Blackboard obligingly showed them as follows:

"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:

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? And a body to fix them in, and tanks for oil and petrol, and a tail, and," archly, "one of those dashing young Pilots, what?"

"Well, we are getting within sight of those interesting Factors," said the Lift-Drift Ratio, "but first of all we had better decide upon the Area of the Surfaces, their Angle of Incidence and Camber. If we are to ascend as quickly as possible the Aeroplane must be slow in order to secure the best possible lift-drift ratio; for the drift of the struts, wires, body, etc., increases approximately as the square of the speed, but it carries with it no lift as it does in the case of the Surface. The less speed then, the less such drift, and the better the Aeroplane's proportion of lift to drift; and, being slow, we shall require a large Surface in order to secure a large lift relative to the weight to be carried. We shall also require a large Angle of Incidence relative to the horizontal, in order to secure a proper inclination of the Surface to the direction of motion, for you must remember that, while we shall fly upon an even keel and with the propeller thrust horizontal (which is its most efficient attitude), our flight path, which is our direction of motion, will be sloping upwards, and it will therefore be necessary to fix the Surface to the Aeroplane at a very considerable angle relative to the horizontal Propeller Thrust in order to secure a proper angle to the upwards direction of motion. Apart from that, we shall require a larger Angle of Incidence than in the case of a machine designed purely for speed, and that means a correspondingly large Camber.

"On the other hand, if we are thinking merely of Speed, then a small Surface, just enough to lift the weight off the ground, will be best; also a small Angle to cut the Drift down, and that, of course, means a relatively small Camber.

"So you see the essentials for Climb or quick ascent and for Speed are diametrically opposed. Now which is it to be?"

"Nothing but perfection for me," said Efficiency. "What I want is Maximum Climb and Maximum Speed for the Power the Engine produces."

And each Principle fully agreed with her beautiful sentiments, but work together they would not.

The Aspect Ratio wanted infinite Span, and hang the Chord.

Maximum Climb.

Maximum Speed.

The Angle of Incidence would have two Angles and two Cambers in one, which was manifestly absurd; the Surface insisted upon no thickness whatever, and would not hear of such things as Spars and Ribs; and the Thrust objected to anything at all likely to produce Drift, and very nearly wiped the whole thing off the Blackboard.

There was, indeed, the makings of a very pretty quarrel when the Letter arrived. It was about a mile long, and began to talk at once.

"I'm from the Inventor," he said, and hope rose in the heart of each heated Principle. "It's really absurdly simple. All the Pilot has to do is to touch a button, and at his will, vary the area of the Surface, the Angle of Incidence, and the Camber! And there you are—Maximum Climb or Maximum Speed as required! How does that suit you?"

"That suits us very well," said the Surface, "but, excuse me asking, how is it done without apparatus increasing the Drift and the Weight out of all reason? You won't mind showing us your Calculations, Working Drawings, Stress Diagrams, etc., will you?"

Said the Letter with dignity, "I come from an Inventor so brilliantly clever as to be far above the unimportant matters you mention. He is no common working man, sir! He leaves such things to Mechanics. The point is, you press a button and——"

"Look here," said a Strut, rather pointedly, "where do you think you are going, anyway?"

"Well," from the Letter, "as a matter of fact, I'm not addressed yet, but, of course, there's no doubt I shall reach the very highest quarters and absolutely revolutionize Flight when I get there."

Said the Chalk, "I'll address you, if that's all you want; now drift along quickly!" And off went the Letter to The Technical Editor, "Daily Mauler," London.

And a League was formed, and there were Directors with Fees, and several out-of-service Tin Hats, and the Man-who-takes-the-credit, and a fine fat Guinea-pig, and all the rest of them. And the Inventor paid his Tailor and had a Hair-Cut, and is now a recognized Press Expert—but he is still waiting for those Mechanics!

"I'm afraid," said the Slide-rule, who had been busy making those lightning-like automatic calculations for which he is so famous, "it's quite impossible to fully satisfy all of you, and it is perfectly plain to me that we shall have to effect a