Lawrence Hargrave

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.