The ice was thick on the Potomac yesterday, the trees still bare, the thawing earth just a soggy, brown mess.

Even Charlie Brown would not climb the Peanuts' pitchers' mound on such a day.

But for 1,500 fanatics the Washington-area baseball season opened yesterday in the Fairfax High gym.

For six hours the boisterous, sophisteated Fairfax Baseball Clinic was a chance for those who have been cooped up with the Sporting News for months to see Brooks field a grounder, Dick Bosman toss off a slider, Paul Blair lay down a bunt or Jim SPencer hit a homer.

Squads of kids in hats of every color/and size scurried through the Fairfax halls playing lethal games of catch and gathering autographs from the big leaguers ("I know the other four, Daddy. But who's Larry Haney?")

In the gym a thousand eyes watched attentively as the speakers took turns passing out more esoteric, technical tidbits than the hot-stove leaguers dared hope for.

The pencils of dozens of coaches scribbled furiously as former Oakland farm director Syd Thrift informed them that a fast ball gripped with the thumb and two tingers crossing all four seams traveled at least 4 per cent faster than a fast ball grasped along two seams.

"You coaches have ruined more pitchers than two world wars by teaching it the other way," growled Thrift.

No one seemed able to get enough of what baseball calls "inside the inside." Bosman, who once won an earned-run-average title for a team named the something-or-other Senators, and who now works for the Oakland A's, followed by a one-hour dissertation by moving to a smaller auxiliary gym for two more hours of gratis teaching.

The hundreds of young pitchers who followed him could only look at each other in disbelief as Bosman taught them - in five minutes - such basic distinctions as the difference in release between the "whole-handslip pitch," the "slippery change" and the "window shade" change up.

"I've been living in darkness,' said Herndon High pitcher Mike Doleski. "I thought this would be old stuff I already knew."

Brother, was he wrong. Anyone who didn't learn a thing a 20 yesterday was asleep.

Somehow, amid the discussion of whether a fast ball rotates 14 or 16 times on the way to the plate, the hundreds of 8-to-12 year olds never seemed to lose interest.

"It's got to be over their heads," said one Little League coach. "It's over mine. But none of them want to go home."

The players, however, were not surprised. "Little kids know as much about your career as your wife," said Oakland catcher Haney. "They read every baseball story in the world.

"An 11-year-old boy came to my house this week and said he was sorry that (Minnesota's) Mike Cubbage would not be at this clinic because Cubbage had had a tooth knocked out at second base last June and he wanted to see how he looked now."

Amidst such adulation, the players felt as confident as so many little Bob Hopes. Bosman not only demonstrated the four-seam, three-quarter arm slider ("thrown like a football") but then showed how to duck the ensuing line drive when the pitch hung.

"Usually it's tougher to talk to 500 people than to play before 50,000," said Bosman."This is the exception."

The entire Herndon team, which played a 40 frigid winter league games under the name of the Reston Reds, showed up three hours early and stayed all day.When smart-guy outfielder Vic Tachi announced, "I ani't learned nothin' yet. I want my $2 back," his teammates booed him into submission.

"I learned how to break in a catcher's mitt and so much other stuff I've already forgotten it," said Herndon catcher Danny Lee. "We can't wait forthe season."

They don't have to wait long Virginia AAA baseball practice opens today. Herndon holds its first workout at 6:30 a.m. "I played all winter when we had to start fires in trash cans to keep warm," said pitcher Doleski. "It's great outside now. I'm ready to go."