Some of the aeroplanes have their propeller behind the pilot and some have it in front. The latter, to distinguish them, are called "Tractor" machines, since in their case the propeller pulls them along. Now it is easy to see that a difficulty arises in such cases through the best position for the gun being such that it throws its bullets right on to the propeller. But that has been overcome in a most simple yet ingenious way. The gun is itself operated by the engine with the result that a bullet can only be shot forth during those intervals when neither blade of the propeller is in the way. The propeller is moving so fast that it cannot be seen and the bullets are flying out in a continuous rattle, yet every bullet passes between the blades and not one ever touches. It is easy to see that when an aeroplane is manned by a single man, as is often the case, he must have his hands very full indeed, what with the machine itself and the gun as well. In fact, he often has to leave the machine for a short time to look after itself while he busies himself with the gun. Now there we see a sign of the wonderful work which has been done in the course of but a few years in the perfecting of the aeroplane, the result of a series of improvements in detail which make but a dreary story if related but which make all the difference between the risky, uncertain machine of a few years ago and the safe, reliable machine of to-day. Modern machines are inherently stable. The older ones had the elements of stability in them but they were so crudely proportioned that these inherent qualities did not have a chance to come into play. If one drops a flat card edgewise from a height it seems as if it ought to fall straight down to the ground. Yet we all know from experience that it seldom does anything of the kind. Instead, it assumes a position somewhere near horizontal and then descends in a series of swoops from side to side. There we see the principle at work which, in a well-designed aeroplane, causes inherent stability. The explanation is as follows. The aeroplane is sustained in the air through the upward pressure of the air resisting the downward pull of gravity. That has been fully explained already. Now gravity, as we all know, acts upon every part of a body whether it be an aeroplane or anything else. But for practical purposes, we may regard its action as concentrated at one particular point in that body, called the "centre of gravity." Likewise, the upward pressure of the air acts upon the whole of the under surface of the plane or planes, yet we may regard it as concentrated at a certain point called the "centre of pressure." Further, we all know from experience that a pendulum or other suspended body is only still when its centre of gravity is exactly under the point of suspension. If we move it to either side it will swing back again.