Of the various flyers and models thus far studied, some manifest fairly good, others very imperfect projectile stability. Many inventors have been more alert to the gravitational stability and safety of the parachute than to the kinetic stability and keen, direct flight of the arrow. Some of the most pretentious machines imitated the thistle down more nearly than the dart or swallow. But the exigencies of actual flight would easily rectify such imperfections of design. Tractional balance also is a property of some importance in fluid navigation. This requires that the line of propulsive thrust coincide with the line of fluid resistance. It is a property, however, that inventors readily apprehend, and usually provide for. In general a flyer is subject to four forces: weight, thrust, air pressure and inertia. When these balance about any axis the craft has equilibrium about that axis; when they balance about the three axes the craft is completely balanced, and preserves its orientation in flight. Devices for preserving this complete balance have already been described; as also provision for propulsion and sustentation, launching and landing safely. Thus at the close of the nineteenth century all the essential principles and contrivances of pioneer flight were worked out, except one—a suitable motor. This was the real problem of the ages. The rest was easy by comparison. A light enduring motor, if available to the old time inventors, would have brought dynamic flight centuries ago. That only could have baffled Da Vinci, Cayley, Henson, Wenham and the long line of pioneer aviators. Eventually, of course, steam engines had come, endowed with ample power; but costly to build and wasteful to operate. The light automobile engine appeared in the latter nineties; promptly thereafter followed the dynamic flyer, the snow-winged herald of the twentieth century. CHAPTER IX AËROPLANES OF ADEQUATE STABILITY AND POWER