ign if this works out, as I hope it will."

"I'll certainly make every effort to be here, Arcot," Fuller assured
him.

"I can promise you a tough problem as well as an interesting one." Arcot
smiled. "If the thing works, as I expect it to, you'll have a job that
will certainly be a feather for your cap. Also it will be a change."

"Well, with that inducement, I'll certainly be here. But I think that
pirate could give us some hints on design. How does he get his glider
ten miles up? They've done some high-altitude gliding already. The
distance record took someone across the Atlantic in 2009, didn't it? But
it seems that ten miles straight up is a bit too steep for a glider.
There are no vertical air currents at that height."

"I meant to say that his machine is not a true glider, but a
semi-glider. He probably goes up ten miles or more with the aid of a
small engine, one so small it probably takes him half a day to get
there. And it would be easy for a plane to pass through the lower
traffic lanes, then, being invisible, mount high and wait for the air
liner. He can't use a very large engine, for it would drag him down, but
one of the new hundred horsepower jobs would weigh only about fifty
pounds. I think we can draw a pretty good picture of his plane from
scientific logic. It probably has a tremendous wingspread and a very
high angle of incidence to make it possible to glide at that height, and
the engine and prop will be almost laughably small."

* * * * *

The next evening the men got together for dinner, and there was
considerable speculation as to the nature of the discovery that Arcot
was going to announce, for even his father had no knowledge of what it
was. The two men worked in separate laboratories, except when either had
a particularly difficult problem that might be solved by the other. All
knew that the new development lay in the field of short wave research,
but they could not find out in what way it concerned the problem in
hand.

At last the meal was over, and Arcot was ready to demonstrate.

"Dad, I believe that you have been trying to develop a successful solar
engine. One that could be placed in the wings of a plane to generate
power from the light falling on that surface. In all solar engines what
is the greatest problem to be solved?"

"Well, the more I investigate the thing, the more I wonder which is the
greatest. There are a surprising number of annoying p