jumps was led by Derrick Pitts, chief astronomer at the Franklin Institute, who counted down with 500 students at Greenfield School in Center City.
"Three . . . two . . . one . . . jump!" Pitts shouted, microphone in hand.
And jump they did, for a full half-minute before erupting in cheers. More than 100 city public schools participated, along with an undetermined number of private and suburban schools.
Similar mass jumps have been tried in the past, including an even bigger one in 2001 in Britain, which was said to involve close to one million children. But even that jump didn't register much of a seismic impact, said David Galloway, a seismologist with the British Geological Survey.
Jonathan Nyquist, the Temple professor who consulted the Ambler seismometer readings, said the impact of the Philadelphia jump was not immediately distinguishable from the daily tumult of city life. He found a small blip on the graph about the time the students jumped, but couldn't say for sure what it was. Blips are seen all the time.
"There is a little something at 11," Nyquist said. "I can't tell you if that isn't a train going by or something else."
One seismometer is not enough to get a true measure of a given seismic event, he said. That's because a spike recorded on one instrument could represent a big event far away or a small one nearby. A true calculation of the children's impact would require three instruments, but even then it would be tough, because they were spread around the city and beyond.
Nyquist estimated that the jumpers' activity registered below magnitude 1 on the exponential scale used to measure earthquakes. By comparison, the Japan quake had a magnitude of 9 - generating many billions of times the energy of what Nyquist estimated for the jumpers.
Many of the schools were nevertheless able to measure their own jumpers' impacts up close, using the sensors called accelerometers that are embedded in many laptops.
The original point of these devices was to determine when a machine was dropped, so the hard drive could be protected. But accelerometers have long been exploited for other uses, including the various iPhone games and applications that require the user to tilt the phone back and forth.
And in Philadelphia, as a few thousand schoolchildren are now aware, the things can be used for honest-to-goodness science.
The Philadelphia Science Festival runs through April 28 at locations around the city and beyond.
Details: 215-448-1128 or www.philasciencefestival.org
To see a video of the jump, go to http://go.philly.com/jump
Contact staff writer Tom Avril at 215-854-2430 or firstname.lastname@example.org.