The problem of keeping pilots and observers warm when flying at high altitudes was brought into prominence during the late war. At a height of 20,000 feet the normal temperature is approximately − 25°C. or “45° of frost,” Whilst in winter at an altitude of 30,000 feet temperatures of the order of − 5o°C. or “90° of frost” are frequently encountered. When flying at probably 100 miles per hour in temperatures of this order, all ordinary means of keeping the body warm are quite inadequate and no padding or lagging capable of being worn as a suit will keep the body warm for more than a few minutes.