On Project Blue Book our problem was to identify these stimuli. We had methods for checking the location, at any time, of every balloon launched anywhere in the United States. To a certain degree the same was true for airplanes. The UFO observer's estimate of where the object was located in the sky helped us to identify astronomical bodies. Huge files of UFO characteristics, along with up-to-the- minute weather data, and advice from specialists, permitted us to identify such things as sun-dogs, paper caught in updrafts, huge meteors, etc. This determination of the stimuli that triggered UFO sightings, while not an insurmountable task, was a long, tedious process. The identification of known objects was routine, and caused no excitement. The excitement and serious interest occurred when we received UFO reports in which the observer was reliable and the stimuli could not be identified. These were the reports that challenged the project and caused me to spend hours briefing top U.S. officials. These were the reports that we called "Unknowns." Of the several thousand UFO reports that the Air Force has received since 1947, some 15 to 20 per cent fall into this category called unknown. This means that the observer was not affected by any determinable psychological quirks and that after exhaustive investigation the object that was reported could not be identified. To be classed as an unknown, a UFO report also had to be "good," meaning that it had to come from a competent observer and had to contain a reasonable amount of data. Reports are often seen in the newspapers that say: "Mrs. Henry Jones, of 5464 South Elm, said that 10:00A.M. she was shaking her dust mop out of the bedroom window when she saw a flying saucer"; or "Henry Armstrong was driving between Grundy Center and Rienbeck last night when he saw a light. Henry thinks it was a flying saucer." This is not a good UFO report. This type of UFO report, if it was received by Project Blue Book, was stamped "Insufficient Data for Evaluation" and dropped into the dead file, where it became a mere statistic. Next to the "Insufficient Data" file was a file marked "C.P." This meant crackpot. Into this file went all reports from people who had talked with flying saucer crews, who had inspected flying saucers that had landed in the United States, who had ridden in flying saucers, or who were members of flying saucer crews. By Project Blue Book standards, these were not "good" UFO reports either. But here is a "good" UFO report with an "unknown" conclusion: On July 24, 1952, two Air Force colonels, flying a B-25, took off from Hamilton Air Force Base, near San Francisco, for Colorado Springs, Colorado. The day was clear, not a cloud in the sky.