Manhattan mansions provide a peek at elegance that is no more

Most everyone visiting New York City tends to think BIG. The International Trade Center, Rockefeller Center, Madison Square Garden, the Metropolitan Museum of Art, even the Statue of Liberty. They are all imposing designs and structures.

But tucked amid the super-skyscrapers, monumental museums, and bustling commercial districts are smaller-scale mansions and houses. In their way, they are as exciting as the awesome Empire State Building or Radio City Music Hall. They are the New York of an earlier, more tranquil and romantic day; the homes of the rich and influential, the politically ambitious, the comfortably wealthy, the flamboyant, the historically known.

Some of the old mansions offer a tantalizing peek at a kind of lavish elegance that is no more. Their ornate interiors now feature collections of books and papers, fine art, or decorative treasures. The Pierpont Morgan Library , at 36th Street just off Madison Avenue, which harbors rare books and papers, was an extension of the great financier's home. The Frick Collection, a breathtaking display of art at the corner of Fifth Avenue and 70th Street, was built as the gallery portion of steel magnate Henry Clay Frick's American ''palace.'' The Smithsonian Institution's National Museum of Design - The Cooper-Hewitt Museum - at 2 East 91st Street, was the magnificent residence of yet another steel tycoon, Andrew Carnegie.

The greatest spellbinders, however, may well be the houses that have been carefully nurtured to give visitors a ''living'' glimpse of life as it was in the days when the New Yorkers had yet to feel the rumble of subway trains underfoot or be bombarded by the never-ending fumes and noises of gasoline-powered vehicles. There is this trio, for example:

Morris-Jumel Mansion, West 160th Street and Edgecombe Avenue:

Age, romance, and historic importance are built-in facets of this beautiful mansion, built by Col. Roger Morris in 1765 as a retreat for his family far from the oppressive summer heat of the Morrises' year-round home in downtown Manhattan.

Morris, a Tory, fled to England at the start of the Revolution and the house became headquarters for George Washington and his men. When the Continental troops vacated the building, it was occupied by British Gen. Sir Henry Clinton and his officers. Later, Hessian mercenaries made it their quarters.

But probably the most romantic period of this elegant mansion's history was during the occupancy of the lovely Eliza Jumel. Her husband, a wealthy French wine merchant, bought the house for her in 1810 and Mme. Jumel restored the by-then run-down property to its former beauty with wallpapers and fine furnishings imported from Paris, some formerly owned by members of Napoleon's family.

After her husband's death in 1832, it was said that Eliza was ''the richest widow in the country.'' Always outgoing and often referred to as ''a social climber,'' the widow Jumel waited only a year before marrying Aaron Burr, former vice-president of the United States. The bridegroom was 77 years old. The bride was 59. Gossips said she married him for the prestige of his name and that he married her for her money. Six months later, the marriage ended in divorce.

Today, visitors see the octagonal drawing room that served as a council chamber for George Washington. They see his bedroom on the floor above and the handsome dining room where, every day at 3 o'clock, Washington and his staff gathered for their main meal. The front parlor, the scene of Mme. Jumel and Aaron Burr's marriage, is also intact.

The house is furnished as it appeared over a span of its finest years. The drawing room, with hand-painted Chinese wallpaper and English furniture, reflects the taste of the Morris family. Original Jumel furnishings are in the front parlor. Objects that once belonged to Napoleon's family and Empire-style furnishings decorate Mme. Jumel's bedroom. Furnishings that were once Aaron Burr's are in his bedroom.

Special exhibits, concerts, and lectures are often held at the mansion. It is open Tuesday through Sunday from 10 a.m. to 4 p.m. Admission is $2.

Theodore Roosevelt Birthplace, 28 East 20th Street:

Theodore Roosevelt was a boisterous, active president, one who was greatly beloved. The house where he was born in 1858 and lived until he was 14 offers a detailed insight into his secure background and also of how a respected ''first family'' of New York lived comfortably yet conservatively in 19th-century Manhattan.

The first home of the 26th President of the United States is a four-story building with a front-stair stoop entrance similar to those that fronted the many other so-called ''brownstones'' which, in those days, were favored by New York's solid middle class. The adjoining ''attached'' house was an uncle's home until Theodore Roosevelt's father bought it and joined the two residences with a door at the rear of the second floor, which led to connected back porches.

Roosevelt's description of his childhood home brings his surroundings to life: ''It was furnished in the canonical taste'' . . . in which men of substance liked to have their homes reflect the dignity of their traditions and lives. ''The black haircloth furniture in the dining room scratched the bare legs of the children as they sat on it. The middle room was a library, with tables, chairs and bookcases of gloomy respectability.''

Five of the rooms are now as they were in 1865. The high-ceilinged parlor with crystal chandelier, blue damask draperies, and huge mirrors is typical of the stately period. To children, this was ''a room of splendor . . . open for general use only on Sunday evening or on rare occasions when there were parties.'' The original furniture and a portrait of his mother are in the front bedroom, where Roosevelt was born. The nursery and the porch where the frail child, Theodore, exercised are also there.

The house is open daily from 9 a.m. to 4:30 p.m. Closed on New Year's Day, Thanksgiving Day, and Christmas. Admission is 50 cents. Senior citizens and children are admitted free.

Old Merchants House, 29 East Fourth Street:

It is a precious gem of a house, one that remained miraculously within the same family and with the same furnishings from the day it was purchased in 1835 until Gertrude Tredwell, daughter of the house owner, Seabury Tredwell, passed on in 1933.

The Old Merchants House is tucked amid warehouses, garages, and other gaunt, impersonal commercial buildings that belie the fragile yet warm loveliness that waits inside the columned front door. What is most unusual in this superb example of Classic Greek Revival architecture is that both the furnishings and interior details are the same as those selected, arranged, and used by the Tredwells, who made it their home for almost a century.

The Tredwells were a prosperous merchant family, and their home is a proud reflection of their station. Their furnishings as well as the house itself have been lovingly and carefully refreshed and restored, so that today visitors might be calling on the Tredwells themselves.

The double parlors have been described as ''two of the most beautiful rooms in America.'' They are exquisitely proportioned, even to a false mahogany door placed to achieve a balanced effect with a matching real mahogany door on the same wall. The rooms are separated by columns that disguise sliding doors.

The furnishings are a blend of early 19th-century styles in a red and gold color scheme highlighted by off-white walls (a shade that was rediscovered by microscopically analyzing six or seven layers of later paint applications). The only replacements in the two grand rooms were faithful reproductions of the carpeting, a red and gold design accented by tiny blue flowers, and of the crimson damask draperies with their ornate tassels and trim.

The Old Merchants House is open on Sundays from 1 to 4 p.m. Admission is $2. Closed during August.