A low-flying jet, enveloped in an aura of cloud made by the jet itself, can look like a strange object. This condensation phenomenon, called a contrail, occurs when areas of low pressure develop on the wing surface; the air cools by expansion in the slowly moving boundary layer in contact with the wing. Both the depth of the boundary layer and the drop in pressure increase with increasing air speed, but each depends very closely on the aerodynamic qualities of the wing. An excellent photograph of one such disk produced by a Canberra jet was taken on February 4, 1956, along the coast of Africa near Accra on a morning when the condensation phenomenon occurred several times during air maneuvers. The weather was fine, the sky cloudless with a few patches of haze over the sea, and visibility was more than eight miles. During the display the air speed of the jets was usually too low or the air too dry for the aura to form. “But over the cliff edge where the sea-breeze was just beginning to break through in patches the air would be moist enough to condense about 1½ gm. of water droplets in each cubic metre of air, quite sufficient to produce the observed effect. The effect is increased by higher speeds at the end of a dive (when the angle of incidence of the aerofoil is least) ... but it is likely that the patchy onset of the sea-breeze was the most important contributing factor.”[III-7] A flying saucer reported from Johannesburg, South Africa, on April 11, 1958, belongs in this category. Hundreds of witnesses reported a mysterious starlike object maneuvering in the northern sky on three successive nights at speeds in excess of 2000 miles an hour. Most observers agreed that “The Thing” could not have been any known aircraft because its speed was too great; it sometimes hovered stationary in the air, and repeatedly changed color from white to red to deep scarlet. One member of an Interplanetary Club who watched it through binoculars described the UFO as saucer-shaped, with a rim like a soup plate around the edge. Members of the Council for Scientific and Industrial Research Minitrack Station, near Johannesburg, were amused by the variety of reports on “The Thing.” The mysterious object in the night skies was in fact a South African Air Force Dakota aircraft, flying back and forth so that the Minitrack Station could test the calibration of its tracking instruments. In addition to the usual navigation lights, the aircraft had carried a bright, flashing light so that it could be photographed[III-7a]. A flight of bombers refueling in mid-air at night can be a startling spectacle and more than once has been reported as a gathering of flying saucers.