Right next to the Great Baltimore Brewery Dig, the Carroll mansion offers a fully furnished look at how the richest man in Maryland lived a century and a half ago.

It's Baltimore's only surviving example of a prosperous merchant's townhouse, and boasts a grand, curving, three- story stairway illuminated by light streaming in through white-shuttered windows. But the house's main claim to fame lies with its one-time owner, Charles Carroll, the longest-lived and sole Roman Catholic signer of the Declaration of Independence. Much celebrated in his later years -- he lived to 95 -- Carroll was a human bridge between the Revolutionary era and the early industrial age of 19th- century America. The wealthiest gentleman in Maryland, he owned plantations and merchant ships, city properties and some of the first shares sold in the B&O Railroad.

Apart from the grand gesture of the stairs, the first impression is that this is a businessman's house: The main ground-floor room is the counting room, austere and spare, with a corner safe encased in a brick vault behind an iron door. Two 19th-century merchants, cast in white plaster, are striking a deal across a heavy wooden table.

"This house isn't an art museum," says Bob Manche, a history enthusiast and volunteer docent. "It's meant to give you a feel for the period."

Manche describes what the neighborhood was like in Carroll's day -- dusty, noisy and often noisome because of nearby Jones Falls, thick with garbage and sewage. It was a mixed neighborhood of merchants' houses, factories, warehouses, the brewery and less-prosperous homes. From his upstairs bedroom window, Carroll could see Federal Hill, where flags were raised to signal the approach of a ship: If a merchant-ship flag was hoisted, Carroll could dispatch a messenger to the harbor to see whether it was one of his ships.

Jones Falls, much sanitized, still flows by. But the stolid, square mansion, fronting right on the sidewalk, sits in rather shabby isolation amid parking lots and a large public-housing project.

Inside, however, it's an early-19th-century world apart. The second-floor formal parlor, for example, is an elegant room with dove-gray brocaded Empire furnishings, a 151/2- foot ceiling, and pine paneling painted to simulate mahogany. The homier ground-floor family dining room has a pewter tea service and a geometrically patterned floor covering of oilcloth, the precursor of linoleum.

And the third-floor bedroom, in which Carroll died in 1832, is furnished with four-poster bed and painted mahogany tables made in Baltimore between 1800 and 1810, when Baltimore and Charleston were the top domestic furniture centers. In the next-door dressing room, near the fireplace is a metal Sitz bath, handpainted with bronze, gold and copper powders in conch-shell patterns; necessities of the day -- washstand with pitcher and basin, and a commode -- add the kind of casual touches that bring the rooms to life. In a small Empire-style lady's bedroom, you'll see gloves and a parasol lying on a chair, a pair of tiny shoes peeking out from under it. In the cheery white- and-green bedroom of Mary Caton, Carroll's married daughter, there's an appliqu,ed coverlet complete with signatures and dates -- one is 1845 -- of the ladies who contributed their "worked" squares.

In all the rooms, an illuminated panel with close-up photographs of furniture details, period prints and paintings and interesting historical details rounds out the docent's commentary. AT HOME IN BALTIMORE CARROLL MANSION -- At 800 East Lombard Street, is open 10 to 4, except Mondays and major holidays. It's free. Tours can be arranged by calling 301/396-4980 or 396-3524. To get there from Pratt Street, turn left at Albemarle and go up one block to the mansion at the corner of East Lombard.