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 exceedingly complex one, and can be solved only by careful experiments. 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 hundred 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 aëroplane, 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 unexpected 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 difference is greatly exaggerated in the case of aëroplanes. According to his experiments, a thin aëroplane of material two thousand 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 distance is greater and greater, until it may become almost inappreciable. The reason is plain. The aëroplane falling straight downward 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 aëroplane 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.