According to a bit of Soviet folk humor, you can calculate by a hair what direction the Soviet state is going in. The state, the pundits say, constantly oscillates between leaders who are bald and leaders with a full head of hair. The bald ones are the reformers; the hairy ones, the conservative tyrants.

The formula works with uncanny accuracy, as Hedrick Smith shows in the opening chapter of "The New Russians." The shiny-headed Lenin, the revolutionary, was succeeded by Stalin, the ruthless dictator with a thick black mane and mustache. Khrushchev, the bald peasant reformer, gave way to Brezhnev, the conservative with the bushy eyebrows.The short, liberalizing reign of the wispy-haired Andropov was quickly followed by Chernenko, the white-haired, senile defender of the Old Guard. Now, of course, we have Gorbachev, "whose birthmark gleams from a naked pate," ushering in "a new era of radical reform."

Should we be looking for a hairy hardliner waiting in the wings?

This question (without necessarily the qualifying adjective) is the inevitable one asked by those of us who have been watching the extraordinary upheavals in the Soviet Union for the past five years.

Will Gorbachev and his reformers be booted out and replaced by the Old Guard? Will the cycle of action and reaction repeat itself?

Will this "era of radical reform" be followed by a Stalinist crackdown and abandonment of glasnost and perestroika?

In "The New Russians," Smith gives us a surprisingly clear answer to at least that final question. The answer is no. The Gorbachev years may very well trigger a conservative reaction, Smith concedes, but a return to a new Stalinist dictatorship? Impossible.

"The Soviet system has already changed too much; too many people around the country in the institutions of government and the press now control elements of power for one man to gather all the strings of control," insists Smith, author of the 1977 best-seller "The Russians. "

Smith's belief that reform in the Soviet Union will survive Gorbachev is not based merely on wishful thinking. Crisscrossing the country in preparation for PBS's recent four-part television miniseries "Inside Gorbachev's USSR," the former New York Times correspondent was able to witness firsthand the effects of Gorbachev's perestroika.

"The breakdown of the economy, the erosion of discipline, the power of striking workers - all these are impediments to dictatorial rule," Smith concluded after two years of traveling more than 40,000 miles and visiting 25 major cities and nine Soviet republics.

Few predicted the sweeping changes now engulfing the Soviet Union. In fact, most Western commentators insisted such a totalitarian state could not by its very nature undergo such fundamental change.

Smith, who won a Pulitzer Prize for his reporting from Moscow from 1971 to 1974, was among those who had thought the Soviet Union would never shake loose its "dual weights of inertia and dogma." He wrote about it in "The Russians."

But he was wrong. And to his credit, he admits it: "Never had I imagined that the Soviet Union would undergo the kind of seismic transformation that became apparent a couple years after Mikhail Gorbachev came to power in March 1985."

Now in "The New Russians" he explains to us in this highly readable book why those changes have taken place - and who has been responsible.

At times "The New Russians" seems to be a biography of Gorbachev, and understandably so. For these past five years, the story of perestroika has revolved around this remarkable figure.

Again to his credit, however, Smith does not fall for the facile conclusion that the revolutionary changes in the Soviet Union are the result of a one-man show. The real impetus for change, he says, has clearly come from below.

"The sullen discontent and stubborn lethargy of millions of Soviet workers and a cynical, disenchanted Soviet intelligentsia have forced Gorbachev to embark on reform - to try to energize his people and revitalize his country," writes Smith. "At each stage, when he has hesitated, popular pressures have impelled him forward."

In "The New Russians" Smith masterfully demonstrates how the Soviet people themselves are forging this revolution at the grassroots level.