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Classification of the Sci.
94

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a current of air of 2 inches per second once set up would continue by its own momentum without any further work being done; or if the word were continued and such a current were set up in a hundredth of a second, with no friction in the machinery, in a second a current of 16 feet a second would have been set up. In point of fact, this does not happen. On the contrary, a rapid current having been set up becomes difficult to sustain owing to the air not flowing in fast enough above nor flowing away fast enough below. Still, the force exerted [is] none the less efficient on that account in sustaining the raft. It remains true that if the force could be properly applied, the expenditure of energy required to sustain the aeroplane would be vanishingly small. What is needed is a rapidity of motion approximating that of sound, so as to give the effect of a mild incessant rocket. It is to be remarked that the motions of birds are excessively rapid, and it may well be that in the soaring of high flying birds, where Langley's "internal work of the wind" may, very likely, not exist,—for it is in part if not wholly the effect of the

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