Ever since windmills were first set up, men have been studying the merits of different propellers. By the time steamships came to be driven through the water by rotary blades or screws, their modeling had become a science. The builders of rotary fans in turn contributed still further to our knowledge on the subject. Drawing largely upon all this experience, the aviator has learned to build fairly efficient propellers, although there is probably no department of aeronautics to-day so little understood. In a windmill a current or cylinder of air flows, of course, against the propeller. The blades must be shaped and spaced with this in view. Reverse the action of the windmill, and the propeller proves inefficient. The broad blades will stir up a current of air, to be sure, but a very weak one. A revolving fan solves a very different problem in detaching a cylinder of air from the atmosphere and propelling it with the greatest possible momentum. Here, again, the propellers must be differently modeled and spaced. Neither the reversed windmill propeller nor the electric fan, however, would serve to drive an aeroplane. [Illustration: A beautiful monoplane built by R. Mungokee] [Illustration: Detail of a model built by R. Mungokee] [Illustration: An ingenious application of the dihedral angle] The propeller of an aeroplane must cut its way smoothly, pressing the air backward without splashing. It is only when an aeroplane is held fast that its propellers kick up such a fuss and blow your hat off. The aeroplane propeller’s work is much the same as that of a steamship, although the air through which it travels has many tricks not yet understood. The density of the air compares to that of water as one to eight hundred, but the friction encountered by the air propellers is much greater than 1-800th that of water. It may be laid down as a general rule, however, that the driving force of an aeroplane propeller varies as the square of the number of revolutions per minute. There is at present no standard form of propeller for the man-carrying or model aeroplane. One school of designers favors a small blade revolved at high speed, while others claim that a larger propeller driven more slowly is more efficient. As a general rule it may be laid down that a model with a span of thirty inches should be driven by twin propellers eight inches in length or diameter. They should have a speed of about 1,200 revolutions per minute, or at the rate of some 200 turns every ten seconds. To test the strength of your motor, give the propeller 200 or 400 turns, and watch in hand, find how long it takes to run down. [Illustration: Diagram Showing How To Make A Propeller From A Wooden Blank]