Picture the Mall a wasteland, the architect-historian commanded the joggers in front of the Smithsonian Castle.

"A railroad switchyard stood where the National Gallery East Building is," said J. L. Sibley Jennings Jr. "A red-light district and rendering plants bordered the tracks. Constitution Avenue was a canal full of sewage, and the Potomac River came all the way up to where the Monument now stands."

Jennings was leading a group of Smithsonian Associates on a jog-and-talk through six miles and 80 years, circling the Capitol and Lincoln Memorial on either end, stopping often to fathom the blunders of the layout. Tightening their Nikes and Adidas in front of the Castle, the group imagined the Mall at the end of the Civil War: City planners were high on Pierre L'Enfant's dignified plans for the new capital. Marble and granite were in, brick and wood were out. Radiating outward from a central triangle -- defined by the President's House, the Capitol and what L'Enfant envisioned as a small equestrian statue as a monument to George Washington -- grand avenues would crisscross the city.

But the scheme was botched from the outset: Early urban sprawl ignored L'Enfant's axles; and because of mushy earth in the right spot, the Washington Monument was located 120 feet south and 360 feet east of the true axial intersection.

Enter Jennings' heroes, the Frederick Law Olmsteds. The senior O. designed Central Park, the U.S. Capitol grounds, the National Zoo and Gallaudet; Junior did Rock Creek Park, the Arboretum and American University, and was the first formally trained landscape architect in history. But more about them on the run.

START: THE CASTLE. Observe the mobs on what were the "Smithsonian Pleasure Grounds" in Andrew Jackson Downing's rejected proposal of 1850. Under Downing's plans, a marble "President's Arch" and a suspension bridge would have linked the Mall and White House grounds over the fetid canal that's now a six-lane roadway.

"Olmsted recognized it wouldn't do to choose elms for the Mall -- you never get the umbrella effect intended," Jennings said. "He wanted an undulating mall, not a flat tabletop." The best-laid plans . . .

At the turn of the century, the red-brick Smithsonian castle was almost moved so it wouldn't intrude on the Mall, but after a terrific fight it stayed put. Originally an English garden grew where the vast slab of concrete and Mall are now.

As a member of the McMillan Commission, a blue-ribbon group of city planners appointed by the Senate in 1902, Olmsted Jr. sweated over the assignment. "From the first of October until about the middle of May the climatic conditions of Washington are most salubrious," he wrote, "but during the remaining four and a half months, the city is subject to extended periods of intense heat, during which all public business is conducted at an undue expenditure of physical force." Accordingly, he pushed for shade, swimming pools and a developed riverfront, Great Falls to Mount Vernon.

Unfortunately, politicians opted for memorials where Olmsted sketched in parklands. Only a few buildings conform to the scale he had in mind, others are too huge or clumsy. But in spots, where stone walls didn't win out over shrubbery and trees, light plays on the eye just as Olmsted calculated.

FOUNTAINLESS THIRD & CONSTITUTION. As you run east to the Capitol, it's easy to imagine the approach alive with grand jets of water cascading where today there's a traffic island. L'Enfant pictured a fountain to wash the silt out of the canal (on what's now Canal Street behind the House Office Buildings) to keep it clear; Olmsted wanted it there for dramatic effect.

CAPITOL PROPS. L'Enfant's Capitol was to sit back on the artistic crest of the hill, where the serious-looking Library of Congress and Supreme Court are today. But it was built larger and farther forward, and the front seemed to be falling into the marsh. Jennings' man to the rescue: To prop up the whole structure visually, the Olmsted West Front terraces, artificial bases that don't touch the original building, were added in 1884.

Straining up the hill to the back (which is really the front) of the Capitol, give Olmsted credit for the landscaping. He achieved depth by "darkening the foreground with foliage, then opening the view to expose naturally undulating fields. The mushroom dome added after the Civil War peeks through clusters of shade trees. The whole thing is a visual composition all the way around," our guide gushes.

TRAINS & THE GROTTO. Olmsted wanted the buildings surrounding the Capitol to be very plain, using the same stone as the Capitol itself. Instead, after he died, the Supreme Court was built from a different stone and "became a self-assertive monument," Jennings pointed out as he rounded the northwest corner. Note the dark greens hemming in the corners of the east lawn and next behold opulent (if leaky) Union Station.

In what Jennings calls the first junket in history, Olmsted was sent to Europe to check out designs for the depot. "He took the Frankfurt Station model, put the imperial Roman Baths of Caracalla on top and that was Union Station." Two railroad terminals polluting the Hill and what was to become the Mall were removed to the neoclassical-style station. In the process of creating the vista from the Capitol, the private houses on Delaware Avenue were ripped down, including two owned by George Washington. An oversight, General.

Catch the Indian atop the dome facing east, the direction the city was expected to take. Then you might stop off at the Spring Grotto, a little-known retreat locked behind an iron gate at the western foot of the Capitol, designed to cool legislators' tempers. Once fed by a spring, it now flows with city water.

The mood shifts from rigid to laid-back as you continue down the Mall, with the foliage loosening up on the approach to 14th Street. The area becomes "more landscaped, less architectural. The Lincoln Monument is the only purely architectural element down there," Jennings noted. Except for the Freer, the elegant Pan American Union and the National Academy of Sciences, all the buildings along the Mall are "super-scale." As Jennings pointed out, they overwhelm the Mall because they're bigger and more monumental than Olmsted had planned. "The Mall wasn't supposed to be Mussolini- scale," Jennings laments, pointing to the fortress-like National Gallery, FTC and National Archives.

MINI HA-HA. The "ha-ha," an English landscaping device, is best observed by joggers crossing 14th Street on the Mall. You can dip down a gentle hill to cross the roadway and climb back up to the level of the Mall to note how the ha-ha effectively hides the unpleasantness of traffic, once horse-drawn, now fuel-injected.

