In common with other machines, each new type aircraft must be put through comprehensive and well-seasoned tests. Unlike other machines, however, the aircraft exists to fly and by so doing severs its contact with the earth. Even this would not present a peculiar test difference if the atmosphere were rigidly connected to the earth and had a constant pressure and constant temperature at all heights, for then the engine power would not change with height or place and the speed over the ground would equal the speed through the air. Likewise the rate of climb, the manœuvrability and the behaviour of the controls would be independent of height. In such an atmosphere, tests would be required at one height only and measurements such as speed, rate of climb, take-off and landing could all be obtained accurately and directly from ground apparatus.