NEW LIGHTS ON THE PROBLEM OF FLYING. 753 



gaged in the generation and transmission of force. What we want 

 more specially is (1) a material stronger in proportion to weight 

 than bone. This, no doubt, we have in steel tubes.* Here, then, is 

 a positive gain. But (2) we also want a force more intense than 

 muscular contraction, which is, I believe, about a hundred to a 

 hundred and twenty pounds per square inch of cross-section. This 

 can doubtless be surpassed, but not without increased weight of 

 containing and transmitting parts. If anything can be gained in 

 this direction (which is doubtful), against it must be set over the 

 greater economy of the natural machine. The problem is an ex- 

 ceedingly complex one, and can be solved only by careful experi- 

 ments. But let us admit that, by greater strength of material 

 and greater intensity of force, the limit of weight of machine and 

 fuel which can be lifted in the air may be pushed to several hun- 

 dred pounds. This, I am sure, is as far as we can go on this score. 



3. But the most important new light is found in the effect of 

 motion on the sustaining power of an aeroplane, and the greatest 

 flaw in my previous reasoning is the imperfect recognition of this 

 principle. 



As already stated, this principle was first brought out by 

 Marey, and was alluded to in my previous paper, but its supreme 

 importance was not fully appreciated until the experiments of 

 Langley. In his hands it becomes almost a new principle, and 

 one which must modify not only our theory of flying, but even 

 our theory of projectiles. Langley's experiments bring out the un- 

 expected result that in air a body does not fall the same distance 

 in a given time whether it falls straight downward from rest or 

 is affected with horizontal motion that its motion in the latter 

 case is not a resultant of distance of downward fall from rest and 

 horizontal motion. The same is true of all bodies, but the differ- 

 ence is greatly exaggerated in the case of aeroplanes. Accord- 

 ing to his experiments, a thin aeroplane of material two thou- 

 sand times the specific gravity of air, say aluminum, in perfectly 

 horizontal position and free to fall, would take four times as 

 much time to fall a certain distance if moving horizontally twenty 

 feet per second as it would if falling directly downward from 

 rest. With still greater velocities the time of falling a given dis- 

 tance is greater and greater, until it may become almost inappre- 

 ciable. The reason is plain. The aeroplane falling straight down- 

 ward must press the air out of its way. It takes time to do this. 

 Now, if it is moving horizontally edge on, before the air can move 

 appreciably, the plane is already on to new still air. Or, to put it 

 more definitely, supposing the aeroplane to be one foot square, 



* It is probably a mistake to suppose that aluminium or any alloy of that metal is 

 stronger, weight for weight, than steel. 

 VOL. XLIV. 56 



