As you might expect from its name, the "Difference Engine" is a strangely difficult object to describe. You might start by imagining the side of a large crib with uprights ringed by small metal wheels — or rather, spools — but it's better to see the thing for yourself.

Well dusted and with all brass fittings polished, it is displayed in the first gallery of the "Information Age" exhibit at the National Museum of American History. Though an amplified voice indicates the machine's importance in the history of science, it rarely draws a crowd. Never doubt, though, that the Difference Engine is a link to high-powered intellectual excitement, and to an astonishing man whom the British government has lately honored with his own postage stamp. He is Charles Babbage, the man who more than 150 years ago first faintly glimpsed today's computer age and strove to reach it.

The Difference Engine is a calculator. It prepares numerical tables using a mathematical technique known as the method of difference. Today such tables — the kind often used in navigation and astronomy — would be computed and stored electronically. Nearly a century and a half ago, the Difference Engine did much the same work, but slowly and mechanically.

Two Swedes, Georg Scheutz and his son, Edvard, built the Smithsonian's machine in 1853. Each of its long shafts holds disks, and each disk has wheels with ten teeth that correspond to marks in the disks. A scientist could set the disks with known figures, odd or even, turn a crank, and by reading down on each shaft, find the result of a calculation. This particular "engine" could also print out its answers. Sold to an observatory in Albany, New York, it was given to the Smithsonian in 1963.

The Scheutzes had no interest in pleasing design. Their device worked well, though, for they had followed to practical completion the concepts of one of the 19th century's most brilliant minds. Inventor and philosopher, Babbage produced a prototype of the original Difference Engine as early as 1822, then kept adding refinements without ever quite finishing it. He enthusiastically endorsed the work of his friends Georg and Edvard Scheutz. But during the years it took them to complete their machine, the inventor's mind was groping toward a mechanical device that would go far beyond calculation. It would actually store the data that it produced, then reuse the information to add more. Babbage described this process as "the engine eating its own tail."

What he foresaw was a primitive computer. As his biographer, Anthony Hyman, wrote, "Babbage worked by himself, far ahead of contemporary thought. He had not only to elaborate the designs but to develop the concepts, the engineering, and even the tools to make the parts. He . . . stands alone: the great ancestral figure of computing."

Charles Babbage was born in 1791 to a Devonshire family of wealth and leisure. He went to a good school, then set out for Cambridge with little inkling of what to expect there except for a warning that it was a bad place to buy wine. Naturally brilliant at math, he found that his mathematics professors actually knew less than he did.

Clearly a genius, Charles appears also to have been a charming young man, filled with a youthful determination to improve math teaching at Cambridge. With his close friend John Herschel, son of the famed astronomer William Herschel, Babbage helped found the Analytical Society.

Like the Lunar Society in the days of Josiah Wedgwood and Erasmus Darwin (grandfather of Charles), two generations before, the "Analyticals" gathered in noisy sociability to discuss, among other things, the manufacture of cloth from cotton and wool, and the iron forges and steel plants then filling England's green Midlands. Their aim was to calculate how science could best support the continuing Industrial Revolution with new techniques, better tools, more accurate planning.

Long before going up to Cambridge, Babbage devised a way to walk on water. "My plan," he wrote, "was to attach to each foot two boards closely connected together by hinges themselves fixed to the sole of the shoe." The thing had worked well enough for young Charles to squish downriver on an ebb tide. But something went wrong, and he had to swim for his life.

He left Cambridge, obsessed with the idea of using machines to speed up time-consuming mathematical calculations. Thus the idea of a Difference Engine was born. Charles also envisioned a machine that would handle more decimal places to speed the process of "carrying" and "borrowing."

"He was always the great improver," says Peggy Kidwell, curator of the Scheutz Difference Engine at the Smithsonian. Kidwell, coauthor of Landmarks in Digital Computing, thinks Babbage was constantly goaded by the urge to improve not just his Engine, but the quality of 19th-century life. Among other examples, she cites his experiments with printing tables in different colors on different shades of paper (black print on white paper was hard on the eyes). In 1826 he had one page of tables published in 13 different inks on 151 different colors of paper.

More important, he endlessly sought ways to take the killing drudgery out of factory work. Metering devices, for example, would automatically do the mindless counting of some repeated action in a mill. He invented a time clock for punching in; suspicious workers called it the "tell-tale." He designed a device to record the direction of shocks in earthquake-prone areas, an inking roller for printing and, thinking perhaps of those boyhood "water shoes," proposed an idea for a hydroplane.

He tried to get the government to change the traditional values of pounds, shillings and pence for a decimal system. He got about as far as American scientists have today after years of pleading in vain to introduce the metric system. Still, the British adopted his proposed two-shilling piece, or florin, making ten florins equal to a pound sterling.

Babbage never fully finished the expanded Difference Engine, which he began calling the "Analytical Engine," but parts of the original ran smoothly in displays and kept bringing him more attention. "Now Mr. Babbage," said one woman after listening to his explanation of it, "there is only one thing that I want to know. If you put the question in wrong, will the answer come out right?" People eventually learned that a computer is no smarter than its programmer. As the saying goes, "Garbage in, garbage out."

Babbage was a splendid host. The Duke of Wellington came to call. So did Charles Dickens. Babbage talked shop with Sir Charles Wheatstone, inventor of the Wheatstone bridge for measuring electrical resistance; with Joseph Whitworth, whose rifle cannon with hexagonal bores were bought by the Confederate States of America and used with deadly accuracy on unfortunate Union troops; with Isambard Kingdom Brunel, builder of the giant iron ship Great Eastern (Smithsonian, November 1994).

Above all, there was Augusta Ada Byron, daughter of the poet. She was a brilliant and beautiful woman, whom Byron had named "Augusta" after his half-sister, who was also his mistress. Though Augusta Ada was her daughter, Lady Byron never forgave the girl for having the same name as the woman she despised.

Ada was skilled at mathematics and one of the few people able to understand and explain what Babbage's inventions were all about. It was a chaste affair — Ada was married to the Earl of Lovelace. But she devoted years to helping Babbage, writing explanations of his achievements and dreams, admiring him with professional as well as filial devotion. She wrote up some of his notes so well that he wanted to publish them under her byline. She declined. Yet when he rewrote a bit of her copy — just changing a word or two — she made it clear that no one ever rewrites a Byron.

Like a number of Victorians, Ada became an opium addict. During her grim death from cancer, her mother hid the opium she was then using to ease the pain so that Ada would suffer more — and repent. Her death left Babbage bereft of the woman whom Anthony Hyman describes as "his beloved interpretress." His plans called for a punch-card system that would command the functions of the still-theoretical machine. He got the card idea from a famous French loom introduced in the early 1800s by Joseph Marie Jacquard that used selected cards to automate the weaving of multicolored patterns. It was Ada who could best express what the card system would do for Charles' machine: "We may say most aptly that the Analytical Engine weaves algebraic patterns just as the Jacquard-loom weaves flowers and leaves."

Though Babbage's ideas for storing information exist only in his voluminous plans, his concepts kept nudging closer to our computer age. A card system was vital to the earliest electronic computers, post-World War II devices that filled a whole room.

The Scheutz Difference Engine also links us with the early days of the Smithsonian. Joseph Henry, the first Secretary of the Institution, visited Babbage in 1837 and wrote: "He, more, perhaps, than any man who ever lived, narrowed the chasm [separating] science and practical mechanics." A mild assessment. Judging Babbage today, as computers whir all around us, making possible a life experience that extends from spaceflight to the Internet, it is hard not to regard this 19th-century prophet with bewildered awe.

By Edwards Park