If you were to start a blog to record...ooh, the number of times you scratch your head, for example, or the registration numbers of all the red cars you see driving past your house, or the size of your bananas, then you might find you're attracting a very limited number of readers. You'd be broadcasting your blog all the same, though, because you'd be trying to reach the whole world with it.
To make it narrowcasting you'd have either to get people to subscribe to your blog before they read it (good luck with that, folks) or you'd have to use some other method of avoiding the casual visitor having access to the information.
You might, for example, be aiming to communicate specifically at Catholics, under-fives, or the people who work in your office - or, for all I know, you might have a personal message for Robinson Crusoe. To reach these groups you might narrowcast by means of a pulpit, a television programme, or a noticeboard; as for getting through to Robinson Crusoe...a message in a bottle, perhaps?
Narrowing down your audience in this way can save money and effort. If you're watching a football match, for instance, the advertisements will be carefully directed at football fans: there won't, I should imagine, be images of sewing machines strewn around the stadium.
Still, even narrowcasting has its joys. I'm not very flattered to be thought the target audience for all those incontinence pad catalogues, but the thought of all the trouble someone's taken to offer me the chance to spend £1500 on a pair of jeans can't help but conjure up a wry smile.
Thing To Watch Out For Today: narrowcasting. This is the opposite of broadcasting, which originally meant chucking seeds about as widely as possible in order to sow a large area. The term was first used of radio programmes in the 1940s, but it became commonly used after JCR Licklider used it in a 1967 report, in which he claimed to have coined it.