In nearly 25 years of amateur excavations, Mumford had never found a coin. "I didn't expect I was going to find them," he said.

But last month, the retired Anne Arundel Community College economics professor found a threepence coin minted by John Chalmers, an Annapolis politician who made some of the country's first coins in 1783.

"I always thought that if somebody was going to find a Chalmers coin, it would be the threepence because it's so small,"Mumford said yesterday as he held up the mottled coin, about half the size of a modern penny. "I just didn't think it would be me."

Historians and anthropologists are excited about Mumford's discovery for other reasons. Although experts have long known that Chalmers made coins, they had never found evidence that he worked in Annapolis, and they think 10 Cornhill St. may have be the site of one of the country's first mints.

They hope Mumford's find can shed more light on minting practices during the 1780s, when the new republic didn't have its own currency and some independent contractors, such as Chalmers, made coins in their homes.

Mumford "is a preservation hero," said Mark Leone, an anthropology professor at the University of Maryland, College Park.

Chalmers was a silversmith who began working in Annapolis in the 1760s. He served in the Continental Army and was elected to the town council in 1783. He died in 1817.

By 1783, the United States had won its independence but did not have its own money. People would often use English coins or Spanish dollars that were cut into eight pieces. Many people were paid in tobacco. Congress created the national mint in 1792.

About 100 Chalmers coins have been discovered in collections. And although many historians assumed Chalmers had worked in Annapolis, no one had ever found one of his coins there.

Historians say property records show that Chalmers owned 14 Cornhill St., but many old-time coin collectors believed that he worked a few doors down at 10 Cornhill St., in a small red-brick building with a large chimney. It had never been excavated.

This past summer, the building's owners, Fred and Renee Marshall, decided to renovate their basement. Neither city nor county archeaologists were able to excavate the site, so the 70-year-old Mumford - who had volunteered with archeaological groups for years and collected coins since he was a child - began digging.

Al Luckenbach, Anne Arundel County's archeaologist, had heard the rumors about Chalmers coins at 10 Cornhill St. but was doubtful that Mumford would find anything.

"I thought it was an incredible long shot," he said.

But Mumford, who lives across the Severn River from Annapolis spent three weeks excavating the basement. He hit a brick floor after digging a few feet. Beneath that, he began finding broken bits of pottery, rusty nails, shoe buckles and other artifacts. He also found several British coins and three small cannonballs.

Mumford did as much as he could at the site and took the rest of the dirt to his home and to a friend's home in West Annapolis.

On Oct. 16, Mumford was sifting through some of the dirt in his driveway when he found a small object. At first he thought it was a button. But when he washed it off, he said, he realized "there was no hole in it."

When he put the coin under a magnifying glass, he noticed the inscriptions: "Annaps." and "I. Chalmers." During that time, most printing presses and dies used "I" instead of "J."

"I couldn't believe it," Mumford said.

Coin experts are giddy over the discovery, even though the Chalmers coin is not in good condition and they say it probably would fetch only about $1,000 at an auction.

Some Chalmers coins have sold for as much as $20,000.

"It makes us want to dance," said John Kraljevich, Jr., who works at American Numismatic Rarities, a New Hampshire company that specializes in rare coins, and has seen Mumford's find.

Mumford still has to go through several more wheelbarrows full of dirt but has found 20 coins.

And although he hasn't found any coin-making equipment, Mumford still believes that 10 Cornhill St. was a mint because he had found so much money. Chalmers would often take old coins and refashion them into new ones.

"Why would people throw money away?" Mumford said. During Colonial times, three pence represented a day's wage for many people.

In the basement, there is a large chimney that could have been used to heat metals, and many of the misshapen metal pieces might have been used to make money.

Leone, the anthropology professor, also believes that 10 Cornhill St. was once a mint. He plans to go over historical records to see if there is any evidence that Chalmers had a coinmaking "license" with the state, a question that has puzzled historians.

Said Leone: "We'll get some answers now."