In its most marked form this condensation, starting from the engine exhausts, forms a band of many times the width of the aircraft, stretching across the skyl like a wisp of well-marked cirrus cloud. From the ground this trail appears to come to a point sharply defined at the exact position of the aircraft, so that although the machine itself may not be visible, every movement it makes is easily visible to the naked eye of an observer on the ground and thus may be accurately plotted enabling accurate AA fire to be brought to bear. Eat a small breakfast in the morning, followed if possible, by a good bowel movement, this being necessary to allow as much food as possible to pass out of the stomach. If the flight is to be made after lunch it is advised that one should not have any lunch whatever, except possibly a bar of chocolate, and do not drink very much of any liquid. At great heights it has been found that the stomach is inclined to distend or at least to give the impression that it is distended, and although not in any way dangerous, it is uncomfortable and causes considerable wind.
Gaseous drinks are also very much inclined to give one the same feeling as too much food or liquid.
Thereafter the Special Survey Flight Spitfire was modified to carry extra oxygen. I got a bit of a pat on the back for installing that second oxygen bottle and another for warming up the pilots a little. They complained of extreme cold at high altitude, with everything frosting up. By January 10, the Special Survey Flight had wn 15 sorties, on ten of which photographs had been obtained. The remaming five were abandoned due to bad weather.