This experimental engine, which was merely a “patched-up” affair, was first equipped with a sparking arrangement built on the wiping-contact principle. With this sparking arrangement several important difficulties presented themselves, among which may be particularly mentioned the great difficulty of so adjusting the sparking arrangements that the explosion in each cylinder occurred at exactly the same point in its cycle that the explosions occurred in all the other cylinders, it being necessary to secure this result to a reasonably accurate degree in order to cause the engine to run smoothly enough to be used in the aerodrome. Where an engine has a large and heavy fly-wheel running at a high rate of speed, the nicety of adjustment of the sparking arrangement is not so essential, for the fly-wheel acts as a reservoir of energy and tends to smooth out the rough and jerky impulses which would be otherwise introduced by slight variations in the force of the explosions in the cylinders. In constructing an engine for an aerodrome, however, the permissible weight of the engine is so very small that the use of a fly-wheel having sufficient weight to act as an energy reservoir is practically prohibited. Another serious difficulty which was encountered with the wiping-contact type of sparking arrangement was that of keeping the stuffing boxes around the rotating contact rods tight enough to prevent leakage, without at the same time binding and causing excessive friction. Although it seemed probable that the difficulties which have been mentioned, and other minor ones which were apparent, could be remedied by further experiment, yet the high tension or “jump-spark” type of sparking apparatus seemed to offer much greater advantages. Since it had fewer moving parts, and furthermore since the wiping-contact sparking arrangement would have to be considerably modified in order to permit the construction of water jackets around the cylinders, it was decided to construct a [p221] new sparking arrangement for the engine on the jump-spark principle. After introducing this change in the engine it was found to run very much more smoothly and to require a minimum amount of care in adjusting it.