It's 1861. The troops at Fort Washington, south of the young capital city, are jumpy -- and with good reason. Only a few miles away in Confederate Virginia, the secessionist armies have recently fought the Union troops at Bull Run. The soldiers who peer across the Potomac into the gathering darkness wait and wonder what will happen.

But right now they're occupied in talking to curious city- dwellers who have come down Indian Head Highway to inspect their brick-and-masonry fort, begun in 1814 and completed in 1824. The visit takes their minds off the work at hand, and off the boredom of waiting for President Lincoln's orders.

This Saturday will be the second time this summer that the city folk turn out for one of Major Haskins' "evening soirees," as one soldier calls them. Major Haskins, the fictional commandant of Fort Washington, lives with his family in a large yellow house built in 1821 just north of the Fort. The house is being restored and should be open to the public sometime this fall, says Ray Ashley of the National Park Service.

The tourists' first stop is for a briefing beside the bonfire near the ruins of one of the fort's eight concrete coastal batteries. Ashley warns that walking over the hill and across the small drawbridge takes you not just into the fort but out of the 20th century. Guards will lead you to the fort's watch office, and then you'll be on your own to scramble in and out of walkways and public rooms or to peer down at the Potomac. If it's the 8 o'clock tour and the skies are clear, you'll get a majestic sunset over the Virginia hills.

But the soldiers, along with the lieutenant's wife rocking on her porch and the laundress seated demurely inside her quarters, will talk only of the concerns of 1861. The new republic seems to be on the verge of breaking up and they're worried. There are only 92 men stationed at Fort Washington now -- nearly 200 Pennsylvanians who swelled their ranks for 90 days have just left. They've heard rumors that the Confederates are moving northeast in their direction. Tourists are not allowed along the eastern wall of the fort -- "It's dangerous -- rumors of saboteurs," says one soldier.

The defenders' fears are reflected in the neat, hand-written letters left on their desks beside small photos of wives or sweethearts. (Postage is up to a penny a letter now, complains one soldier, who has heard that Congress may vote to let the soldiers write home for free.) But the checkerboard left on a bench and the soldier inviting sightseers to join him in a card-and-dice game called Chug-a-Lug indicate that not all is grim.

Still, if it's light enough to peek in the door of the darkened cells off the guard office, the rough dirt floor and the rickety bunk bed are reminders that these people take war seriously. Their pay, they say, ranges from $11 to $13 monthly -- 40 cents a day. One stocky sergeant who's been in the Union Army 11 years sights a lad of about 17 in T-shirt and cut-offs and hails him: "Want to join up? Three meals a day and a roof over your head, guaranteed. Your country needs you!" The 20th-century teenager looks startled; his parents smile.

As the evening light fades after a spectacular sunset, the soldiers -- all volunteers in uniforms provided by the National Park Service -- bring out torches made of cannon wadding soaked in coal oil and place them around the yard. As they're lit, acrid smoke fills the air. But the bonfire, the torches and the rifles add drama. Even the markers that tell the story of Fort Washington have been covered with burlap so the future won't interfere with 1861.

Although the 8 o'clock tour allows a lovely view of the river, a view down to the District and a golden-red sunset, Ashley says the later tours provide a better glimpse into the 1860s, mainly because the dark helps obliterate reminders of the 20th century -- weeds that have grown too high and the construction trailers near the commandant's house. But if you go when it's light, you can walk down to the water's edge where the original fort was located. That fort, a soldier says, was destroyed by American soldiers when they realized they couldn't hold off the British and set fire to the powder magazine.

The Torchlight Tours are one of the fort's most popular programs, says Ashley. If he had more volunteers to play the roles of soldiers, he could open more areas of the fort during the tours. Most of the volunteers are professionals with an interest in history. Some are members of reenactment societies; many provide some of their own accoutrements, such as period guns. The park also offers such special military programs as rifle drills each Sunday from noon to 5. Unlike the Torchlight Tours, these do not require reservations. HISTORY BY TORCHLIGHT The next torchlight tours at Fort Washington will be held this Saturday at 8, 9 and 10. For reservations, call 292- 2112. If you would like to participate as a volunteer, call Roy Ashley at the same number. To reach Fort Washington, take Beltway Exit 37S (Indian Head Highway) and turn right at Fort Washington Road. Follow signs into the park.