Not only are Civil War buffs refighting mock battles of 125 years ago. They're also discovering some long-lost facts. Now we learn that researchers have pinpointed the never previously identified spot where 113,000 federal soldiers and a 50-mile wagon train crossed the James River downstream from Richmond on a 101-pontoon bridge in June 1864. Under Lt. Gen. U.S. Grant, they were en route to the siege and pivotal Union victory at Petersburg.

The keys to finding the precise crossing place were (1) an old cypress tree on the south bank of the James, and (2) a fascinatingly simple photographic technique developed by a photographer and archeologist from the University of California at Berkeley.

Tom Kiser, staff archeologist at the Flowerdew Hundred Plantation, had noted similarity between the old cypress on the river bank and a tree pictured by Civil War photographer Mathew Brady. Eugene Prince, the Berkeley photographer, was field-testing a technique he had devised for checking historical photos against current scenes.

Prince slipped a transparency of Brady's 1864 bridge picture into a tripod-mounted camera and focused it on the cypress. The melding of the two scenes proved that to be the crossing site, a point underscored Monday when Prince let others look through the camera.

"It's stupidly simple," said Prince. "The method is a lot of fun, but when you create the ghost picture, it's a strange emotional experience."