May 4 (UPI) -- On this date in history: In 1494, on his second expedition to the New World, Columbus explored the Caribbean island of Jamaica. In 1886, four police officers were killed when a bomb was thrown during a meeting of anarchists in Chicago's Haymarket Square protesting labor unrest. Four leaders of the demonstration, which became known as the Haymarket Square Riot, were convicted and hanged. In 1904, construction began on the Panama Canal. Panama celebrated the 100th anniversary of