A Byte Out of Time : THE DIFFERENCE ENGINE, <i> By William Gibson and Bruce Sterling (Bantam Books: $19.95; 435 pp.)</i>

What if the Victorians had built computers--huge, mechanical contraptions based on the calculating engines of Charles Babbage? What if the Information Age had arrived a full century ahead of schedule?

Such is the alternative world of “The Difference Engine.” It’s a tempting alternative, given that the historical Charles Babbage really was the high-tech genius of his age. The “Difference Engines” he actually built were large, complex calculators, well ahead of their time. But Babbage also planned another type of engine, far more ambitious, containing all the elements of a modern digital computer. It even used punched cards, an idea Babbage borrowed from the Jacquard loom. More to the point, it could alter its own sequence of operations: Like all true computers, it could change its mind.

Alas, the British government could change its mind, too, about financing Babbage’s project. Almost no one understood what he was trying to do except Lady Ada Lovelace. Ada, daughter of Lord Byron and the first woman programmer, wrote essays explaining how the Babbage engine could weave algebraic patterns, just as the Jacquard loom weaves flower patterns.

The real Babbage died in obscurity without completing his great engine. In this novel, however, the British government is happy to build Babbage’s steam computer, and even happier to use it. Gibson and Sterling’s alternative Britain is a technocracy, ruled by an elite of Radical Industrial Lords, with Lord Byron as prime minister. Scientists and other “savants” are elevated to the peerage as Lord Darwin, Lord Brunel and, of course, Lord Babbage.

The young paleontologist Edward Mallory has hopes of becoming a lord himself. He has returned to London from digging up a “leviathan,” or brontosaurus, fossil in America’s Wild West. Mallory is a Catastrophist, believing that evolution proceeds by sudden leaps. He is unware that his own life is about to undergo catastrophic changes.

Mallory meets Lady Ada Byron at a steam-car race, where she seems to be a drugged hostage. She manages to slip him a pack of punched Engine cards. Simply having these cards places Mallory at the center of a web of intrigue involving police spies, Luddite thugs, murderers and arsonists.

The melodrama is played out in an appropriately Victorian, yet strange, setting. In 1855, Britain is experiencing all of the heaven and hell of technology. The good life produces a wealth of inventions ahead of their time, including subways, steamships, cash registers, credit cards and automatic rifles.

Enormous steam-driven Babbage engines are in constant use by the government, operated by young computer “clackers.” There are clackers everywhere, working on independent computer tasks. We find John Keats clacking away in computer graphics, while Benjamin Disraeli is a hack speech writer wielding his word processor. Life is so good that even Engels has joined the establishment, as Lord Engels. (Marx has fled to America, to take part in the Manhattan Commune.)

On the other hand, everyone has a government-issued identity number and a corresponding file at the Central Statistics Bureau--the state police. Victorian poverty, disease and pollution are as bad as ever. Keats is still dying of consumption. There are still plenty of syphilitic prostitutes and cholera-infested water supplies. Discontented mobs are roaming the streets. London has become a place as thick with intrigue and surveillance, as it is with smog and stench.

As Mallory makes a nightmare journey back and forth across the great city, the sewage and smoke finally become life-threatening. Nor are they the only threats to his life: Luddite rioters are looting and burning the city. Mallory, the savant, finds himself a primary target of the revolution.

While the great Babbage engine never is seen, its dark force is felt throughout the novel. The title refers not only to Babbage’s Difference Engine but also to the way computer power--possessing and manipulating information--can make all the difference.

For good or ill, Britain has become an absolute power in the world. France has tried to follow, but the French giant computer, the Great Napoleon, suffers a mysterious breakdown. Russia has been quelled by a successful campaign in the Crimea. America remains divided into four weak nations: the Union, the Confederacy and the Republics of Texas and California. This weakness evidently has been engineered by British diplomacy, backed by a superior system of information-gathering and -processing.

Pitted against this powerful system, the Luddites too have grown smarter. They no longer try to break machines with hammers but now fight with the weapons of high technology. They have their own clackers, who cook up the mysterious pack of punched cards.

The pack comprises a program that might be fatal to the government’s great Babbage engine--a virus, if you will. In principle, there is such a deadly program for any computing machine. The program presents a proposition that is true but unprovable within the machine. In principle, the machine will worry away at such a problem forever, until it breaks down (or until someone switches off the power; the authors do not explain why the owners of a team-driven Babbage machine in trouble cannot just stop stoking, but never mind, it makes a good story). We see the fatal pack of cards pass through several hands--a Luddite gives it to a London prostitute, who mails it to Paris; Lady Ana Byron hands it to Mallory, and at one point, it’s hidden in the skull of the brontosaurus.

“The Difference Engine” is an intelligent novel, taking on weighty themes: information science, catastrophe theory, scientific responsibility, the nature and limits of mathematics. And for the most part, it takes them on with a light touch. There are intellectual jokes aplenty here, from clackers to Lord Engels. Even the name leviathan can mean not only a monster dinosaur but also a huge, repressive bureaucracy. The authors repeat the (true) story of the man who stopped a community’s cholera epidemic by removing the handle of the town pump.

But the novel isn’t always as accessible as we might hope. Some characters, scenes and subplots seem to have no other function than to convey the pack of punched cards from one place to another. Sam Houston turns up to no purpose. And in the final portions, the narrative breaks down into brief notes, speeches, even a synopsis. This may be an attempt to somehow emulate the breakdown of a Babbage engine in prose, or it may simply mean that the authors grew tired of the enterprise.

Even so, it’s fun to follow the mixture of real and imagined history, worked up into a ripping adventure yarn.