The correction being done by an internal shoring and jacking process amounts to about a eighth of an inch a day - not enough to be perceived by the naked eye. But but when the effort concludes, the upper section of the tower should be much stronger and somewhat straighter, according to Maggie Rosa, chairwoman of the City Hall Restoration Commission.
"The jacking of the tower has begun," Rosa said. "Each day the screws get turned."
Rosa said the jacking is proceeding cautiously, straightening the tower as much as prudent each day while construction workers install massive, 40- to 60-foot steel beams inside the tower's wooden frame.
Structural work on City Hall, built in 1871 and on the National Historic Register, and its tower has been going on intermittently since October 2004.
On the same day the Red Sox won the World Series, City Hall was declared structurally unsafe, forcing a year-long dispersal of City Hall workers to other buildings around the city, and repairs began.
The current work was spurred last March when rot was discovered in an 8-by-12 inch, west-to-east beam that separates the clock section of the tower from the bell section.
Building inspector William Sanborn ordered the building emptied again in anticipation of 40 mph winds. Workers were allowed back the next day when the wind died down.
Meanwhile, the council gave the restoration committee a $1.3 million budget to secure the tower.
The architectural firm McGinley Kalsow & Associates was retained to design the repair and restoration of the ornate tower, which, investigation showed, was a set of modular components - essentially assembled like a wedding cake.
An aerial inspection identified "extreme deterioration" of flashing, roofing and decorative elements, the firm wrote in a report to the restoration committee.
The design team also identified and calculated two distinct tilts - 19 inches to the south, and 41/2 inches to the east, which is toward the center of the building.
The most worrisome problem, the big tilt to the south - toward the harbor and most easily visible from the central fire station - was caused by decay in the beam below the floor of the clock level.