the right side of the wall to the left wing. At
the end of each tube there is what we call a 'venturi tube.' This is a
kind of suction device operated by the wind. The wind which blows
through the left venturi tube sucks the air out of the right-hand side
of the mercury tube, and the right venturi tube sucks the air out of
the left-hand side of the mercury tube. The stronger the wind, the
greater the suction. Now, when making a turn to the right the left
wing must travel faster than the right wing, and so there must be more
suction in the left venturi. This produces a greater suction in the
right-hand side of the mercury tube, which draws the mercury up on that
side and down on the other, until the proper electrical contact is
broken and the ailerons are returned to neutral position."

"Can the mechanism be thrown out of gear when desired? I should think
such a feature might be desirable," remarked Mr. Giddings.

"Indeed it is desirable, sir," declared Paul. "No red-blooded pilot
wishes to sit still and let his machine run itself all the time, no
more than an automobilist. That would spoil all the sport. By merely
disengaging the automatic pilot's wires here at the sector--the work of
a couple of seconds--the airplane is ready for hand control."

"How much does it weigh?" was the gentleman's next query.

"A trifle less than a hundred pounds."

"That oughtn't to handicap an airplane any."

"Not a bit," said Paul.

CHAPTER V

AN AIR RACE FINISH AND A CHALLENGE

All in all, Mr. Giddings expressed himself as more than pleased with
the drawings for the Sky-Bird II. At the end of the explanation, he
put the papers back in the envelope, and asked:

"Have you another set of these drawings in ink, Paul?"

"Yes, sir; this is a copied set; the original drawings from which we
will make our tracings and blue-prints are at home."

"You had better leave them there in a safe place, and work from your
blue-prints in the old exhibition building at the fair-grounds, being
careful to lock them up in your workbench every time you depart. I
think you boys have a valuable thing here, and it is to your interest
to keep others from knowing your plans or seeing the airplane until we
have full government protection in the shape of patent rights. I shall
turn this set of drawings over to a patent attorney in the city and ask
him to make application to the Patent Office in Washington without
delay."

The next morn