Paul touched the little valve in the tank chamber of the fuselage.  The next moment there was a quiver, and then the propeller began fairly to hum.  A strong, steady gust of air began to blow in the faces of the Giddingses, while they had to hang on grimly in order to keep their little charge from jumping out of their arms and dashing away into the air.  For fully three minutes the propeller continued to whirl with undiminished speed, then slowly it began to slow up, and finally stopped.