dmiring him. He was right there when it came to
grit."

After a while they began to discuss technicalities, and the boys learned
a great many things they had never known before. The pilot happening to
mention that there were sometimes a number of airplanes equipped with
radio operating within a restricted district, Joe wanted to know if they
did not have a good deal of trouble with interference.

"No. There was at first some interference by amateurs, but these soon
learned to refrain from using their instruments during patrol periods.

"You see," he explained, "we use a special type of transmitting outfit
aboard our fire-detection craft. It's called the SCR-Seventy-three. The
equipment obtains its power from a self-excited inductor type
alternator. This is propelled by a fixed wooden-blade air fan. In the
steam-line casing of the alternator the rotary spark gap, alternator,
potential transformer, condenser and oscillation transformer are
self-contained. Usually the alternator is mounted on the underside of
the fuselage where the propeller spends its force in the form of an air
stream. The telegraph sending keys, field and battery switch, dry
battery, variometer and antenna reel are the only units included inside
the fuselage.

"The type of transmitter is a simple rotary gap, indirectly excited
spark and provided with nine taps on the inductance coil of the closed
oscillating circuit. Five varying toothed discs for the rotary spark gap
yield five different signal tones and nine different wave lengths are
possible.

"So," he finished, looking around at their absorbed faces, "you see it
is quite possible to press into service a number of airplanes without
being bothered by interference."

"It sounds complete," said Bob. "I'd like a chance to see one of those
sets at close range sometime."

The time passed so quickly that finally the visitor rose with an apology
for staying so late. The radio boys were sorry to see him go. They could
have sat for hours more, listening to him.

"That fellow sure has had some experiences!" said Joe, as, a little
later, the boys mounted the stairs to Bob's room. "It was mighty lucky
we happened along while he was here."

"You bet your life," said Herb. "I wouldn't have missed meeting him for
a lot."

"Say, fellows," Jimmy announced from the head of the stairs, "I know now
what I'm going to do when I'm through school. It's me for the tall
timber. I'm going to pilot an airplane in t