In their after-school confabs, Weston's experience and Hal's theory and hopes fought many an acrimonious friendly battle. Raynor, who as advanced-flying pilot was for the present out of Hal's school sector, sometimes of a night joined in these air battles fought out on the ground. All manner of past and modern experiments came under the fire of their discussions. Aeronautical engine builders seemed to have tried out many varieties of motors. There were the long ago experiments with steam engines for airplanes. The engine itself could be made much lighter than the gasoline engine, but the fuel and water added so much weight that the whole combination was far too heavy for air purposes. Mercury and other liquids were tried out without much success. Then came gasoline and mercury-vapor turbines, fine in principle but somehow unsuccessful. There was a flaw that needed some genius to find it.