On Independence Day, Marcia Farabee, head librarian for 13 years for the National Symphony Orchestra, becomes a conductor.

For a few minutes during the annual

"A Capitol Fourth 1999" concert on the Mall, she directs the Presidential Salute Battery of the 3rd U.S. Army Infantry, the Old Guard, stationed at Fort Myer.

It's Farabee's commands of "Fire!" -- relayed via two-way radio to an officer -- to which the cannoneers will respond Sunday during the NSO's production of Tchaikovsky's "1812" Overture.

Although Farabee's task is unusual for a music librarian, and fell to her mainly by default, she does not seem unduly burdened by her responsibility.

"It's just a hoot," she said. "It's just so much fun, and the men are so phenomenal. I wish there were a way for John Q. Public to see them."

Now hear this, John Q: You can see them -- and you may be able to see Farabee as well. She'll be standing behind the performance shell, stage right, and the cannoneers will be located at the Capitol Reflecting Pool not far from the statue of Gen. Ulysses S. Grant, a soldier who made it to the White House.

"A Capitol Fourth" begins at 8 p.m. Sunday, airing live on PBS stations.

Sgt. Jack Klott of the Presidential Salute Battery said for this appearance, his unit uses four 2,000-pound cannons, modified 75mm howitzers with a 105mm chassis, each loaded onto a separate truck with a Humvee leading the convoy.

At the Mall, spectators are roped off from the cannons and trucks for safety. When the cannons fire, there's a lot of smoke, but little actual flame, said Klott.

The platoon positions the cannons in a canted row ("roughly toward the Department of Labor," said Farabee) so reverberations will not break windows in the Capitol and nearby arboretum.

Musicians with the more conventional percussion instruments will be onstage with the strings, reeds and brass, playing mightily under the baton of concert conductor Erich Kunzel.

Farabee said Marines provided the gunfire for the NSO in the 1980s, but the orchestra has paired up with the Old Guard each year since 1990. To the PBS audience and to the crowd on the Capitol's west lawn, the combined effort has proved fascinating.

Not every orchestra's presentation of the "1812" Overture gets to use real artillery, of course, although the Old Guard Presidential Salute Battery also fires its cannons for the Army Band's production and in Annapolis for the Naval Academy's rendition, both in August.

"I would love to have them do it when we do the `1812' at Wolf Trap," said Farabee. Instead, that NSO concert uses electronic cannon fire, as do most orchestras.

Still, Washington's "A Capitol Fourth" is special to the Old Guard guns platoon.

"This is our most high-visibility ceremony that we do all year, unless a president were to die," said Klott. "So only the best and most-experienced soldiers go on this mission."

Klott has been with the unit since 1994, serving a one-year tour in Korea in 1998.

All Old Guard soldiers are volunteers

5 feet 11 or taller. They have passed a background check and scored well on a general technical test. But unlike other Old Guard units, the Presidential Salute Battery has no female soldiers. The platoon also fires between 11 and 17 rounds at the funerals of general officers, depending on rank, but they do their work away from the grave site because of the noise level of the cannons. They also fire rounds for the retirements of general officers and for the arrivals of foreign dignitaries at the Pentagon, Fort Myer and the White House, said Klott.

The Salute Battery arrives at least 90 minutes before a program begins, said Klott, "to set the guns up, clean them, and do any last-minute preparations to be sure we are ready for the real thing. It's standard for every ceremony that we do."

Farabee said she tries to alert spectators at the July 3 rehearsal and at the July 4 performance that the noise will be loud.

"We warn the people on the lawn both days that this is going to happen and so don't be surprised, but there's no way we can tell everyone on the Mall," she said.

Last year, there was an unfortunate incident involving a carriage horse nearby.

"Yes," sighed Farabee. "It literally had a heart attack and keeled over. The Old Guard took it to heart, because they work with horses all the time."

Tchaikovsky intended the cannon fire he wrote into his composition to be realistic, so the guns are not fired in a rhythm. There are times when the cannoneers must load and fire rounds very quickly.

"I do get a little worried," admitted Farabee. "Sometimes Mr. Kunzel speeds things up. The cannon men practice to a cassette tape -- this is a very serious business for them -- so they're used to a particular tempo. They have timed out their particular ceremony. It's a big deal."

The guns platoon practices indoors at Fort Myer and outdoors at Fort Belvoir's Davison Army Air Field. For the NSO dress rehearsal, they wear a ceremonial dress green uniform, said Klott. For the performance they wear dress blue uniforms. They also wear ear plugs, which is why they depend on Farabee, who is reading from her marked score. Klott holds a Motorola two-way radio to her ear; the cannoneers' officer in charge, about a quarter-mile away, listens from a radio or earpiece.

"When I tell him we have started, that is kind of their five-minute warning, and that's when they march on and proceed to the cannons," said Farabee. "They're at parade rest. Then they get a two-minute warning. When it's 30 seconds out, that's when they're at their ready position. They pair off. Each of the four cannons has two men.

"One man actually loads the cannon -- puts the shell full of gunpowder into the chamber and then slams the chamber door shut, just missing the other guy's fingers, and then the other guy fires the cannon. They have to unload it right away because they have to fire it right away. It's so fast. I yell `Fire!' in my lowest voice -- which is hard for a soprano."

Farabee said she cues the guns platoon when the orchestra begins the overture. "When we call the cues, we're calling them a second and a half before the moment to fire. I have it marked in the score. Tchaikovsky was very specific in the score where he wanted them fired."

The officer in charge, listening to Farabee and standing where the cannoneers can see him, drops his arm, signalling the men to fire.

"We do exactly what we've been trained to do," said Klott. "She relays the command to fire to the officer in charge. I will advise the officer when to be ready to fire -- the rest is up to her. I just hold the radio for her. We fire when she tells us to."

"The first time I saw this split-second precision, I was awed," said Farabee. "They are just wonderful. I've often kidded them that they really don't need me at all. They're so good. They take it seriously and practice a lot. We have a good time."

About three years ago, she said, she and the cannoneers added a final volley at the last note of the overture -- a surprise to maestro Kunzel.

"He looked over at me," she recalled. "It was like, `I'll get you.' But it was great, and now we just do it every time."

CAPTION: Presidential Salute Battery soldiers fire cannons on a signal from their officer (below), listening to instructions from the National Symphony Orchestra's Marcia Farabee.