At first glance, the tufted, tasseled contraption in tthhe advertisement appears to be an overstuffed Turkish sofa. But wait. Not only is the "common sense invention of the age" the last word in household furniture. It's much, much more. It also happens to be a full-sized bathtub, complete with an 18-gallon water tank, heating appliances and waste-water attachments -- a "combined" sofa and bathtub. "A piece of furniture that isn't a combination of something or other is no good nowadays," groused its inventor, C. A. Baker. The year was 1883.

Baker's unintentionally hilarious safa/bathtub and other Victorian furniture combinations of its ilk -- spiton/foot-benches and billiard table/davenports -- are long gone, but their contemporary counterparts are going strong. multipurpose furniture and furniture designed to make the most of small living spaces and lack of storage are selling like hot cakes.

This year, for instance, the showstopper at the International Furniture Fair in Cologne was a wall system that beatured both a bed that turned into a couch and a sliding track that let a storage unit move around corners as well as from one end of the system to the other.

Elsewhere, tthe story is the same. Roche-Babois has "Surprise," a chair concealing a mattress and armrests., Castelli has its "Plia" folding chairs, which hang on the wall when not in use, and one of Castro Convertibles' best sellers is a 33-inch square ottoman hiding a twin bed with an innerspring mattress for $199. The firm is about to revive and and update its most ingenious design -- a chair that contains an iron, an ironing board and a bonnet-style hair dryer. It apparently got its old name, "The Ready Lady," prior to women's liberation. Its new name has not been officially announced, but, according to company president, Bernadette Castro, it will reflect the times.

Murphy beds have been dropping out of walls for 82 years, but, nowadays, beds are just as likely to be hidden in hassocks, coffee tables and credenzas. Concealed storage cabinets drop from ceilings, televisions rise from floors, remote control switches and sprocket mechanisms turn coffee tables into dining tables, stainless steel cylinders revolve to reveal unsightly video equipment or a bar. Brave New World? Not at all.

"It is very important that furniture do more than just sit there," says Dan Droz, a 31-year-old Pittsburgh furniture designer who typifies the new space-conscious attitude. Droz is responsible for the "Universal Table," which recently won a Daphne, the furniture industry's equivalent of the Oscar. his invention, which comes with a variety of reversible tops and an X-shaped base, can be adjusted to sox different heights, so with its two-way tops, "its actually 12 tables in one," he says.

"Space is getting smaller and demands more of furniture than just aesthetics," insists Drox, whose credits include the Popsycle Table -- notable for legs that fold completely flat and, not surprisingly, look like popsicle sticks -- and a tea cart that collapses to a width of 1 1/2 inches. (Last year, the Italian trade magazine Abitare pronounced the Popsycle table one of the best pieces to come out of America.) Clearly, Droz's designs meet his own criteria that furniture should either store easily for occasional use or serve more than one function. The Universal Table, for example, can be anything from a 16-inch high cocktail table to a 34-inch buffet.

Droz's sentiments are echoes by Paul Evans, whose electrically operated multi-functional cabinetry is sold only through interior designers. Evans has designed contemporary furniture for 31 years, but the fururistic tangent is a relatively recent development: a product of his son's graduation with a degree in electrical engineering and his own boredom with furniture that "does nothing."

"I got so sick and tired of decorative furniture that I could spit," laughed the designer, who now specializes in custom items, like a footboard that conceals a TV -- push a button and up it rises -- or a carpeted daia that goes to a prescribed hight when you stand on it -- perfect for lofts or out-of-reach cabinets. At The Paul Evans Studio on New York's East Side, the dais goes 14 feet up. "Consider it a contemporary version of the old library ladder," says Evans. "Its the future of furnishings. Once they're over the initial shock of pushing a button and having things happen, people adore it or it scares them to death."

A one-room apartment out-fitted for an exercise fan is another ready example of Evans' brand of furniture that "does something." It has a series of pedestals that hide everything," he said. "There was a bike in one, barbells in another, a little telly in anoter." They all opened by remote control.

'With electronics, video and audio, we're in tthhe position we were in when indoor plumbing cam along. No one knew what to do with it. Do you put it in a closet? In a living room? Do you set flowers on it? The real trick is to take space you'd ordinarily diregard and to use it well."

Frank Gehry is a Los Angeles artchitect best known for working with materials most would consider junk. His own house sports a rough plywood and chain-link-fence-covered facade.

True to his maverick reputation, he has designed a ffull line of furniture out of corrugated cardboard. The collection includes a compact nest of three S-shaped chairs called Sleigh Chairs. "They're great in apartments becarse you can put three chairs in the space of one," he said of the design, which stretches a room's usefulness while saving floor space.

"It had its altruistic aspect in that I used recycled material and built a good design -- cheap," he remarked. "It was like the VW . . . beautiful and sculptural sshapes out of material that looks richer than it really is. The finished fabric," he said of the design, which uses up to 60 pieces of cardboard per piece, "is an engineering feat" -- a structure that holds up weight and uses a material thhat would otherwise have been wasted.

If you're trying to find a way to stretch your space, doubleduty furniture is a good place to start. And it has other advantages. Lurking beneath that innocent ottoman there may be a guest bedroom, comfortable yet just spartan eenough ttokeep house guests from settling in for an interminable visit. CAPTION: Picture 1, the 44 inch Part Time Ottoman hides a cot-size mattress, (By Thayer/Coggin; available at Chase Furniture, Ursells, and Sloane's, $660 starting price); Picture 2, the Sofa/Bath Tub ad appeared in an 1883 decorating magazine. Its inventor noted that, among its virtues, it "seldom explodes."; Picture 3, the Universal Table's wood base adjusts like an ironing board and turns on its side. (At Bloomingdale's, $250; for table base alone, $119, write: Dan Droz and Associates, 4614 Fifth Ave., Pittsburgh, Pa. 15213; Picture 4, the unopened Stair/Chair. Lower, the chair's back flips over, becoming the support for a stepladder. (At Bowl and Board; unfinished, $129.)