That compass was the most remarkable of all the splendid equipment features of the Wind Bird. It was based upon the principle of the relation between the earth's magnetic field and the magnetic field generated in the airplane. When the plane's course had been set so that the needle registered zero on this compass, any deviation would cause the needle to swing away from zero in the direction of the error. Such an error could be corrected by flying the plane with the needle at the same distance on the other side of zero, and for the same time that the error had been committed. This would set the aircraft's nose back on her course again.