All baseball fans can be divided into two groups: those who come to batting practice and the others. Only those in the first category have much chance of amounting to anything.

Those who arrive just in time to hear. "Oh, say can you see. . ." will never truly appreciate what happens after". . . the home of the brave."

Baseball gives up its secrets in drips and drabs. Conceals its tensions and tendencies well. Some say too well. Perhaps no sport rewards its devotees in such direct proportion to the quality of their attention.

Batting practice is the Rosetta Stone that unlocks many of the game's hieroglyphics. But "BP." as the players call it is also entertaining theater in its own right.

Finally, and most important batting practice -- including all the fielding drills, pepper games and pitching warmups that fall under that generic term -- is a sort of 90-minute tone poem that sets the mood of the sport.

The fan's leisurely arrival while the ballpark is nearly empty, the slowly-paced first hot-dog-and-beer, the meticulous filling out of the scorecard, the calm perusal of the visitors taking their licks. . . all this is part of adapting the spirit to baseball's deliberate speed and its demand for heightened awareness of detail.

Especially at evening, when the sky itself darkens like the stage lights dimming, there is a coexistence of total relaxation and keen anticipation that is totally lost on the fan who rushes to his seat to beat the first pitch.

It is commonplace, but essential, to note that the longest home runs, the best outfield catches, the grandest infield pirouettes are almost always displayed in batting practice.

The fresh memory of those harshly batted balls and brilliant catches keeps us aware of the potential behind each pitch during the actual game. Surely the gent who reached the upper deck at 6 p.m. can still do it at 10 p.m.

Although batting practice is a circus with many side shows, the center ring is always the batting cage.

The rules of the cage are inflexible: time, down to the second, is of the essence. Each team hits for 45 minutes and not an instant more.

The subdivisions of that 45 minutes are minutely guarded. A player who takes an extra swing is cursed to his face. A starter who invades the few precious minutes of the benchwarmers is threatened with an instant visit to the disabled list.

Only a few princes can ignore the ethics of the cage. Ted Williams, in his later years, disdained the hitter's coffee klatch around home plate, preferning the cool of the locker room.

When he stepped from the Boston dugout, the water parted and whichever Bosox had the misfortune to be swinging fled. Williams hit for as long as he wanted and whenever he canted, then returned to his subterranean solitude.

In recent years, Boston's Carl Yastrzemski has tried to cast himself in a similar role -- with little success.

"When are you guys going to run that Yastrzemski out of there. He's had 16 swings already," snapped Boston's Rick Burleson last month by the Fenway cage, more than loud enough for the Boston captain to hear.

Yastrzemski, known for a slight case of rabbit-eared paranoia, anyway, departed in haste, perhaps remembering an incident in Baltimore a year before when the feisty Burleson had gotten in a fistfight with strongman Jim Rice over a matter of just one extra swing.

The batting cage is at the same time baseball's most gregarious gathering spot where two teams can mingle like zebras and gazelles at an African watering hole, and also the most serious of workshops.

Hitters are that rare athletic breed that must do their plotting in public.

"It's annoying that anybody, especially the other team, can get a pretty good idea about what you're going to try to do in the game by what you work on in BP," said Boston's Denny Doyle recently.

"Most of us are constantly changing our stances, looking for different kinds of pitches, trying to protect our old weaknesses and steal a few hits until they figure out our new ones," said Baltimore's Mark Belanger. "Maybe a few great hitters can dictate to the pitchers, but the rest of us have to work on things we'd like to keep secret with everybody watching."

For weeks, the Orioles' Ken Singleton field, watching his average soar until it is the third highest in baseball. "I'm going to the plate knowing they're going to pitch to the old book" on me," he said before one Detroit game. "It's a joke. They just haven't figured me out yet."

It is as easy to figure out who is hot and who's not, who has changed his tendencies, when a player takes 30 swings in a condensed 10-minute period as it is hard to get a clear pattern from watching the same fellow's four at-bats in the game.

Of all the batting-cage spies, New York's Thurman Munson is perhaps the most blatant, leaning on the cage for a half hour at a time starting at the stances and swings of the opponents against whom he must call pitches in an hour.

