t I say is, make the surface of Infinite
Span with no width or _chord_, as they call it. That's all I require,
I assure you, to make me quite perfect and of infinite service to Miss
Efficiency."

[Illustration]

"That's not practical politics," said the Surface. "The way you talk one
would think you were drawing L400 a year at Westminster, and working up
a reputation as an Aeronautical Expert. I must have some depth and chord
to take my Spars and Ribs, and again, I must have a certain chord to
make it possible for my Camber (that's curvature) to be just right for
the Angle of Incidence. If that's not right the air won't get a nice
uniform compression and downward acceleration from my underside, and the
rarefied 'suction' area over the top of me will not be as even and clean
in effect as it might be. That would spoil the Lift-Drift Ratio more
than you can help it. Just thrust that chalk along, will you? and the
Blackboard will show you what I mean."

"Well," said the Aspect Ratio, "have it your own way, though I'm sorry
to see a pretty young lady like Efficiency compromised so early in the
game."

"Look here," exclaimed a number of Struts, "we have got a brilliant
idea for improving the Aspect Ratio," and with that they hopped up on
to the Spars. "Now," excitedly, "place another Surface on top of us.
Now do you see? There is double the Surface, and that being so, the
proportion of Weight to Surface area is halved. That's less burden of
work for the Surface, and so the Spars need not be so strong and so
deep, which results in not so thick a Surface. That means the Chord
can be proportionately decreased without adversely affecting the Camber.
With the Chord decreased, the Span becomes relatively greater, and so
produces a splendid Aspect Ratio, and an excellent proportion of Lift
to Drift."

"I don't deny that they have rather got me there," said the Drift; "but
all the same, don't forget my increase due to the drift of the Struts
and their bracing wires."

"Yes; I dare say," replied the Surface, "but remember that my Spars are
less deep than before, and consequently I am not so thick now, and shall
for that reason also be able to go through the air with a less
proportion of Drift to Lift."

"Remember me also, please," croaked the Angle of Incidence. "Since
the Surface has now less weight to carry for its area, I may be set
at a still lesser and finer Angle. That means less Drift again. We are
certainly getting o