It is 3:30 p.m. and the thermometer is climbing to 90. Impervious to the heat, Spec. 4th Class Mark Arnold, 23, from Memphis, Tenn., clicks the heels of his spit-shined shoes and raises his rifle to the shoulder of his winter-weight woolen uniform.

The honor guard is about to change at the Tomb of the Unknown Soldier in Arlington National Cemetery and that means Arnold will soon be relieved.

While many must spend long hours in Washington's paralyzing heat and humidity, few are required to do it standing at rigid attention, encased in tightly-buttoned 10-pound uniforms. But that is the daily regimen for Arnold and his fellow Tomb guards, who present arms and parade at 30-minute intervals as many as eight times a day.

I'm on my second blouse (uniform jacket) and pair of pants today. Sometimes I go through three uniforms and four is not uncommon for some guys," says Arnold's comrade, Sgt. Eugene Harris, 24, of Indianapolis. As Harris speaks, the second shift's relief commander helps him to buckle into a white cummerbund, producing the perfect military silhouette.

After his dark woolen jacket passes inspection, Harris wets his white gloves as a finishing touch. "This helps you handle your rifle," he said.

He looks quickly in the mirror and then forges into the heat to trade places with Arnold.

Sweat still dribbles down his face as Arnold, the senior sentinel among these honor guards, enters the air-conditioned guard quarters a few minutes later.

Arnold and the other guards defend their superheated attire, even though regulations forbid them to sit down while wearing the wool dress uniforms. When they are bused to a ceremonial function, they stand up.

"We wear them because they look much better," Arnold says. "Summer uniforms bag and sag where they shouldn't."

Considerable care is taken to keep each guard's four sets of ceremonial blues in immaculate condition.

"We have a laundry service," says David Jones, 20, a private from Georgia, "but it really doesn't do that good a job. So most of the guys use a civilian cleaner." Jones estimates that his laundry bill runs $15 to $16 a week" out of his own pocket during the heat of summer.

Arnold notes proudly that the guards are an elite corps who must spend six months with the 3rd Infantry (the "Old Guard") at Fort Myer before volunteering for duty at the Tomb.

Height counts, Arnold adds: all guards must be at least 6 feet but none may be taller than 6 feet four. Weight must be proportional to height and complexion should be clear. In addition, all guards must display "unquestionable loyalty to the United States," he said.

Arnold, however, makes no mention of any written requirement that the guards be immune to the rigors of a Washington summer. When asked whether any of the guards had ever collapsed, Arnold responds with a frown.

"Some have come close," he says finally, "but we've never had a sentinel faint out there. Numerous times, people will feel faint or dizzy. But then you remember that you're a tomb guard. It would be a disgrace to tradition."

Arnold says that while it is impossible to be unaware of the heat, other factors govern his thoughts while he stands duty.

"Sure it's hot," he says. "Sometimes I think, 'I sure will be glad to be downstairs (where the guard post is located).' But then you remember . . . there's probably someone watching -- maybe a middle-aged woman who lost her husband or brother in war."

Shortly after 4 p.m., Harris reenters the guard post, as drenched in perspiration as Arnold had been half-an-hour earlier. As he cools off, Harris is asked to recapture the most important thought of his walk.

"I've been wondering," he says as he finally catches his breath, "why the breeze always blows on your back and not on your face."