"The fuselage is divided into three sections," said Paul.  "The forward section contains the engine here; the rear section is an airtight chamber containing helium; and the central section is also an airtight chamber, but contains ordinary air which has been pumped into it through a valve, using the bicycle pump John is carrying, until it is under strong pressure.  When I turn this little valve an outlet is opened for the air to escape by a tube into branches communicating with each of these four cylinders.  This works the tiny pistons, much the same as gas in a gasoline-motor, and they turn the little crank-shaft to which they are connected, and the crank-shaft in turn revolves the propeller on its end."