"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."