Huck Finn, barefoot drifter on the Mississippi, was conceived in a millionaire's mansion. What a wonderful paradox!

This year marks not only the 150th anniversary of the birth of Samuel Langhorne Clemens, best known as Mark Twain, but it also marks the 100th anniversary of the publication of "The Adventures of Huckleberry Finn." It's a dual occasion that is prompting scores of Mark Twain devotees to explore the house in Hartford, Conn., where Huck first stirred into literary life.

The house at 351 Farmington Ave. stands in a leafy residential area of Hartford, on a tract of land originally known as Nook Farm. It looms three stories high, a massive earth-colored structure of porches, chimneys, balconies, turrets, unexpected gables and oversized windows. "Excessive" sniffed the neighbors, but exuberant seems more like it.

Sam Clemens, barefoot child of midwestern rural poverty, married the daughter of an achingly respectable, wealthy upstate New York family. It was Olivia Langdon's inheritance, not her husband's royalties, that built the elaborate slate-roofed mansion. In 1874, when they moved into the house, Sam Clemens was 39, his wife, 29.

A wide porch runs the full length of the front of the house, an ideal rainy-day playground for Susy, Jean and Clara, the three Clemens children. (An infant son, Langdon, died at the age of 2, the same year Susy was born.)

An oversized front door opens into an oversized entry hall. Its walls are meticulously painted in a trompe l'oeil fashion to suggest hand-carved inlay. In the center of the hall is a circular, high-back settee, just like the ones that used to stand in the lobbys of early movie theaters. In a closet off the hall is one of New England's very first telephones, a wall-hung model whose single wooden hand piece served as both a speaking device and a listening outlet.

"A wild extravagance," Sam Clemens called it, "and a splendid toy."

The drawing room, with its heavy velvet portieres, its oriental carpets and its elaborate, dark-wood moldings, could have been lifted intact from any handbook of Proper Victorian Interiors. Here Huckleberry Finn's creator, author as well of "The Gilded Age," "Life on the Mississippi" and dozens of other perennial titles, often entertained his family and a never-ending parade of guests by singing "Swing Low Sweet Chariot," "Go Down Moses" and other favorites of his Missouri boyhood.

Directly above the fireplace is a window. It's an architectural frill, made possible by a divergence of the flue, and it permitted the family simultaneously to enjoy, as he often pointed out, the flickering flames within, the snows and rain without.

Off the drawing room is the library with its requisite bound sets of the classics. This room was generally reserved for those rare occasions when Sam and his darling "Livy" were alone with just their children for after-dinner company. They purchased its enormous, intricately carved 18th-century mahogany mantlepiece from a castle during a vacation in Scotland. When the top fifth of the structure proved too massive for the ceiling height of the library, Clemens ordered it sliced off and placed over the doorway, where it remains, looking altogether appropriate.

The end of the library opens out onto a glass conservatory. Here flowering plants, herbs and "table greens" were cultivated; the ficus that still flourishes here is a cutting taken from a plant Sam Clemens himself tended. It was in this pleasant, fragrant space that Susy, Clara and Jean starred in their own dramatic creations, often based on tales their father told them.

But what the little girls most enjoyed was not the stuff for which their father would become a literary immortal. They found Tom Sawyer, Huck Finn and his other Mississippi River creations offensive. It was "The Prince and the Pauper" that was his daughters' favorite; and they delighted in acting out, all in costume, scenes from its pages.

The guest room opens directly off the library. It's not hard to understand why guests to the Clemens' household came frequently and stayed as long, or longer, than propriety permitted. It's a large room with its own dressing room and large bathroom, this in an era when indoor plumbing was in itself very much a novelty. The bathroom fixtures are fastened to mahogany paneling, and there are two side-by-side hand basins set into a mahogany dressing-table top. Sumptuous indeed, even by today's over-plumbed standards.

It was in this room that Susy, eldest of the three girls, died of meningitis at the age of 24, a family tragedy of such monumental proportions that her mother, who was abroad at the time, could never bring herself to pass a single night in the house thereafter.

Upstairs the house divides up into large, airy bedrooms, each with a fireplace and oversized windows that look out onto broad lawns and handsome shade trees. In the master bedroom stands the huge mahogany bed whose sculpted cherubs and flying angels had so delighted Sam and Olivia Clemens when they bought it in Venice.

But once reassembled in their bedroom they discovered that its ornately carved headboard made it miserably uncomfortable to read in bed. Rather than abandon their find, the couple simply moved to the other end, a solution they insisted permitted them all the better to admire the cherubic assemblage flying across the headboard.

Even though Mark Twain's writings amply and nostalgically reflect his own school days in the one-room schools of his boyhood, he was curiously at pains to shield his own children from any similar experiences. It was their father's firm decision that all three girls should be taught at home by a governess.

Accordingly, the children's nursery across the hall from the master bedroom was converted to a schoolroom. Their desks, texts and copy books still litter its shelves. In one corner stands a loving father's gift to Susy on her third birthday: a hand-carved replica of Noah's Ark with 200 wooden animals " . . . such as only a human being could create and only God call by name without referring to the passenger list."

Even though the house had 19 oversized rooms (not to mention five full bathrooms), it almost seemed to have been built with no thought of providing some retreat or corner within its massive walls where Sam Clemens could find the requisite solitude in which to write. Somewhat in desperation he appropriated the third floor of the house, originally intended for storage.

Here in a large room, he had both a billiard table and a writing desk. When his power of concentration failed at one, he turned to the other. Here, too, away from Livy's reproachful gaze, he indulged his passion for cigars. Deprived of tobacco and liquor, he insisted, his creative wells ran dry.

On display in the room is a "Standard Visible Writer," a fancy name for a typewriter. This was the same machine on which the author prepared his manuscripts, the first American writer to submit typed pages to his publisher.

It's interesting to note that its keyboard, built in the last half of the 19th century, is identical in the placement of the keys to those in use today. The layout of the letters was based on the need to make the typist's fingers move at the slowest possible rate to prevent the primitive mechanism from jamming.

Mark Twain's own words are framed above the strange-looking contraption: "After writing for 15 years it struck me that I had no talent for writing but I couldn't give it up. By that time I was already famous!"

It was in this room atop his extravagant mansion that Sam Clemens dug back into his memory to bring Huckleberry Finn to life. The Finn was borrowed from his Hannibal, Mo., days where James Finn was the town drunk. The Huckleberry was a souvenir of his first visit to Hartford in 1868, when he saw children, pails in hand, gathering huckleberries on the hillsides. Until that day, he enjoyed telling friends, he thought that huckleberries were "a kind of turnip."

Perhaps because his family thought too little of it, a portrait done in 1889 by Dora Wheeler Keefe hangs in this third-floor retreat. It shows a bushy-haired, pink-cheeked man with a luxuriant red mustache, seated with a corncob pipe between his fingers and a book open on his knees. There is about the painting a distinct air of health and well-being.

The house and its maintenance proved ruinously expensive. It required a full-time staff of six. Twenty years after he moved his family into 351 Farmington Ave., Sam Clemens, world-famous author, lecturer and publisher, was forced to declare bankruptcy. The house was sold at a fraction of its original cost.