Even if the nervous system could be schooled into endurance of the wear and tear of consciously balancing an aeroplane for many hours, it is still imperative that the task be not left to the exertion of human wits, but controlled by self-acting devices responding instantly to unforeseen conditions as they occur. [Illustration: Diagram showing turbulent air currents produced when a flat plane is forced through the air at a large angle of incidence in the direction A-B.] [Illustration: Diagram showing smoothly flowing air currents caused by correctly shaped plane at proper angle of incidence.] Some of the problems of which the model-builder may find the solution are: whether large screws revolving slowly, or small screws revolving rapidly, are the more effective; how many blades a propeller should have, and their most effective shape; what is the “perfect” material for the planes (Maxim found that with a smooth wooden plane he could lift 2½ times the weight that could be lifted with the best made fabric-covered plane); whether the centre of gravity of the aeroplane should be above or below the centre of lift, or should coincide with it; new formulas for the correct expression of the lift in terms of the velocity, and angle of inclination--the former formulas having been proved erroneous by actual experience; how to take the best advantage of the “tangential force” announced by Lilienthal, and reasserted by Hargrave; and many others. And there is always the “paradox aeroplane” to be explained--and when explained it will be no longer a paradox, but will doubtless open the way to the most surprising advance in the art of flying. It is not assumed that every reader of this chapter will become a studious experimenter, but it is unquestionably true that every model-builder, in his effort to produce winning machines, will be more than likely to discover some fact of value in the progress making toward the ultimate establishment of the commercial navigation of the air. The tools and materials requisite for the building of model aeroplanes are few and inexpensive. For the tools--a small hammer; a small iron “block” plane; a fine-cut half-round file; a pair of round-nose pliers; three twist drills (as used for drilling metals), the largest 1/16 inch diameter, and two smaller sizes, with an adjustable brad-awl handle to hold them; a sharp pocket knife; and, if practicable, a small hand vise. The vise may be dispensed with, and common brad-awls may take the place of the drills, if necessary.