The closer the bird is to the surface of the water, the firmer and more inelastic is the uplift of the rising air. The bird appears to almost feel the surface with the tip of its weather wing.
From Lawrence Hargrave
And from a poise at this station the plane may swoop down, at great disadvantage if close to the back of the wave, at various slopes and directions till it cuts into the air that is being raised by the face of the following wave, which again enables it to resume its velocity.
As to the effect of the wave on the air, we will suppose the water to be quite flat and the air motionless, a heavy undulation comes on the scene, it has to pass, so it pushes the air up with its face, letting it fall again as its back glides onwards.
Bent metal is worse than bent wood and weight for weight is more flexible.
Common sense steps in here and says: Separate the parts you want to be mobile from the parts you want to be inert. You have seen the result, and I know many have the skill to apply it.
If you direct your attention to the position of a bird with regard to the wave surface, it will speedily be noticed to be nearly always on the rising side or face of the wave and moving apparently at right angles to the wave's course, but really diagonal to it.
It becomes a giant's task to compute the result when the effect of cross seas, wind at all angles and ever varying force, arched surfaces, head resistance, ratio of weight to area, and the intelligence of the guiding power crop up.
The most ordinary conditions for observing sailing birds are then the wind and sea are both aft.
The plane is simply abstracting the power stored in the wave by a distant gale, and using it to counteract gravity. And if the work be continued long enough, or a multitude of planes be continually drawing on the reservoir of power, the wave must inevitably be flattened.
The wings are moved several times by hand to charge the crank chamber with mixture, which flows on through the external pipe and inlet valve to the compression space and cylinder.
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