Manuscripts/Mixed Material Article by Alexander Graham Bell, June 5, 1892
In this connection it is obvious that the greater the specific gravity of the machine the less effect will wind have upon it; and conversely the less the specific gravity the more will it be at the mercy of atmospheric currents.
The essential condition therefore of the successful flying machine is, that it shall be specifically heavier than the air, and the greater its specific gravity compared with the air the more independent it will be of atmospheric conditions.
Any machine that is specifically heavier than the air can only be supported by mechanical motion of some sort, and the question is what sort of mechanical motion we are to use, and how it shall be applied.
It is significant in this connection that all the flying creatures of the earth, from the humblest insect up to the soaring eagle, all fulfil the above conditions. They are all specifically heavier than the air; and in all, flight is accomplished by the agency of extended surfaces called wings. Hence we may conclude that the flying machine of the future will have wings; it will consist of a car, or a body specifically heavier than the air and be supported by the motion of flat, extended surfaces or wings.
Of course, an engine of some sort will be employed to give motion to the wings.
Of the different motive powers available, that one will be most suitable that has the least weight proportionate to its power.
Electricity, so far as our present knowledge extends, is out of the question, as all the machines for utilizing electrical force yet known have great weight; the choice lies between steam and some form of gas engine, with the probabilities in favor of gas.