Dec. 9, 1948.  About 12 miles SE of  Pittsburgh, Pennsylvania (BBU)

3:21 p.m. (EST).  USAF Pentagon officers pilot Capt. Eugene G. Mulling, Jr., Instrument Flight Group, USAF Office of DCS/Operations and copilot, with senior pilot rating, Col. John K. Brown, Jr., Guided Missile Group, DCS/O, flying in a C-45 at 6,000 ft heading 300° Mag [295° True] at 180 mph IAS saw a "perfectly round"� chalky white object, much whiter than the cirrus and alto-stratus clouds, first seen at about 1 o'clock position (about NNW) at about 60 degrees elevation (at about, on a straight-line level flight heading estimated at SW or 250 degrees [actually closer to about , a converging course with the C-45.  At 3:24 p.m., C-45 turned left to heading 270 degrees Mag (W) to move away from UFO which had pulled almost directly ahead, reduce convergent headings and make more nearly parallel the course of the object.  UFO estimated to be above cloud bank at 12,000-16,000 ft it later disappeared behind, at about 4-5 miles away at first sight, traveling about 250 mph [a great underestimate, speed actually ~ 700 mph] at constant speed, pulling ahead of C-45 at an increasingly greater distance to about 60 miles [increased relative distance about 55 miles in 1/10 hour or about 550 mph faster than C-45's 180 mph and probably moved to the left of C-45's heading by about 15 degrees to 20 degrees thus about 245 to 250 degrees True].  Object had horizontal shimmering or jittery motion on edges extending inwards about "a ¼ of the object's diameter" possibly due to ... directed beam of light or extreme speed� and/or a distortion of light waves� according to Project SIGN investigators.  Object repeatedly disappeared and reappeared behind clouds, and when it reappeared it was on the same straight-line flight path.  Object seemed to have a physical effect on the clouds, at one point a stratus cloud it disappeared behind seemed to change from long narrow shape to a "series of small strips"� of cloud oriented perpendicular to original long axis.  Angular size initially about 0.5 degrees, "slightly less than the sun when first seen", a� size of quarter coin (0.955 inch) on windshield, diminishing to less than size of BB (0.17 inch) hence <18% of initial, or based on decrease of angular elevation by factor of about 12x SIGN confirmed as a distance increase of aboiut 12x to 15x [thus <0.03°], when it disappeared behind large cloud S of Akron at known distance/location/altitude about 60 miles away [cloud extending about 20 miles S of Akron] at 12,000 to 16,000 ft, at about 5 degrees to 10 degrees elevation, location known becsuse C-45 flew over it as they landed at Akron at 3:39-1/2 p.m.  [Actual size about 200 ft based on angular size of 0.5 degrees at 4 miles.]  (Sparks;  McDonald papers;  Jan Aldrich;  Saunders/FUFOR Index;