Manuscripts/Mixed Material Letter from Alexander Graham Bell to Mabel Hubbard Bell, May 14, 1899
For example — consider a surface of only one square inch. If the propeller could displace a layer of air above this inch surface — so that for a moment a slice of the atmosphere resting on this surface should be removed — the atmospheric pressure acting on the under side would exert an upward above equal to 15 pounds unbalanced by downward pressure from above.
Or again suppose that the propeller could cause a condensation of air under the surface so as to double the density of the air there — even for a moment — then at that moment — an upward pressure or “lift” equal to 15 pounds would be experienced. Think of the possibilities of lift due to this cause — 15 pounds to the square inch! — 144 pounds for only one square foot of surface!
And if it could draw air from above surface and condense it below — the possibilities would be doubled without increase of surface.