"When we begin to take up the construction of a glider or an aeroplane, we must, like the Wright brothers, reluctantly enter upon the scientific side of it, because in model building we cannot simply make exact reproductions of the great man-carrying fliers, but must meet and overcome new problems. The laws that govern the standard aeroplanes apply a little differently to models, so it is necessary for the model builder to figure things out for himself. "For instance," explained the scientist, "most amateurs have decided that monoplane models fly much better than biplanes. The reason for this is probably that with the miniature makes the air is so disturbed by the propeller that its action on the lower plane tends to make it unsteady rather than to give it a greater lifting capacity. This could be avoided by placing the two planes farther apart, but they would have to be so far separated that the machine would be ungainly and out of all proportion. Moreover, the second plane, with the necessary stays and trusses, adds to the weight of the machine, and this is always bad in models. "There are as many different types of model aeroplanes as there are of the big man carriers, but you had better make a small flier first, experiment with it, and then work out your own variations just as you think best." "Will you help me build one?" asked the boy. "No, for you don't need my help and you will have more fun doing it alone. I will tell you how to go about it, and with what you know of the principles of aviation from our conversations it will be easy to make a successful model." Then taking a piece of paper and a pencil the scientist began to draw rough plans for the building of a little model monoplane something like the Blériot, except that it was driven tail first, with the propeller at the rear. As he worked he explained how the plan shown below should be followed, saying that the beginner would find that a length of about one foot would be the most convenient for this first model. Later on he can make the big ones with a spread of wings of three feet, and a length of forty or more inches. [Illustration: A SIMPLE MODEL AEROPLANE]