WHEN WATERGATE SPECIAL PROSECUTOR Archibald Cox was fired 10 years ago last week, he didn't know that the last head to roll so unceremoniously at the whim of a U.S. President was Salvador Allende's only months before. While Cox's booting was a boon to his career. Allende, the Chilean leader murdered in a CIA-backed military coup, was not so lucky. That abuse of presidential power, like Cox's firing, was discovered only in its wake. Sometimes that's too late.

But memory fades, and attention shifts in a busy republic. On October 20, 1973, President Richard Nixon ordered that Loeb University Professor Cox be fired from his post as Special Prosecutor for his persistence in seeking the Watergate tapes. In what was dubbed the "Saturday Night Massacre." Nixon went through two Attorney Generals before finding one who would carry out that order.

Ten years after the Saturday Night Massacre, Cox is still the familiar, almost folksy figure who wouldn't compromise with the devil. He sits somewhere among the folds of an enveloping tweed jacket, with a bright red bow tie, and a crewcut that went out of fashion with a bright red bow tie, and a crewcut that went out of fashion with the two-term President, in a dusty, book-lined office in a corner of Langdell Library. If Harvard academics could design a hero of their-own, who met the real world and won, surely he would look like Archibald Cox.

"The most important thing at stake was whether the chief executive is subject to the rule of law," he reminisces, supping into tutorial tones. "Or to put the point a little differently, whether our Constitution is an operative restraint on our President or whether, if he chooses to disobey, he has the power to do so."

The Executive executes the law. Would the Presidency move against the President? In the fall of 1973, Nixon faced a court order to surrender his tapes. Instead that day he fired his prosecutor, announced he was abolishing the Special Prosecutor's post, and sent F.B.I. agents to seal off Cox's office. "I'm going home to read about the Reichstag fire," cracked a bitter Cox aide.

Cox remembers: "I wasn't sure what would happen. It was far from clear whether the people would see what the issue was." But public outery was immediate and intense--letters and telegrams poured in: horns honked through the night in front of the White House, reportedly waking Nixon up. Nixon's staff called it the "firestorm". "The only power that could have compelled him would have been the political and moral power of the people," Cox says, adding "What ought to be celebrated about the Saturday Night Macssere is the people's responses." Within days Nixon appointed a new prosecutor and released the first tapes. He dared not act otherwise.

THE STANDARD MORAL OF WATERGATE is that Nixon had to abide the law, for after all, ours is a government of laws and not men. But 10 years ago this week we can see that the ultimate check on his transgressions came not from the justice system but rather from an extra-legal eruption of public opinion. The system worked, but only because the public was mobilized. It was left to the people, Cox says, to "rise up morally and politically and force a President to comply."

If so, the sole consolation left by the Watergate affair may be deceptive. For public opinion mobilizes less effectively in some cases than others, and firestorms gather slowly over arcane or complex issues. Secrecy or distance shrouds excessive power from its best restraint. And if a President's action is swift enough, decisive enough, reactive outcry may be too late. Allende could benefit from no fire-storm, when the Nixon White House contributed to his downfall.

Americans have stopped a President from a flagrant and clumsy attempt to obstruct an investigation against himself. "A President cannot command," Cox comments. "He can only persuade," But when opinion is the bound of his powers, he can still deceive. When another President contends our troops in Lebanon are at peace, so he can avoid the restraints of the War Powers Act; when he alludes to vague security threats in Grenada, but bars reporters and obstructs the public's critical vision of threat; when he distorts details and invents rationalizations for his policies, what effect can a firestorm have?

Reform is cyclical, Cox says; public interest advances then abandons it. Without public interest--or without public knowledge-- "The standards have a way of sagging." That Saturday night in 1973, Richard Nixon feared a chain of resignations in his Justice Department before somebody in the succession would fire Cox. Solicitor General Robert Bork agreed to do it; today, Yale Law School Professor Bork is widely named as Ronald Reagan's top choice for the next opening on the Supreme Court. What does Cox think about that?

"I don't think about that," he reples with a laugh.