President Donald Trump declared a national emergency on Friday, the most significant move yet by the U.S. government to head off the coronavirus outbreak. Trump’s declaration came as many public and private institutions have taken action — including canceling major events, temporarily banning large gatherings, closing schools and telling people to work from home — in an effort to slow the spread of the virus. The Dow Jones Industrial Average tumbled, soared, and then closed with a gain of 1,900 points after the emergency declaration. Wall Street had reeled Thursday afternoon after coronavirus fears drove the markets to their worst...