The motion should preferably be taken from an eccentric on the cam shaft of a single cylinder four cycle engine, or the crank shaft of a single cylinder two cycle or a two cylinder four cycle engine. On a two cylinder four cycle engine, it is sometimes more convenient to drive the igniter from the cam shaft, using a two throw cam to produce the required number of sparks. In this case the shape of the cam should be such as to duplicate the motion of the eccentric. That is, it should start the driving bar slowly from its lower position, move it most rapidly at mid stroke and bring it to rest gradually at the upper end of the stroke, exactly as is done by the eccentric motion. When an eccentric is already on the engine the motion may be taken from it to an igniter with a driving bar through a properly proportioned lever that will give the required length of stroke. Where a plunger pump is used on the engine the motion can usually be taken from the pump rod. Where an eccentric has to be provided especially for the igniter, the driving bar is generally used with its roller running on the eccentric. (104) Starting On Magneto Spark. A four-cylinder engine in good condition will come to rest with the pistons approximately midway on the stroke and balanced between the compression of the compressing cylinder and of the power cylinder. When the cylinders of such an engine are charged with a proper mixture, the engine will start by the ignition of the mixture contained in the compressing cylinder, for the pressure produced by the ignited gas will be sufficient to rotate the crankshaft. [Illustration: Fig. 114. Bosch Dual System. ] It is essential, for the ignition system to be so arranged that a spark can be produced at any point in the piston travel, and in this the Bosch dual, duplex and two independent systems are successful. The Bosch dual system, Fig. 114, is part of the equipment of many of the cars and engines marketed, and is composed of two separate and distinct ignition systems, one supplying ignition by direct high-tension magneto, and the other by a battery and high-tension coil. These two systems consist in reality of but two main parts; the dual magneto, incorporating a separate battery timer, and the single unit dual coil with its battery. The sparking current from either battery or magneto is brought to the magneto distributor, so that the only parts used in common are the distributor and the spark plugs; the common use of the latter for both magneto and battery systems is cause for the popularity of the dual system for motors having provision for only one set of plugs.