“He told me that plans are now on foot to fly mail across the continent, daily, both ways, in something like twenty-four hours. Just think of that! From coast to coast in twenty-four hours! That’s five times as fast as an express train does it, and a hundred times as fast as the old pioneers with their prairie schooners could do it.
“But in order to do this, a gap of about a thousand miles must be flown at night. And here is where the radio comes in. In order to be able to find his way in the dark, the flier uses his ears instead of his eyes. He wears a radio-telephone helmet that excludes the noise of the motor. A coil of wire is wound on his plane and is connected to a radio receiving set on board. Along his route at stated intervals are transmission stations whose signals come up to the aviator. When the pilot’s direction finder is pointed toward these stations that mark out his path the signals are loudest. The minute he begins to get off his path, either on one side or the other, the signals begin to get weaker.
“Now, you see, all that the pilot has to do is to keep along the line where the signals are loudest. If he goes a little to the right and finds the signals getting weaker, he knows he must shift a little back to the left again until he gets on the loudest sound line. The same process has to be followed if he gets off to the left. You see, it’s just as if the plane were running along a trolley line miles below it. Only in this case the trolley line instead of being made of wire is made of sound. That loudest sound line will stretch right across the continent, and all the flier has to do is to run along it. If he does this, he’ll get to his destination just as certainly as does the train running along the rails that lead to the station.”
“It’s wonderful!” exclaimed Bob.
“Sounds like witchcraft,” commented Joe.
“You see how easy that makes it for the aviator,” resumed the doctor. “It may be as black as Egypt, but that makes no difference to him. He may be shrouded in fog, but that can’t bewilder him or shunt him off his course. He can shut his eyes and get along just as well. All he’s got to do is not to go to sleep. And when the dawn breaks he finds himself a thousand miles or so nearer to his destination.”
“Suppose he gets to his landing field in the night time or in a heavy fog,” said Joe thoughtfully. “How’s he going to know where to come down?”
“Radio attends to that too,” replied the doctor. “At each landing place there will be a peculiar kind of radio transmission aerial, which transmits vertically in the form of a cone that gains diameter as it goes higher. At a height of about three thousand feet above the field, such a cone will have a diameter of nearly half a mile. In other words this sound cone will be like a horn of plenty with the tip on the ground and its wide opening up in the air. The pilot will sail right into this wide mouth of the horn which he will recognize by its peculiar signal. Then he will spiral down on the inside of the cone, or horn, until he reaches the tip on the ground. This will be right in the middle of the landing field, and there he is safe and sound.
“But here I am at my corner,” Dr. Dale concluded. “And perhaps it’s just as well, for when I get to talking on radio I never know when to stop.”
He said good-by with a wave of his hand while the four boys looked after him with respect and admiration.