With air the conditions are entirely different. The boat submergence in the water is practically the same, whether going ten or twenty miles an hour. The head resistance is the same, substantially, at all times in the case of the boat; with the flying machine the resistance of its sustaining surfaces decreases.
Without going into a too technical description of the reasoning which led to the discovery of the law of air pressures, let us try and understand it by examining the diagram, Fig. 7.
A represents a plane at an angle of 45 degrees, moving forwardly into the atmosphere in the direction of the arrows B. The measurement across the plane vertically, along the line B, which is called the sine of the angle, represents the surface impact of air against the plane.
In Fig. 8 the plane is at an angle of 27 degrees, which makes the distance in height across the line C just one-half the length of the line B of Fig. 7, hence the surface impact of the air is one-half that of Fig. 7, and the drift is correspondingly decreased.
Fig. 7. Equal Lift and Drift in Flight.
Fig. 8. Unequal Lift and Drift.
MOVING PLANES VS. WINDS.—In this way Boisset, Duchemin, Langley, and others, determined the comparative drift, and those results have been largely relied upon by aviators, and assumed to be correct when applied to flying machines.
That they are not correct has been proven by the Wrights and others, the only explanation being that some errors had been made in the calculations, or that aviators were liable to commit errors in observing the true angle of the planes while in flight.
MOMENTUM NOT CONSIDERED.—The great factor of momentum has been entirely ignored, and it is our desire to press the important point on those who begin to study the question of flying machines.
THE FLIGHT OF BIRDS.—Volumes have been written concerning observations on the flight of birds. The marvel has been why do soaring birds maintain themselves in space without flapping their wings. In fact, it is a much more remarkable thing to contemplate why birds which depend on flapping wings can fly.