SINCE its incarnation in New England more than 150 years ago, the institution of the county fair has endured through the changes of the growing nation. County fairs might have a slightly different feel from the East Coast to the West, but their rural theme is the backbone of their popularity.Over the past few years, travel and tourism have taken a huge hit because of the threat of terrorism. In these unsure times, many Americans have returned to these more traditional, homegrown forms of entertainment.The fairs are places where old-timers reunite each year to see a familiar face — and to lament the ones no longer in their company. They are places where children marvel as local ballfields or pastures are transformed into oases where anything seems possible, if only for a week.At its most effective, a county fair can raise revenue and help define a community. At its most basic, it’s a place to go, close to home, for safe and familiar entertainment.The lure of the fairs varies from the spectacle of the lighted rides and the sound of crashing metal and muscle car engines to the taste and smell of the nutritionally challenged food of the midway. Either way, expect the county fair to be around for another 150 years, but don’t expect to see it change.