Travelers passing by the Hopkins Center this past fall mistook it for a wrecking ball, a bomb, even a bulldozer. But what it was, was art. Part of an exhibition featuring sculptures by John Van Alstine, the steel and granite work, entitled Buoy, was supposed to go in the Darling courtyard, the enclosed space in the middle of the Hop. It got a much more prominent booking when Gerald Auten, director of studio art exhibitions, had second thoughts about lugging the 800-pound work from the basement. "I was afraid it would roll down on me when we brought it up the stairs," he says.