"It is this way, sir," responded John. "The regular big biplane of the bomber type carries two propellers with an engine for each propeller. If one motor fails them when flying, about all the other is good for is to make a landing with. By reason of the great lightness of our airplane one good 400 horse-power motor is all we need for pulling purposes. But suppose this should fail, as any motor might do? We could not continue, any more than the other fellow, and would have to volplane to the ground. Again, suppose we wished to fly continuously more than twelve hours? We could not do so, as such a steady run would heat the best motor and ruin it. These two Liberty motors, which we have, overcome all these troubles. Both are so arranged that a simple switch connects and disconnects either one with the propeller, and both can be put at work at the one time if needed in a bad storm. If one stalls, the other can immediately be thrown in and a forced landing obviated. Moreover, if we could get fuel when needed, with this arrangement I am safe in saying we could fly steadily day and night, resting one motor and working its mate, for a week or more." "What is this?" As he spoke the publisher touched a peculiar-looking helmet hanging from a hook near the pilot's seat. Bob laughed. "Why, don't you recognize the products of your talented son, dad?" he cried, as he took the object down and clapped it over his father's iron-gray head. "That's my new wireless telephone headpiece, and right underneath it here is the 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.