The latest and most ambitious of sculptor Ed McGowin's "inscapes" (sculptures with an interior dimension) has been commissioned for installation in a park in Hattiesburg, Miss., where he grew up before moving to Washington in the '60s and New York in the late '70s.

A much more elaborate variation on the basic idea embodied in his obelisk on 19th Street (outside the Bread Oven), it will be a 30-foot, three-sided pyramid celebrating the centennial of Hattiesburg, with a fountain running down one side and terra-cotta bas-reliefs on the other two sides. Inside the pyramid will be a tableau of Hattiesburg a century ago carved in yellow pine, and at the top will be what looks like abstract shapes--bars of metal set at various angles, which will cast shadows that form words when the sun strikes them in the right way. A committee is now at work to decide what words will be formed. "The handicap," says McGowin, "is that this will only occur for a few minutes per day, and if the weather is bad you're out of luck. But we're hoping for word-of-mouth on this; people don't have to see the event but know that it happens." The project is budgeted at about $150,000, and McGowin thinks there will be "precious little left for me when we get through," but he doesn't sound unhappy as he says it.