The great hall of the Pension Building -- an astonishing, stupendous, preposterous room -- looks better now than it did 100 years ago when it was rushed into service for Grover Cleveland's inaugural ball. Of course, on that occasion the building, begun in 1882, still lacked a roof, and U.S. Navy sailors were required somehow to rig a temporary canvas top to cover its vastness.

In a way, getting the hall ready this time was harder. Saving and restoring so distinguished a structure can be chancier than building it from scratch. Even though its fate finally seemed secure in 1980, when Congress agreed to help turn it into a National Building Museum, the place has been caught in a budget crossfire between the executive and legislative branches, and continues to suffer physical deterioration and institutional uncertainty.

But the building looks good inside and out, which has to be encouraging to all concerned -- Monday's dancers, Building Museum enthusiasts, and those who love the structure just for its oddly lovable self. The leaky original roof at last has been replaced and the building has been given a long-needed clean-up, paint-up job, so that once the dance is over many more people will be able to step back and see just how exceptional the possibilities are.

The great hall makes the most impressive ballroom in the city, but, more important, it makes an extraordinary home for a promising institution. Sen. Daniel P. Moynihan was hardly exaggerating when he observed that had Gen. Montgomery Meigs, the Pension Building architect, set out to design a building museum, he could not have done a better job.

What a fabulous space the great hall is! You can't help but describe it with circus adjectives. You want bigness for the sake of bigness, you get it here: Those colossal Corinthian columns, 75 feet high, are two feet higher than the highest known classical colonnade, that of the late Roman Temple of Jupiter at Baalbek in Lebanon. Did Meigs know this? My bet is that he did. He was a great engineer (the Cabin John Bridge is his), a patriot, a man of his expansive times and well-versed in classical architecture.

The Pension Building itself was adapted from the plan and facades of the Renaissance Palazzo Farnese in Rome. Bates Lowry, the director of the National Building Museum who, in his role as "curator of the building," keeps unearthing more information about the architect, feels strongly it can be no accident that one side of Meigs' court corresponds exactly to the length (116 feet) of the interior chamber of the Parthenon. The beautiful side rooms of the Pension Building, with their impressive brick vaults (largely plastered over, like so much else in the building), are indebted, Lowry believes, to Michelangelo's daring vaults in the church of Santa Maria degli Angeli in Rome.

And yet, what an obviously American building it is. Today we may treasure the great hall for its almost nutty grandeur, and Meigs certainly intended a big spatial effect, but the main reasons for the room, he stressed, were practical: It was his way of bringing light and air to the some 1,500 clerks who spent their days keeping records and dispensing checks from the rooms surrounding the great hall. (Fresh air was pulled in through holes under each window, and exhausted through the clerestory windows high in the great hall. Clerks complained about the draft but Meigs dismissed such complaints. It was good for their health, he felt.)

I must say I really don't like the sweet-and-sour combination of warm, rich rose tones and acerbic, almost bilious yellow-greens that was chosen for the restoration of the great hall -- a combination so strange it is hard to accept as historically accurate, though experts insist that it comes close. But even this unpleasant contrast is a significant improvement upon the government-grim flatness of what was there.

Besides, there are many wonderful touches. The entrance vestibules, not quite complete, are becoming the sort of dark, elegant tunnels to the dramatic space that Meigs intended. (The embarassing old rug is still there, but a new one is on the way.) The colossal solid brick columns have been marbleized again, and their capitals cleaned up and repainted. The vast ceiling has been painted, as Meigs wanted, a summer-sky blue. The rows of niches just above the roof trusses have again been filled with busts (sculpted by Washingon artist Greta Bader), and they glitter up there like a little pantheon. Seventy-six planted urns, duplicated from a single long-lost original, are being put back in place atop the double arcade surrounding the hall. The columns of these arched arcades today stand crisply defined. In general, one can now see much better how Meigs modulated the space to reduce its vastness.

Outside, the bricks are clean again (more than 15 million were used) and one can admire again the precision of the design and the workmanship. And, of course, the never-ending terra-cotta parade of Civil War veterans on the frieze above the first floor are now etched cleanly in the winter air.

These are, as Lowry ruefully (and realistically) points out, simply cosmetic changes. The building, though structurally sound, is in terrible shape, as any century-old structure would be that has been so poorly maintained for such a long period. Needed is about $30 million (in addition to the $4.65 million already spent) for the kind of basic reconditioning work that will make it usable as a modern museum and office structure. The administration has balked at this expenditure; some have even suggested the government simply give the building away, and with it any responsibility for and control over its upkeep. The museum's strong supporters in Congress so far have nixed that nonsense, but it is becoming an annual standoff.

Perhaps this will change soon, now that the building itself is looking splendid and the programs of the Building Museum are becoming more visible. Although Lowry's talented professional and volunteer staff has done well under the circumstances -- they have produced a national quarterly publication ("Blueprints") and several excellent traveling exhibitions -- it is indeed difficult to run a museum without walls. Next fall, with the opening of exhibitions in six renovated galleries on the ground floor, including a major exposition of architectural drawings of federal buildings, the Building Museum will become a physical fact, and its audiences, I think, will grow and grow.

It is a daunting enterprise, starting a museum that has, in this country, no precedent. Many people (including especially The Post's former architecture critic, Wolf von Eckardt) have worked long to make this one a reality. The formula eventually worked out -- a public-private partnership, with the government providing the physical facilities and some start-up program money (almost none of which has been appropriated) and the private trustees (representing all segments of the building industry) providing the operating funds -- is a complicated one. But it is not unworkable, and if given a chance most likely will earn wide support.

Of all the arts and trades, none affects the lives of more people than architecture and building, and of all the structures in Washington none suits the teaching of these subjects better than the Pension Building. Visit it sometime soon. The entrance for the next few weeks is on Fifth Street, between F and G streets NW. There are tours, by appointment, on Tuesdays, Thursdays and Saturdays (phone 783-0690 for reservations).