Before there were smart bombs, there were smart bombardiers. The battery of artillery cannons that protected San Francisco Bay from Civil War days until the end of World War II wasn't guided by lasers or computers but by the Pythagorean theorem, which was used to chart the distance of a floating target — with chalk upon slate. Just one such gun remains: a 6-inch cannon weighing just shy of 50 tons and mounted at Battery Chamberlin, overlooking Baker Beach. Volunteer docents — in period garb, naturally — are there on the first weekend of every month to demonstrate how the "disappearing rifle" works. The recoil from firing the cannon rocks it back behind the parapet, dropping a massive counterweight into a 12-foot hole, and then propelling it back into firing position. The only comparably gargantuan item to disappear so effectively in San Francisco was the city's onetime budget surplus.