then, if falling straight downward, one square foot section of air must be moved in a given time; but if moving onward twenty feet per second, then twenty square feet must be started in motion in the given time. Thus with increasing velocity the air becomes more and more rigid because more and more must be started in motion in given time, until, if velocity is infinite, the air becomes immovably rigid.
We have spoken thus far of a perfectly horizontal plane moving edge on, and therefore with no front resistance. But if the plane be slightly inclined upward in the direction of motion, then the onward motion would tend to sustain the plane. The whole air pressure may be resolved into two parts, one resisting onward progress and one sustaining the plane; and when this latter is equal to the weight the plane will not fall at all. Now, as velocity increases, less and less inclination is necessary to get the requisite sustaining force. But with less inclination comes also less front resistance. Thus at very high velocity the aëroplane may be placed nearly horizontal with proportionally small front resistance and yet sufficient sustaining power. Thus it follows from this important principle that instead of force increasing as the square of the velocity attained (or even higher rate), as in a steamboat, the increase of force with increasing velocity is unexpectedly moderate. This, of course, applies only to the aëroplane. Resistance to the attached machine follows the usual law. But this is small in comparison with the sustaining power of the aëroplane. Therefore, once get a flying machine, even one of great weight, with its aëroplane well up in the air and moving onward, and there seems to be no physical impossibility of sustaining it indefinitely and giving it by means of suitable propellers a great velocity, say of forty to sixty miles per hour.
In the light of this new principle (for such it may be called) Langley and Maxim have constructed models of flying machines, and expect eventually to solve the problem of flying. A small model of a machine which he calls an aerodrone (air-runner) has been constructed by Langley, and was to have been exhibited at
- A striking illustration of this principle is seen in the extreme rigidity of the jet issuing from the nozzle of a hydraulic pipe. The water is under a pressure of three hundred or four hundred feet head, and is projected with a velocity which would cut in two a man's body. If the jet is struck with a crowbar, the bar rebounds as it would from steel. In penetrating, say, half an inch, the bar encounters an immense quantity of water at once. It is evident also that the same principle must apply to all bodies moving in the air, and therefore also to projectiles. There is, then, a kind of truth in the popular notion that velocity holds up or prevents the fall of a rifle ball—not, indeed, that the velocity itself holds up the ball, as popularly supposed (for it would not do so in a vacuum), but that the air is more effective in sustaining a moving body than one falling directly downward from rest.