n splendidly! Show us how it looks now, Blackboard."
And the Blackboard obligingly showed them as follows:

[Illustration]

"Well, what do you think of that?" they all cried to the Drift.

"You think you are very clever," sneered the Drift. "But you are not
helping Efficiency as much as you think. The suction effect on the top
of the lower Surface will give a downward motion to the air above it and
the result will be that the bottom of the top Surface will not secure as
good a Reaction from the air as would otherwise be the case, and that
means loss of Lift; and you can't help matters by increasing the gap
between the surfaces because that means longer Struts and Wires, and
that in itself would help me, not to speak of increasing the Weight.
You see it's not quite so easy as you thought."

At this moment a hiccough was heard, and a rather fast and
rakish-looking chap, named Stagger, spoke up. "How d'ye do, miss," he
said politely to Efficiency, with a side glance out of his wicked old
eye. "I'm a bit of a knut, and without the slightest trouble I can
easily minimize the disadvantage that old reprobate Drift has been
frightening you with. I just stagger the top Surface a bit forward, and
no longer is that suction effect dead under it. At the same time I'm
sure the top Surface will kindly extend its Span for such distance as
its Spars will support it without the aid of Struts. Such extension will
be quite useful, as there will be no Surface at all underneath it to
interfere with the Reaction above." And the Stagger leaned forward and
picked up the Chalk, and this is the picture he drew:

[Illustration]

Said the Blackboard, "That's not half bad! It really begins to look
something like the real thing, eh?"

"The real thing, is it?" grumbled Drift. "Just consider that contraption
in the light of any one Principle, and I warrant you will not find one
of them applied to perfection. The whole thing is nothing but a
Compromise." And he glared fixedly at poor Efficiency.

"Oh, dear! Oh, dear!" she cried. "I'm always getting into trouble. What
_will_ the Designer say?"

"Never mind, my dear," said the Lift-Drift Ratio, consolingly. "You are
improving rapidly, and quite useful enough now to think of doing a job
of work."

"Well, that's good news," and Efficiency wiped her eyes with her Fabric
and became almost cheerful. "Suppose we think about finishing it now?
There will have to be an Engine and Propeller, won't there