On Dec. 6, 1884, reporters and members of the Washington Monument Society gathered on a wooden platform at the monument's peak to witness the setting of the 3,000-pound aluminum capstone atop the world's tallest structure.

It was a great day for the memory of the first president and the city named in his honor, and the event seems even greater today, now that we've had a century to judge the lasting value of the soaring stone edifice. To say that the Washington Monument has aged well is to understate the case dramatically.

Of course, living here, we see the monument every day, and at night, without leaving the living room, we see it again in spectacular helicopter tracking shots understandably admired by editors of the late news shows, and we see it everywhere on post cards and in the papers, and once a year we see it as the splendid punctuation mark for the bursting of the fireworks, and we dutifully take visiting friends and relatives to see it. In Washington we see the monument so often that we often don't see it anymore, or, in certain moods, actually resent it or poke fun at it or generally get real tired of it.

But every once in awhile, when a mood and the city's pristine light intersect just so, we do see the monument again, with fresh eyes, and are moved once more by the sheer beauty of the object and the place it inhabits. When this happens to me I am always amazed anew at the fact that the Washington Monument, in its present century-old form, is a stroke of fantastic good luck.

Completion of the monument on that cold, windy day 100 years ago meant that a long embarrassment finally was over -- the Continental Congress had proposed a monument to "General Washington" in 1783, the perspicacious Maj. L'Enfant had selected its magnificent if swampy site eight years later, and the unfinished obelisk Mark Twain likened to a "factory chimney with its top broken off" had stood untouched for 26 years before construction recommenced in 1880.

Still, praise was far from universal. Victorian architects were especially unkind. The American Architect and Building News, a professional journal inaugurated in 1876, referred to the unadorned monument as "this monstrous obelisk, so cheap to design but so costly to execute, so poor in thought but so ostentatious in size," and concluded that it would inspire "no thought worthy of thinking."

The magazine's editors were right in one respect, though. They predicted that "the result of plainness, squareness, simplicity and extreme height will doubtless assert itself to the common mind as a clear achievement (in the vernacular, a big thing) . . ." The monument retained the world height record for just five years, until Gustav Eiffel's tower of steel was completed in Paris, but it was an immediate popular hit and has, justifiably, remained so.

What caused the architects' vituperative complaints was explained with patience and economy by Robert Belmont Freeman Jr. in an amply illustrated 36-page article, "Design Proposals for the Washington National Monument," that appeared in the 49th volume (1973-74) of the Records of the Columbia Historical Society. (An entertaining and informative exhibition of 19th-century materials relating to the history of the monument will open on Dec. 14 at the society's headquarters in the Heurich mansion, 1307 New Hampshire Ave. NW.) What happened, simply put, is that because 40 years elapsed between its design and completion, the monument suffered from a sort of architectural time warp.

The monument was designed in the early 1840s by Robert Mills, an architect who already had contributed several notable structures to Washington (including the Treasury building, the Old Patent Office and the Tariff Commission building) and who also had done the columnar Washington Monument for Baltimore. Mills' design was by far the best entered in the competition sponsored by the Monument Society -- other entries foresaw a giant pyramid, a domed cylindrical temple, a castellated "Palladium" and a Gothic Revival setpiece with a high spire -- but, like much else in the early history of the capital city, Mills' idea was almost unbelievably ambitious.

In addition to a rather flat-topped obelisk that would rise 600 feet, Mills proposed a Pantheon-like base, itself 100 feet high, to house statues of the nation's founders and patriots -- an eclectic marriage of Egyptian and neo-classical motifs perfectly fitting for a city that from the time of Jefferson and L'Enfant had eagerly adopted classical architecture as the expression of its symbolic aspirations. But it was frightfully expensive, and when the time came to actually build the monument, the Society, hard-put for private donations, deferred any consideration of the Pantheon and simply pressed on with the obelisk.

Construction began in 1848 and was halted in 1854, due to lack of money and one of the more bizarre of the many low moments in the history of Americanism, when members of the anti-foreign, anti-Catholic American Party invaded the unfinished monument to protest the presence there of a block of marble donated by the Pope, one among many given by states, organizations and individuals. This political fiasco, and the Civil War, put an end to congressional thoughts of bailing out the Monument Society, and nothing effective was done until the centennial year of 1876.

By this time architectural thinking had of course changed a good bit -- classicism was decidedly out -- and the decision by Congress to build the monument with federal funds stimulated another round of (unsolicited) design proposals, most of them curious, several preposterous and even the best of them inappropriate to the location. Fortunately, this turned out to be no more than a lot of sound and fury, for Congress sensibly had called upon the Army Corps of Engineers to oversee the construction. Under the no-nonsense guidance of Lt. Col. Thomas L. Casey, the corps proceeded with dispatch to firm up the foundations of the obelisk and then, simply, went ahead and built it.

To his everlasting credit, Casey asked the American minister in Rome, George Perkins Marsh -- the right man in the right place -- to research the proper proportions for an obelisk. Ancient Romans, having stolen a dozen obelisks from Egypt, had begun the process of incorporating the form into the vocabulary of classical architecture, and Marsh was able to determine that a "true" obelisk should be 10 times the height of its base (thus the final product, 55 feet and 1 1/2 inches at the base, is 555 feet and 5 1/8 inches tall) and that the facets of the pyramid -- called a pyramidion -- at the top should be set at angles of 60 degrees.

That this fortuitous combination of can-do determination, engineering skill and archeological exactitude is responsible for a result more thoroughly satisfying than the proposals of Mills or any of the other architects is, as I said, an amazement. Mills must be credited, though, with the inspired realization that the site could take an object of such tremendous height -- a leap of faith and imagination that we can far better appreciate today, after the city has filled out around the "big thing."

If the audacity of the monument's size fully matched the audacity of L'Enfant's plan, it also solved one of the plan's big problems: namely, the distance between the "Congress House" high on its hill and the low-lying presidential mansion more than a mile away. Although soil conditions prevented its location precisely on axis with the White House, as L'Enfant had planned, the towering obelisk completes the great triangle L'Enfant envisioned with an emphasis even he dared not dream of.

And, thanks to the efforts of the members of the McMillan Commission and many other artists -- not least among them Henry Bacon and Daniel Chester French, creators of the Lincoln Memorial, and Henry Merwin Shrady, sculptor of the Grant memorial at the opposite end of the Mall -- the Washington Monument has become the visual and symbolic centerpiece of one of the most beautiful and moving ensembles of parkland, public statuary and buildings that mankind has been able to place upon the planet. And it gets better year by year.