e mahogany cabinet containing the
sending and receiving instruments. You see, these two wires run from
the plug up to the receivers, there being one receiver in each side of
the helmet, right over your ear, pressing against the ear tightly by
means of a sponge-rubber gasket."

"A man looks like a padded football player with this thing on," said
Mr. Giddings with a smile. "Why is a helmet required at all?"

"We wouldn't require it so much with these motors, as they are equipped
with a new kind of muffler which shuts out about four-fifths of the
noise other airplanes get," explained Bob. "But for all that there are
always noises in airplanes; for instance, they say the whirr of the
propeller when it is revolving about 1450 revolutions per minute, or at
the full speed of this one, makes quite a roar; so you see the need of
the helmet to shut out all undesirable sounds possible. In ordinary
planes the crew cannot talk to each other except by using phones or
putting their lips to each other's ears and yelling at the top of their
voices, according to what John and Tom tell me. But we don't expect to
have that trouble in this enclosed cabin and with this new muffler
working, do we, fellows?"

"I'm sure we won't," said John.

"Not if I'm any judge," grinned Tom.

"Can you talk with a ground station when you're flying, say a couple of
miles high?" asked Mr. Giddings, examining a transmitter attached to a
yoked wire support which his son slipped over his shoulders.

"Farther than that. With this particular vacuum tube, which will
amplify sounds three or four times over any other I have tried, we
expect to talk with ground stations or other aircraft at a distance of
three thousand miles. Notice what a simple thing it is, dad," and Bob
indicated a little glass bulb which looked a lot like an ordinary
incandescent light, but which had a peculiar arrangement of wires and
substances inside.

"Is the transmitter or receiver made just like the ordinary kind?"
asked Mr. Giddings.

"Practically the same, dad. The wireless transmitter, like that of the
wire telephone, contains a sensitive diaphragm which your voice strikes
and sets to vibrating. These vibrations compress and release a capsule
of carbon granules which agitate and set in motion an electrical
current in two magnets connecting with them. The magnets convey the
sound-waves in the form of electrical waves, along wires out to the tip
of each wing, where the wi