Yes it is difficult On land we are able to compare our rate of travelling with that of other moving objects, such as trains, motor cars, cycles, pedestrians, etc. Allowance must also be made for the rate at which the air is moving, e.g., if the air is moving at forty miles per hour, an aeroplane travelling against it at sixty miles per hour only appears
to be doing twenty miles per hour. An airman cannot compare his speed with that of other moving objects,
as these are generally too far away to be seen. Or they may be hidden by clouds.
