Since the power exerted by a single cylinder four stroke cycle engine is intermittent, the explosive force exerted on each power stroke is much heavier than would be the case if the power application were continuous, as the explosions must be heavier to compensate for the idle periods. To reduce the strain on the engine and the vibration as well and to obtain an even turning moment it has been customary to provide more than one cylinder on engine of over 10 horse-power capacity. In this way the total power is divided among a number of cylinders, and as no two cylinders are under ignition at any one time the turning moment is more even, the vibration is less, and the strain on the engine is considerably reduced. Dividing the power in this way makes it possible to reduce the weight of the engine as less material is required to resist the strains and a small fly-wheel may be used because of the even engine torque. In order to gain the full benefit of this reduction in weight, the builders of aeronautic motors have carried the multiplication of cylinders to an extreme, the Antoinette for example having sixteen cylinders. Engines having more than six cylinders exert a continuous pull as the impulses “overlap,” that is, ignition occurs in one cylinder before another cylinder in the series ends its working stroke. The greater the number of cylinders, the more continuous will be the torque or turning moment. The multiple cylinder engine may be considered as a group of single cylinder engines connected together, and receiving their fuel from a common source, the only difference between the single and multiple being in the inlet and exhaust piping and the ignition system. [Illustration: Fig. F-12. Six Cylinder Maximotor. ] [Illustration: Fig. F-13. Four Cylinder Buffalo Motor for Marine Service. ] As a single cylinder four stroke cycle engine has one working impulse in every two revolutions, a two cylinder engine will have an impulse for every revolution as there are twice as many impulses in the same time. It should be remembered that the number of impulses given per revolution by a four stroke cycle engine is equal to the number of cylinders divided by two. Thus, a six cylinder engine has 6 ÷ 2 = 3 impulses per revolution, and an eight cylinder, 8 ÷ 2 = 4 impulses, providing of course, that the engine is single acting.