754 THE POPULAR SCIENCE MONTHLY. 



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 mo- 

 tion in the given time. Thus with increasing velocity the air be- 

 comes more and more rigid because more and more must be started 

 in motion in given time, until, if velocity is infinite, the air be- 

 comes immovably rigid.* 



We have spoken thus far of a perfectly horizontal plane mov- 

 ing 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 aeroplane 

 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 unex- 

 pectedly moderate. This, of course, applies only to the aeroplane. 

 Resistance to the attached machine follows the usual law. But 

 this is small in comparison with the sustaining power of the aero- 

 plane. Therefore, once get a flying machine, even one of great 

 weight, with its aeroplane 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 issu- 

 ing 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 ve- 

 locity 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.. 



