Its secret lies in rapture - 10 minutes (less once you've dealt with credits and continuity) is not a good length for argument and absolutely hopeless for drama - but it is just the right length for a considered gush. Any more and you might be inclined to think these people were banging on a bit; any shorter and there wouldn't be room for the detail which makes the thing work, the sense that someone is tugging at your sleeve and saying 'Now come and look at this]'
The classic example of the format remains Sir Norman Foster's exploration of a Boeing 747, a little film that oozed glee at the ingenuity and beauty of the plane's design. In this series, Jeff Koons's hymn to the John Hancock building in Chicago set the benchmark - an extravagant attempt to link a skyscraper to the spirit of German Baroque which convinced you only of his rather loopy passion for the building, but convinced you of it so well that you wanted to go and live there.
Not every one quite matches up. Last night's programme, in which Professor Ann Bergren talked about the Nicola Restaurant in Los Angeles was a touch too cerebral in its approach, preached from the head rather than the gut. She had problems anyway, as the building in question looked like a dinosaur's ribcage and it was a bit difficult to know whether you were looking through it or at it. This rather muted the other great pleasure of these films, the sense, in the best of them, that the camera is at play, swooping and gliding round the space so that your view is more privileged than that of an ordinary visitor. Much television could be radio without too much diminution (indeed it often has been radio). Building Sights, as the pun suggests, would be inconceivable without the thrill of looking.
I thought I'd had my fill of aviation history after recent films about the Dambusters and black wartime pilots, but Timewatch's (BBC 2) account of cold war spy flights changed my mind. It looks like a Great Game, now that both sides are old soldiers, but at the time this was potentially lethal sparring. Nobody could be quite sure that the Russians wouldn't interpret a spy-flight as the prelude to a nuclear attack. When a C150 flying an electronic surveillance mission took a short cut over a bulge in the Soviet border it was promptly shot down with the loss of all the crew.
'I would like to know what they made of it,' the pilot said of another spying mission. These days, of course, you only have to ask. General Vladimir Abramov, describing the same raid from the ground, told how he sent his Migs on a direct collision course with the intruder - they had no radar at the time and it was the only chance he had of bringing them down, even though one of his pilots was a close friend. The Soviet archives appear to have been opened up too - you even saw gun camera pictures of that ill-fated C150, flaring to its destruction.
Attempts to find out what happened to the missing airmen have come up against 'the institutional secrecy of Soviet officialdom', the programme noted, which seemed a bit rich when you consider that the British Government still won't reveal how many U2 flights were flown from British bases. The great game may be over but some of the rules still apply.