London's Tower Bridge

Close your eyes and picture London Bridge. Unless you're British or very well traveled, the structure you have conjured up is Tower Bridge, not London Bridge at all. It's easy to understand why - after all, the enormous (it's tempting to call it ''towering'') Tower Bridge with its gothic towers, great swooping cables , and Victorian precision engineering, is a far more fitting symbol for the city than the relatively prosaic London Bridge.

Of course, the fact that the towers aren't true gothic at all upsets the historian, but the effect is splendid, a good match for its neighbor, the imposing Tower of London itself.

On December 31, 1909, the two ''high level walkways'' connecting the towers were closed - permanently, the old guidebooks say. ''Permanently'' in this case meant 73 years, and last June they were thrown open once again by the lord mayor of London. So now any visitor who wants a superb view of London can get one, 139 feet above Thames level. There are stainless steel elevators for the weak-kneed, but those who climb will find restored interiors to enjoy on the way up - and seats to rest on.

Once up there and out on one of the walkways (mercifully glassed in), they will find on the skyline a marvelous sampling of London's architecture - ancient domes and spires (St. Paul's Cathedral, All Hallows-by-the-Tower), the Monument, London Bridge, the tower itself, and the modern British Telecom Tower looking like a space-age version of a Russian cathedral. There it is - London spread before you. Or, to quote a French visitor, simply, ''Tiens!''m

Of course, most visitors are not just sightseeing. They come to examine and admire a beautiful example of Victorian high-precision engineering. The carefully thought-out displays, models, history are cunning enough to convince me that I can understand (for a moment at least) how the bridge operates and even how it was built. Now I can only tell you what the guide book told me: Building began in 1886 and took eight years. The bridge is ''one integral whole'' and has two drawbridges (bascule). Six years ago electro-oil-hydraulic machinery took over from the original water-hydraulic machinery. It takes about 1 1/2 minutes to raise the drawbridges - a tremendous weight (''exerting an upward thrust of about 150 tons''). And I am happy to say the machinery has never failed.

But there's more to the displays here than engineering. There's a whole section devoted to the history of all the bridges. And if you ever wondered about the old ominous-sounding nursery rhyme ''London Bridge is falling down,'' you can learn about that too.

As a matter of fact it's a song of triumph and refers to what happened around 1014, when the Danes had grabbed London and the English, under King Ethelred II, were trying to wrest it back. The Danes were defending the city from the bridge and didn't notice Ethelred's ally, Olaf, rowing under the bridge and attaching cables around the supporting piles until he pulled the whole thing out from under them. ''London Bridge [came] falling down.'' The Danes surrendered and the English acted on the advice of the poem's second verse: Build it up again with stone so strong, Stone so strong, stone so strong, Stone will last for ages long, My fair lady.

Victorian steel should last even longer, despite some rather tart comments about the new bridge from the newspapers of the day.

The Building News for June 29, 1894, insisted that parts of it were ''foredoomed'' to become ''soon obsolete.'' The Builder called it a ''monstrous and preposterous architectural sham.''

The Times, on the other hand, praised the river itself. On July 2, l894, it called the Thames ''the noble river which all true Englishmen love with a proud affection as the chiefest glory of their ancient capital.'' And standing on the walkway, looking across the Pool of London, you know just what the Times writer meant.