NEW YORK - For more than a century, Harper's and The Atlantic were the Tweedledee and Tweedledum of highbrow monthly magazines, so alike as to be almost indistinguishable in their thoughtful coverage of literature, art and public affairs.

But times change, and so in the past decade have these two elite periodicals -- Harper's much more radically than The Atlantic. In 1980, the same year that Mortimer Zuckerman bought the Boston-based monthly, the young philanthropist John R. (Rick) MacArthur -- with the help of his family's MacArthur Foundation and the Atlantic Richfield Foundation -- rescued Harper's from extinction.

For Harper's -- the nation's oldest monthly, at 139 -- the long ...