Manuscripts/Mixed Material Letter from Alexander Graham Bell to Mabel Hubbard Bell, May 14, 1899
This brings me to the focal point of the whole problem — the propeller. What we want is that form of propeller — which, by reaction against the air, can impart great velocity to a heavy mass. My whole thoughts are focussed now upon the propeller.
Not only do we want it to impart great onward velocity to the mass of the machine — but we should also seek to utilize the disturbance of the density of the air caused by its action — as a source of lift.
The propeller acts by shoving against the air — thus compressed it — and rendering it more dense. The reaction moves the machine. It acts by dragging air away from some point (rarifying it) — and shoving it towards another place (compressing it there.) Now the compressed air in expanding, exerts pressure in every direction around it. At one point the machine is pushed by it — thus gaining velocity — at other points it acts only against surrounding air. If, however, there should be a surface above the condensed air — that surface will experience an upward above or “lift.”
If we consider the place where a portion of air is rarified by the action of the propeller in displacing it — the surrounding denser air has a tendency to rush in every