BOSTON (AP) _ Sometime Monday night or early Tuesday, Robert Merry will retrieve ''Old Nancy'' from her secret hiding place and take her to the town common in Rowley, about 30 miles north of Boston.

There the 1,200-pound, Revolutionary War-era cannon will be fired just twice on Independence Day before ging back in hiding for another year - a precaution Merry says is necessary to keep the militiamen in neighboring Georgetown from stealing her.

''We only move her under cover of darkness,'' said Merry, 45, who holds the honorary post of town cannoneer. ''We have to be very careful because of the neighbors. They send spies out once in a while.''

Horsefeathers, replied Alan Aulson, a member of Georgetown's Board of Selectmen.

''We already have the real Nancy,'' said Aulson, 41. ''If you go down to the Georgetown Town Hall, you'll see her right there - mounted on a block so the guys from Rowley can't take her.''

As for Rowley's fat old muzzleloader, Aulson said: ''They put a few firecrackers in it and they make a nice bang, but that doesn't make it real.''

The annual cannon fusillade between Rowley and Georgetown, in which sarcasm and subterfuge long ago replaced black powder and shot, has been a Fourth of July tradition for more than 100 years.

It all started in 1775, when a colonial raider, Capt. John Manley, captured the British frigate Nancy off the Massachusetts coast and Rowley's militiamen were assigned to guard the ship's cannon.

In 1839, the town of New Rowley, now Georgetown, seceded from Rowley and ownership of the cannon came in dispute.

According to one local legend, the Rowley militia hid it in the Rowley River, which kept it out of the hands of the upstarts from New Rowley but somehow ruined its ability to shoot.

Another legend has it that a clever Georgetown carpenter fashioned a fake cannon out of wood in 1877, after ''Old Nancy'' had been stolen back and forth several times.

Merry says the Georgetown militia probably had some kind of devilment in mind, such as switching the cannons and making off with the real one, but the plot was foiled when an observant Rowley resident noticed that Georgetown's horsedrawn artillery carriage wasn't sinking very deep in the mud.

''He just walked over and stuck his jacknife in the barrel of the cannon, and that was the end of that,'' Merry said.

Today, both Rowley and Georgetown have iron cannon. Georgetown claims its cannon can't be fired because it's the original - the one that came out of the Rowley River.

Merry replies that having a non-firing cannon ''is the price you pay for cheap imitations acquired at some Topsfield yard sale.''

Both towns are celebrating important anniversaries this year. Rowley, which has 3,900 inhabitants, is turning 350. Georgetown, with 5,700 residents, turns 150.

Neither side expects the dispute over Old Nancy to be resolved anytime soon.

''I sure hope not,'' said Merry. ''If it did, it would just be an old cannon.''