After trying out the model as a glider by throwing it across a room and making sure it is well balanced both laterally and longitudinally, or from side to side, and fore and aft, the rubber strands can be put on, and the motor wound up. About four strands of rubber one eighth of an inch square, such as is sold for this purpose, would suffice for good flights of more than one hundred feet, if the machine were of the same weight and proportions as the model from which this description was written. In models, however, there are many little details that can change the conditions, and a boy can only experiment, locate his mistakes, and try it over again. This is one of the simplest and easiest model aeroplanes that can be made. A trip to one of the model aeroplane tournaments will reveal dozens of more elaborate ones, which will give any ingenious boy ideas for development of the principles he can learn from the simpler type. Probably the next step of the average boy would be to build a machine with two motors, which can be done by elaborating the single stick backbone or by making a backbone of two or three sticks well braced with cross pieces at each end and in the middle. Then there are interesting experiments with the size of planes, number of planes, their aspect ratio--that is the proportion of their width to their depth--ailerons for automatic stability, and rudders for keeping the machine on a straight course. There are always new things to be done with the motors, because, though the rubber motors have driven models close to half a mile, there are now on the market miniature gasoline motors to drive models, and experiments are being tried with clockwork and compressed air. Indeed the model aeroplane field is as broad in itself as that of the man-carrying machines. Aviation has been reduced to an exact science, but it is yet in its early growth, both in the field of models and in the field of the various kinds of man-carrying machines. Not only are the designers making great headway with aeroplanes, but also with dirigible balloons so any one interested in aeronautics has a very wide field for his work. As we said in an earlier chapter, the boy model designer of to-day may be the inventor of to-morrow who gains undying fame by some now undreamed-of development of the aeroplane. The designers of the man carriers are trying to make their machines stronger, safer, more reliable, capable of carrying more passengers, and they hope at last to bring them to a more practical use in the world than as a sport. The most thoughtful aviators do not favour exhibition flying so strongly as they do long cross-country flights, endurance tests, passenger-carrying tests, and other experiments that will develop aeroplanes beyond their present limitations.