The pilot and his passenger were performing aerial observations, checking paper mill inventories. After approximately three hours of flying, they made an intermediate stop. No attempt was made to purchase fuel at the airport. During the return leg, the left fuel gauge read zero, so the pilot switched to the right tank. The engine began to lose power when the right tank read zero. The pilot switched back to the left tank and was able to restore power for about two more minutes, then the engine quit. Unable to restore power, a forced landing was made in a wooded area. The airplane collided with trees and came to rest inverted. An A and P mechanic who recovered the wreckage reported that there was no fuel in the tanks and no evidence of fuel leakage on the ground.