Once the choice of a _type_ had been made, there were countless other problems. _Stability_ was of prime importance and the airmen of a few years ago labored desperately to attain it. They knew all too little about the airplane from a scientific angle. We have seen in our brief study that the method of obtaining balance in a glider or an airplane is to see that its _center of weight_ coincides with the center of the _upward pressure_ of air. How to bring this happy state of things about was a source of much debate. Some suggested that instead of a tail at the stern a tail in front of the main planes of the machine would help to balance it in flight. Some placed the pilot's seat above both planes of the biplane, while others thought he should sit below. Many of these queer ideas were tried out and by dint of hard practise and many failures certain simple elementary facts were finally weeded out and set down. Probably the addition of a "fuselage" or body to the modern airplane has had something to do with helping in the proper distribution of its weight and increasing its stability. Larger at the bow and tapering toward the stern where a fixed tail piece or horizontal stabilizing plane is attached, it resembled more or less closely the general outlines of a fish or bird. And this "streamline form" greatly reduces the _head resistance_, another important subject on which there was very little known when the first of the airplanes was built. In addition to having only a very slow and inefficient engine the early machine suffered from the head resistance it created as it pushed forward through the air, and this check to its progress ate up the little speed its motor could develop. For if the airman of 1908 or 1909 was made miserable by his fear of winds, gusts and aerial whirlpools which might upset him in mid-air, his fears in this direction were completely overshadowed by his worries about a suitable motor. If the design of his craft was faulty and it proved "balky" when he attempted flight, he had only himself to blame. But for an engine he had to rely entirely upon some one else. The airplane could be a "home-made" article, but the engine had to be chosen from such as were on the market. The Wright brothers in their first flying machine used a made-over automobile engine of 12 horsepower. It was not long before this was improved upon, and later Wright machines had a four-cylinder, water-cooled engine developing 35 horsepower. Its weight had been reduced as far as possible and its simplicity of design was its greatest recommendation.