"The Sky-Bird is really more of a bird than you may think, Mr. Giddings," declared Paul. "Yes," added his brother John. "As you probably know, sir, a bird gets its great buoyancy from the fact that every bone in its body is hollow; in flight it fills these bones with a very light gas, which is formed by an action of its lungs in drawing in air. We have adapted this principle in the wings and fuselage of this little machine. They are airtight and filled with compressed helium-gas, which is non-inflammable and nearly as light as its highly volatile rival, hydrogen-gas." "Hydrogen-gas is surely a dangerous commodity around fire," said Mr. Giddings. "I understand that when the big English dirigible R-34 came across the Atlantic last summer she was filled with hydrogen, and that her commander and crew all wore felt-soled shoes, so that they would not by any chance cause a spark when they walked over her metal floors and ladders just beneath her great bag." "That is true," vouched John Ross. "One little spark reaching any of that stored hydrogen would have torn that great dirigible into fragments in one gigantic blast." "We have handled recent newspaper copy containing mention of this new gas, helium; but I must confess I am in the dark regarding its nature and source," said Mr. Giddings. "What is it, anyway?" "I will refer your question to Paul here," replied John. "He is the one who worked out this idea of using helium in an airplane and giving it the best properties of a dirigible without any of the dirigible's handicap of clumsiness and excessive wind resistance. He has been studying the properties of helium in school, also the flight of birds." "Well, not to get into a tiresome discourse, as Professor Herron would say, I shall make this description very rudimentary," said Paul, with a smile. "During a total eclipse of the sun in India in 1868, Lockyer, a British astronomer, saw in the spectroscope a bright, yellow line of light around the sun. He called it _helium_, after the Greek word for sun. So much for him. Twenty-seven years later an element was found on earth in natural-gas in Kansas, which gave the same bright, yellow light viewed through the spectrum. The people, finding it would not burn, disgustedly let millions of barrels of this valuable element escape into the air, before a scientist told them that it was of untold value for balloon and airship purposes. It is thought the gas comes from radium deposits. It has never been found in any country except the United States, and only here in Kansas and northern Texas, where it occurs in sands from 14,000 to 16,000 feet deep. Our government is now securing about 50,000 cubic feet of helium per day, refusing to sell it to foreign countries, as it is all needed here, besides which it might be used against us in case of another war." While Paul had been telling this, Mr. Giddings had been busy jotting something down in shorthand in a notebook.