the machine is on the ground and the propellers working. In Continental machines, very small screw propellers are used. These screws have probably been made small because the experimenters have found that they encounter a good deal of friction in the atmosphere, but this is caused by imperfect shape and the rib of steel at the back of the blades. In order to use a small screw, experimenters have been forced to use a very quick-running engine which makes it necessary to have the cylinders very short, so, in order to get the necessary power, they are obliged to use no less than eight cylinders. However, by increasing the diameter of the screw and making it of such a form that very little or no atmospheric skin friction is encountered, a much better and cheaper engine of a totally different type may be employed. There is no reason why more than four cylinders should be used, but the stroke of the piston and diameter of the cylinder should be increased. Doubtless Continental experimenters have an idea that, as the engine cannot be provided with a flywheel, it must have a very large number of cylinders in order to give a steady pull completely around the circle, and thus avoid so-called “dead centers”; but, when we consider the enormously high velocity of the periphery of the screw, and also take into consideration that the momentum is in proportion to the square of the velocity, it is quite obvious that there can be no slowing up between strokes even if only one cylinder should be employed working on the four-cycle principle, in which work is only done one stroke in four. Then, again, I find that the weight of these Continental engines can be greatly reduced, providing that they are made with the same degree of refinement that I employed in building my steam engines. Recently there has been a great deal of discussion in _Engineering_ and other journals regarding the comparative merits of the aeroplane system and the hélicoptère. Some condemn both systems and pin their faith to flapping wings. It has been contended that the screw propeller is extremely wasteful in energy, and that in all Nature neither fish nor bird propels itself by means of a screw. As we do not find a screw in Nature, why then should we employ it in a machine for performing artificial flight?