In naval warfare the battle-ship is the biggest and heaviest ship of the fleet, but in the air the battle-planes are the lightest and the smallest of the lot. They are one-man machines, as a rule, little fellows, but enormously speedy. Speed is such an important factor in aërial warfare that there was a continuous struggle between the opposing forces to produce the faster machine. Airplanes were constantly growing speedier, until a speed of 150 miles per hour was not an uncommon rate of travel. It is hard to imagine such a speed as that, but we may gain some idea if we consider a falling object. The observation platform of the Woolworth Building, in New York, is about 750 feet above the ground. If you should drop an object from this platform you would start it on a journey that would grow increasingly speedy, particularly as it neared the ground. By the time it had dropped from the sixtieth story to the fifty-ninth it would have attained a speed of nearly 20 miles per hour. (We are not making any allowances for the resistance of the air and what it would do to check the speed.) As it passed the fiftieth story it would be traveling as fast as an express-train, or 60 miles per hour. It would finally reach the ground with a speed equal to that of a fast battle-plane--150 miles per hour. The battle-plane was usually fitted with a single machine-gun that was fixed to the airplane, so that it was brought to bear on the target by aiming the entire machine. In this the plane was something like a submarine, which must point its bow at its intended victim in order to aim its torpedo. The operator of the battle-plane simply drove his machine at the enemy and touched a button on his steering-lever to start his machine-gun going. SHOOTING THROUGH THE PROPELLER Now, the fleetest machines and the most easily manoeuvered are those of the tractor type, that is, the ones which have the propeller in front; but having the propeller in front is a handicap for a single-seater machine, for the gun has to be fired through the propeller and the bullets are sure to hit the propeller-blades. Nevertheless the French did fire right through the propeller, regardless of whether or not the blades were hit; but at the point where they came in line with the fire of the gun they were armored with steel, so that there was no danger of their being cut by the bullets. It was calculated that not more than one bullet in eighteen would strike the propeller-blade and be deflected from its course, which was a very trifling loss; nevertheless, it was a loss, and on this account a mechanism was devised which would time the operations of the machine-gun so that the shots would come only when the propeller-blades were clear of the line of fire. [Illustration: Machine-Gun mounted to Fire over the Blades of the Propeller] [Illustration: Courtesy of "Scientific American" Mechanism for Firing Between the Blades of the Propeller