You might also like to pay your respects to George Washington outside the nearby National Gallery to pay your penance to fine art. He was a gift from Virginia, and stands on imported American earth because he said that he’d never set foot in London again. And don’t miss Charles I on the west side of the square. This is the finest equestrian statue in the city. Just down the road in the Banqueting House, you can see where his head was cut off, and also the brilliant Rubens painting of the Apotheosis of James I.

The Thames is London’s great secret, hidden in full view. We do very little with it, or on it, except complain how difficult it is to get over and under. It is the reason London is here at all, but the people stand aloof because we have long memories and longer noses. The Thames was so disgustingly noxious and pestilent that Parliament would abandon the Palace of Westminster when the weather got too hot in the summer, because the smell became dangerous.

London was the biggest city in the world, and the river was the biggest sewer on earth. The Victorians finally built an underground sewerage system that was so efficient we still use it. But they also made the Embankment, which lifts the city above the river. Getting access isn’t easy, but if you only do one thing while you’re here, you should take a boat from the center of town and go either downstream to the maritime museum at Greenwich or up toward Oxford and get off at Kew Gardens and Syon House.

The river is the best way to see the city. London glides past you like human geology. It is not a particularly impressive city seen from above; not like Paris or New York, although you can go up to Primrose Hill and Hampstead Heath and look back, and it has a dreamy loveliness brought on by distance. And Wordsworth said that earth had nothing so fair to show as the view of the morning from Westminster Bridge. Two hundred years later he wouldn’t recognize it, but it’s still pretty impressive.

The great problem for visitors to London is size. This is a big place. It’s not a walkable city; there are great walks but you can’t stride from everywhere to anywhere. And it’s easy to lose any sense of where you are in relation to everything else. So it’s best to do what the natives do, and think of London as a loose federation of villages, states and principalities, and take them in one at a time. The oldest bits are in the east. The Tower of London and the Roman Wall mark the beginning of the city. To the east are the docks and the working classes, and what is now the trendiest and most youthful, fashionable bit of London. As the city grew rich, it grew west. Mayfair, Chelsea, Kensington, Notting Hill are mostly Victorian.

You will do all the big-ticket tourist things. I doubt there’s anything I can say that will convince you that the best way to see Tower Bridge is on a postcard, and that the Tower of London is a big, dull box packed with Italian schoolchildren, or that Harrods is much the same. But while the living Londoners are to be avoided, the dead ones should be sought out. St. Paul’s Cathedral is London’s parish church, the single greatest building in Britain, designed by Christopher Wren. It’s light, civilized, rational and humane — everything Londoners aren’t. It has monuments to J. M. W. Turner, the Duke of Wellington and, of course, John Donne, who preached there. Behind the altar is a little memorial chapel and stained-glass window dedicated to America and the help it gave London and the nation in World War II.