It may not surprise some people that the Mystic drawbridge ranks, among posters on the travel website tripadvisor.com, as the second-most popular tourist attraction in Mystic, behind Mystic Seaport but ahead of Mystic Aquarium.

It probably wouldn't surprise many of the downtown merchants who are accustomed to watching tourists stand and gawk at the hourly bridge openings, the best show in town, and free.

The bascule bridge, a description of its clever engineering design, opening and closing with the assistance of 500 tons of concrete weights suspended overhead, which offset the weight of the roadway deck as it goes up and down, is believed to be the first bridge of its kind built in the country.

It wasn't the first modern Mystic bridge, though. A wooden bridge connecting the towns of Groton and Stonington across the Mystic River was replaced with an iron bridge in 1866, according to a Groton historian, the late Carol W. Kimball.

A steel swing bridge, pivoting the road bed to be parallel with the river, to let boats pass, was built in 1904. The steel bridge was built to accommodate trolleys. But it remained troublesome because it often stuck open.

Today's bridge opened in 1922. It cost $254,000 to build, according to Kimball. Groton paid $63,000. Stonington paid $53,000. The rest came from the state.

The bridge underwent some significant repairs in the 1950s and again in 1999. But none of those fixes came close to the $15 million, three-year overhaul that is winding down now.

Keith Schoppe, one of the state Department of Transportation engineers supervising the project, told me this week he hopes to have a another 20 years in his DOT career and never return to Mystic again for more work on the bridge. These repairs, he said, should last a long time.

The most significant issue this overhaul addressed was the settling of the structure. The northwest piece ended up about three inches lower than the rest of the bridge, a small amount, maybe, but not in engineering terms.

No one knows for sure when the settling occurred, but some theories trace it back to the Hurricane of 1938. Engineers have been monitoring it since the 1950s, and it hasn't changed in that time.

Nevertheless, the three-inch differential has been taking its toll over the years on the mechanicals of the bridge, scoring and scratching metal parts.

To address the underlying problem, the contractor, Cianbro Corp., devised a complicated program to lift and level the main steel superstructure. That occurred last year, during the second winter of work. (The work has stopped every April 15, to let the bridge operate for the summer season.)

In addition to the leveling of the bridge, the project included stripping the old lead paint and repainting the structure and replacing the control house and giant bull gears that lift the roadbed. They also moved the electric motors to new water-protected compartments under the roadbed, where they are less visible.

The project engineers hewed to the original plans and made every effort to respect the unique design of the bridge, which is on the National Register of Historic Places.

It was like peeling back an onion, said Kim Sieber, who helped oversee the project for Cianbro, to interpret what needed to be done.

Within the layers uncovered were some trolley tracks that the engineers believed were salvaged from the old steel bridge and reused in the bascule bridge, as support. They were left in place.

The next major disruption from the project will occur next week, with a 54-hour closure, starting Wednesday, for the installation of a new safety barrier gate, which will rise 26 inches out of the Route 1 roadbed, on the Stonington side, when the bridge is open.

The new gate will be installed close to the part of the bridge that made a cameo appearance in Mystic Pizza, as the character JoJo yelled at her fisherman boyfriend, while his boat went under the opened bridge.

Brent Church, another DOT engineer who supervised the project, said the original bull gears, the trademark symbols of the bridge, with their wagon wheel design, have been preserved. One will be given to each town.

The Day's archives tell of a celebration of the bridge's 70th year in operation, which included festivities on both sides of the river and songs sung around a Maypole.

Maybe a 91-year celebration would be appropriate this spring, some time after the April l5 reopening.

And everyone in the two towns that uniquely share a bridge they built together ought to think of what to do with those bull gears, which weigh only 12,000 pounds a piece.

This is the opinion of David Collins