The flight instructor stated that he and the private pilot receiving instruction had planned a long cross-country flight to build flight time. The cross-country flight was uneventful and they landed back at their home airport; however, they performed a touch-and-go to fly more and practice instrument approaches. After two approaches, the pilots were again approaching their home airport when the engine lost all power. The flight instructor was unable to restart the engine and performed a forced landing to a field. He further stated that he had miscalculated fuel consumption and that the engine lost power due to fuel exhaustion. Examination of the wreckage by a Federal Aviation Administration inspector revealed substantial damage to the right wing. The inspector noted that the single fuel tank remained intact and was absent of fuel. One gallon of fuel was then added to the fuel tank and the engine started without hesitation and ran continuously. The inspector stated that according to Hobbs time, the airplane was operated for 3.6 hours since its last fueling.