August is the month for sharks. Up they come, jaws wide, from the depths of third and fourth place to swallow the smaller fish that have dared swim in the more pleasant waters of first place for a hundred games.

For a century the rule of thumb has been that the team on top on Labor Day will win the pennant. That means flags are won in August more often than in the dwindling days of September.

The best thoroughbreds don't wait to make their charge at the wire. The head of the stretch is where the track announcer says, "And here comes the favorite, making his move."

In just this fashion, three divisional races that promised to give baseball a scene of absolute madness on the final Oct. 1-2 weekend now have been at least partially defused.

The New York Yankees, Kansas City Royals and Philadelphia Phillies - defending champions all - have used August to bring down the pretenders and surge from third, fourth and third places, respectively, into noticeable, if not comfortable, league leads.

The sight hasn't been pretty for those who love underdogs, dislike the rich and just generally hate those September days called "playing out the string."

On Aug. 1 the Baltimore Orioles, Chicago White Sox and Chicago Cubs were in first place, swimming along a bit nervously like out-of-shape suburbanities who know they have ventured out over their heads.

Then came the sharks - snap, grind, swallow.

The Phillies made the first attack. They were 8 1/2 games behind the Cubs at the start of July. The deficit was 1 1/2 games as August opened.

Starting on Aug. 3, the Phils - long on power, deep down the bench and loaded in the bullpen - won 13 straight games and eventually an alarming 19 of 20. Victories nine through 12 were a quick brunch of Bruin as the Cubs were devoured four times in Wrigley Field.

By Aug. 23 the Phils led Pittsburg in the National League East by 7 1/2 games. The Cubs were out of sight.

The Yankees were next. After an embarrassing 9-2 loss to expansion Seattle on Aug. 7, general manager Gabe Paul chewed out his players royally, telling them in effect to "shut up and play ball."

From the lecture on - perhaps only coincidentally - the Yanks won 16 or 18 to leap from five games behind to a three-games lead on Friday.

"I won't go into an extreme analsis of the reasons," Reggie Jackson said this week, setting a personal record for self-restraint - once in a row. "We're palying too good. If anything can stop this club from winning, I don't wannt to be part of it."

Jackson, moved up to cleanup, was one reason.

"I can bat Chris Chambliss at any spot in the lineup and he won't complain," manager Billy Martin said after dropping one of his favorites to the No. 5 spot. "I wasn't getting the best out of Reggie," added Martin, admitting that Jackson's ego had been assuaged again.

Besides Jackson's incredible 29 RBI in his first 58 at-bats at cleanup, the Yanks got hot streaks from Mickey Rivers, who was reinstated as leadoff batter, and Graig Nettles, who may win two straight homerun title before anyone notices him.

Most important for the Yankees' September hopes, the Bronx Bickerers finally found a pitching rotation, a truly amazing one that included Dick Tidrow, Ron Guidry and new ace Mike Torrez - three pitchers who were nowhere in the Yanks' five-man starting plans in April.

In their recent streak the Yanks have won 1-0 and 11-10, as well as 11-1, 15-3, and 10-1, a sign that the Yanks may yet jell into that dreams team of all-stars that can win any conceivable style of game.

The Royals were last to strike, perhaps because the teams in front of them were the least frightening.

Kansas City trailed Chicago by 5 1/2 games on Aug. 1, but the BlueClads meandered along until the 17th when they began a 10-game skein that put them into their current three-game lead.

Of the three big fish, it is easiest to imagine the Royals folding - that is, if one of the teams behind them could be imagined making a pennant drive.

The Royals have won 47 of their last 68 games, a phenomenon that can be read two ways. All streaking teams run out of luck and gas before long. Few can sprint for more than 40 games.

The Royals have a four-deep bullpen that leads the League in saves (34).None of the fireman is a legend but on a staff that has only one starter with more than four complete games, it is more important to have quantity.

In a normal season it would seem reasonable to suggest that once the favorite gets ahead, the race is over. But 1977's teams all have gaps - either physical or psychic.

Aside from Martin's penchant for self-immolation, the Yanks have to worry about those curious names - Guidry and Tidrow doing the starting pitching and the one man - Sparky Lyle - carrying the bullpen.

Manager Whitey Herzog of Kansas City probably pronounced the benediction on the fading Orioles today.

"I'd love to see Baltimore or Boston beat the Yankees' behinds," he said, "but you'd have to say it's amazing that Baltimore is where it is (four games back). It's great for them, but . . ."

Only the Oriole's September schedule - eight games with awful Toronto, seven with Detroit and six with Cleveland - could create the illusion of a Baltimore pennant rush.

Boston is far beyond the prediction stage. Any team that can win 11 in a row, then lose nine straight, is demented. The Red Sox and Yanks play home-and-home series next month. Those sets could be the regular season highlight.

One shark has been absent so far this season. Cincinnati. With no one watching, the Reds used August to cut Los Angeles lead from 14 to 8 1/2 game. This weekend they cooled off the proud Phillies three straight. That could have the two-pronged effect of puncturing the streak Phils and awakening Cincinnati.

The Reds' pitching is still awful, 11th in the NL. As Herzog said today, "They gotta hit every single day. If you don't put the blame on the pitchers. I don't know where it would go. It sure ain't (catcher) John Bench's fault. He ain't catchin' many. They don't get to his glove."

The Dodgers, thought since May to be the one sure winner, should survive the long swim to shore. But they will be lucky to get there with all their limbs. Three sharks have already struck. The biggest one may only now be coming out of the deeps.