From afar, or more precisely through a pair of Union Army field glasses, it could be a Rodman naval gun or Columbiad, the kind of iron firepower that could jolt the ground and make a Civil War soldier consider other lines of work. Up close, it looks like what it is: a crudely shaped log with some black paint slapped on. And that's exactly what Dave Purschwitz was pushing into place yesterday at Mayfield Fort in Manassas.Called "Quaker guns" because they couldn't hurt a fly, the camouflaged logs were used by wily Confederate commanders to exaggerate their strength or mask...