Economist: IN A new book, The Frackers, Gregory Zuckerman says of the late George Mitchell, a pioneer of the technique of hydraulic fracturing to tap unconventional reserves of oil and gas, that his impact eventually might even approach that of Henry Ford and Alexander Graham Bell. Yet of late doubters have been making themselves heard too. In October Peter Voser said that one of his biggest regrets as boss of Shell is the $24 billion his firm has invested in North Americas shale beds. This summer, the...