555f, 5!i. In the original plan, terraces led up to the Monument, which would have been a small horse-and-rider number in honor of GW, surrounded by a huge pool and lots of shade trees. "But the bureaucrats came up with a circular temple with a portico. They ran into a cost overrun, only got about 150 feet into the job and ran out of money in 1855," Jennings noted. After teams of oxen knocked themselves out dragging stones up the hill to begin building the obelisk, it sat unfinished for more than 20 years. That explains the different-colored marble at the seam. Fortunately, the circular temple around the base never got past designer Robert Mills' drawing board. The monument slipped off-axis because construction crews hit a band of marine clay that wouldn't support the foundations. "So the entire Mall is bent to the south," Jennings explained. Continuing westward, watch for a narrow but striking vista between clusters of trees: to the left, the Jefferson Memorial, to the right, the iWhite House. "From the President's House, the Jefferson appeared as a backyard decoration," Jennings said. At 17th and B Street (now Constitution Avenue), the old gate house is a monument to the river that once flowed, instead of today's stream of Hondas and VWs, bordering the Mall. "People used to enter the Potomac at Fletcher's boathouse and zip through the gates here," Jennings related. "John Quincy Adams was famous for coming down and swimming in the altogether."

ROLLING & RIGID GARDENS. The landscaping in Constitution Gardens is "as good as it can be lacking the hand of the master himself," Jennings proclaimed. "Give it 25 years to grow in." Across the street to the north, note what Jennings calls the "cookie-cutter arrangement of the Federal Reserve Board. Even in the shrubs, the lack of understanding of what Olmsted was trying to do for the Mall is obvious."

LINCOLN'S WATERGATE. The original water gate, Memorial Bridge, was to be the arrival point to the city. Theodore Roosevelt Island was to have been the waterfront to Georgetown, landscaped like Central Park but more rustic. "Olmsted saw it as a primeval forest, with trails of raw dirt arranged so that the visitor, venturing here by boat and following the paths, would stumble upon a majestic city. He'd have heart failure to see the monument to TR there today." Olmsted is credited with inventing Memorial Bridge, which he designed after photographing every bridge across the Seine during his junket to Paris. Previously, there was one long crossing below 14th Street and Aqueduct Bridge, above what's now Key Bridge. The idea was to tie together Lee and Lincoln, north and south, after the war of the states. "It's also off the axis but loaded with symbolism," Jennings said. Originally the Lincoln Memorial was to have been open in back, so that a sightline would be maintained through the pavilion. But sculptor Daniel Chester French wanted a solid backing to his seated Lincoln and, in 1925, he won. Join Jennings in lamenting the fact that Olmsted's vision of people-oriented buildings, including theaters and restaurants, lost out to monuments to dead politicians. Pause looking toward West Potomac Park for a bit of future shock: Olmsted's plan will be further defeated by the proposed FDR Memorial. The plan that's been approved calls for a huge stone slab to cover the volleyball, soccer, rugby and baseball fields, with underground amusements, shopping-mall- style. "If the money was there, friends, it would be under construction now," Jennings says sadly.

BLOCK THAT VISTA. Back at the Monument, it's hard not to get angry. "Some dolt built a gift shop, refreshment stand and lavatory right in the line of vision of the Capitol, blocking one of the most dramatic lines of sight," Jennings noted.

COULD BE FREER. "These were all to be carriageways and walkways, no pavement," Jennings explained, running east on the south side of the Mall toward his starting point. "And the Freer was to have been set down in a park. But just as they started construction, wouldn't you know, here come the bureaucrats." When the Agriculture Department started to build a structure onto the Mall, it took a presidential order to stop it. As you catch your breath back at the Castle, be thankful for small favors. Without designers like Olmsted, the whole town could look like the FBI Building.

PIT STOPS ON THE MALL BOEHM BIRDS -- At the National Museum of American History, "Birds of North America: Porcelains by Edward Marshall Boehm" in the third-floor Hall of Ceramics and Glass.

FUN & GAMES -- WETA's 20th-anniversary party, with two stages full of entertainment, will be held in Constitution Gardens, Saturday 11 to 4. Food available. Moon bounces for kids, balloons, T-shirts, TV personalities, clowns, dancers, The Airmen of Note, Buck Creek Jazz Band, Raquel Pena Spanish Dance Company, Susan and Gordon from "Sesame Street."

WOMEN OF THE AIR -- The National Museum of American History presents "Silver Wings and Santiago Blue," a film about Women Air Force Service Pilots of World War II, Saturday at 1 in Carmichael Auditorium. Free. At 2, Gene Ashton presents his own music on harps, banjos and pipes of his own making, in the Hall of Musical Instruments.

THE ART OF IT ALL -- At the Freer, a new program of free public tours of Chinese, Japanese, Near Eastern, Indian and American collections is offered Saturdays at 10:30, 11:45, 12:30 and 1:45; Sundays at 12:30 and 1:45. The current exhibit, "The Imperial Image: Paintings for the Mughal Court," continues through January 18.

THE ART OF HERE & THEN -- At the National Gallery's East Wing, more than 90 paintings, drawings, watercolors, pastels and neoclassical statues will be in "An American Perspective: 19th Century Art from the Collection of Jo Ann and Julian Ganz Jr.," an exhibition opening Sunday, continuing through January.

MORE MALL HISTORY: "The Federal City: Plans and Realities," an exhibit at the Smithsonian Castle, 1000 Jefferson Drive SW, daily 10 to 5:30. Also, though Jennings won't commit himself to leading any more jog-talks, the Smithsonian Resident Associates regularly schedule walking tours in various parts of the city. Call 357-3030 for information.