First encounters between American presidents and their Soviet counterparts have generally been of two kinds. The getting-to-know- you summitry comes with loose agendas and no sure purpose beyond easing tensions by a "free, frank and friendly exchange," as the diplomats say. The down-to-brass-tacks business meetings put finishing touches on serious and substantive negotiations already well advanced.

Kennedy-Khrushchev at Vienna in 1961, then, is the first kind. If nothing else, in a bleak way it cleared the air. Nixon-Brezhnev at Moscow in 1972 is the other; out of it came the first SALT agreement limiting strategic nuclear arms. What is both intriguing and worrisome about the meeting scheduled for November in Geneva between Ronald Reagan and Mikhail Gorbachev is that neither model fits. More so than any of the postwar U.S.-Soviet summits, it has the look of a leap in the dark.

If that's still so in November, the best analogue might turn out to be the Geneva Conference of 1955 -- Eisenhower's first brush with the Kremlin high command. Nothing was done to resolve dangerous differences over Germany and Berlin. Gains were measured in matters such as cultural exchange. Under the cover of a spurious Spirit of Geneva, the Soviets launched a crusade for communism in the Third World.

Nobody can say whether that will be Gorbachev's game. Nor does it follow that Reagan would fall for it. But there is also little in the Reagan record to suggest that he will be any readier for constructive confrontation with his Soviet opposite number in November than he has shown himself to be in 41/2 years in the White House.

It is fair to note that the high rate of turnover in the Kremlin leadership had argued against productive summitry. But you have to ask also whether the president's longstanding, rhetorical obsession with "the evil empire" lends itself to free, frank (never mind friendly) exchanges in any event.

Whatever may be said for the consequences of the Kennedy-Khrushchev meeting, Kennedy was eager to learn everything about his adversary; intellectual curiosity has not been a hallmark of the Reagan approach. Khrushchev wanted to test a new, untested American president. By now, Reagan must be a comparatively well-known quantity to the Soviets.

Khrushchev made Berlin the central issue. However he might have read Kennedy (the critics say, soft), he never made good on his crude threats. No such time-bomb awaits Reagan in Geneva. So the purposes, not to mention the personalities, are not the same this time around.

If the Kennedy-Khrushchev model doesn't work, the Nixon-Brezhnev model works less well. Both sides saw something positive to gain in 1972. Nixon was on a roll. He had opened the door to China -- a score against the Soviets. He had boldly escalated the Vietnam war by mining Haiphong harbor -- another gauntlet thrown Brezhnev's way.

Yet the SALT talks were proceeding apace. Nixon's gamble worked. The meeting was a success on its own terms; an arms-control agreement was reached and there was progress on defining supposedly safer rules of engagement for East-West conflct.

Reagan can hardly be said to be on a roll, if Soviet perceptions of American power are calculated in terms of what is done in relation to what is said. Reagan's actions have never matched the harsh language and brave threats that characterized his years as a presidential candidate and most of his first term as president. He is the old hand by now.

But in the past week or so, Gorbachev's swift rearrangement of the Kremlin power structure has given him the look of a man who knows his own mind and how to act upon it. The president, by contrast, has never come to grips with the divisions in his administration's ranks among ideologues, pragmatists and political handlers.

In a real sense, then, both sides are new to the business of preparing for summitry -- but with one important difference. Gorbachev is settling in for a long haul. Reagan has not much more than two years, if you start the clock in November and stop it with the approach of Reagan's last lame duck year, 1988, when the Soviets will be looking over his shoulder for his successor.

Thus Gorbachev, the new boy on the block, is better positioned to claim that the ball is in the old boy's court. And time is not on the old boy's side.