Isn't this kind of obvious, like a detective wearing a white suit on a tail job. Munson was once asked. "That might be part of the idea," he said with a grin.

When a crossbar fell off the cage last week in Memorial Stadium and landed on Munson's head, raising a large mouse, the Orioles awarded the sleuth in their midst a round of mock applause.

For the hundreds of big-leaguers who think their "swings" are serious business, there has to be one, Fred Lynn, who steps into the cage with a blank brain and a light heart.

"They call me Five-Swing Freddy," Lynn, career average 327, said with a laugh. "I don't ask for a particular pitch in a special spot, I don't work on anything or think about anything. and I swing at every pitch . . . even if it bounces.

"I'm just getting loose and sharpening my hand-eye coordination. I don't always swing at perfect strikes in the game, and the bottom line is that when you finally swing you gotta hit the ball square. That's all I try to do.

"Five qitches and they're rid of me. I can't think of the last batting practice pitch that was so would that I didn't hit it."

To those whose eyes wander from the plate, batting practice offers a panoply of little vignettes. Perhaps an ancient coach like California's Jimmy Reese who once roomed with Babe Ruth, is running Nolan Ryan into a puddle of sweat chasing his deftly placed fungos. Perhaps an idle pitcher tries to catch a high fly over-the-head-behind-the-back and manages to get himself conked. Then pretends nothing happened.

For the fan who wants to get a reading on his team's morale, no barometer is better than BP. The pregame ritual is such an accustomed daily event that most players forgot years ago that they are in public. The argument, the glare, the cross words between teammates are almost always exactly what they seem to be. And so are the hijinx, the laughter, the cameraderie. Nobody bothers to fake it in BP.

The manager who stands by the cage needling his players 8 "hey, Kelly's hitting line drives. What time is it? Must not be game time yet," -- usually knows his players will not suddenly choose this moment to show him up, undercut him as they easily can with a word.

And the manager who sits alone usually has his reason, also. So does the beleaguered soul, like Billy Martin at present, who hides from the press by shagging fly balls for an hour and then signs autographs until there is too little time left for an interview.

The athlete, so protected by the shell of professional aplomb that covers any situation on the field, so glib and experienced in showing only one side of himself in interviews, gives at least a glimpse of his disposition in those hours before the game when he counts as though no one could see him.

Batting practice is baseball's gossip mart, its campfire for tall tales. In the wake of the New York Blackout, the story now making the round concerns the Cubs' wild fast-baller, PEte Broberg, who was right at the point of release in the Shea Stadium bullpen when the lights went out.

Chicago catcher Steve Swisher, rather in the dark as to whether Broberg had thrown the pitch, winced and held the mitt where Broberg had last seen it. "Smack!" the ball struck dead in the pocket.

Such a yarn, unadorned, is not worthy of the cage. Only the kicker qualifies it. "That," Swisher reportedly said, "is the first time Broberg has hit a catcher's glove in six years."

Batting practice presents the baseball tableau on its smallest and most intimate scale. It provides the stuff of anexdote, not myth. Instead of citing great deeds, players are more likely to rest their heads on their forearms as they lean on the cage and discuss their disappointments. Ex-teammates meet again and sometimes find themselves faced with capsulizing the most important events in their lives -- a divorce, a bad beaning, a firing8with a quip. "Hey, Billy," a player yellst o his old team's coach, "Can't you keep your infielders married?"

As soon as a batter catches the assembled eyes by pumping a few into the seats, he is brought back toe arth. Brooks Robinson cries, "Put him in the Hall of Shame . . . Two o'clock hitter, "7p. The old-timers shake their heads and remember names: "Yes, Davey May. Best BPhit ter lever saw. Too bad they couldn't put a screen in front of the pitcher during the game."

The easy hours before the call to arms ("Play Ball") are baseball's pastoral idyll and its guarantee of value. Those who leave the park dissatisfied are usually those who unwisely chose to foresake the game's lazy preamble with its graceful delights. Put "em all in that Hall of Shame.