At the present time ignition by high tension electric spark is almost universal on high speed reciprocating internal combustion engines other than the compression ignition type. The growing demands for greater efficiency, higher specific power output and increasing speed range with the associated high compression pressures has made greater demands on the ignition system. The modern magneto and coil, having progressed from the early hot wire and hot tube ignition, have been developed to a high stage of efficiency, although other systems have been suggested. Improvements in performance have been noted with dual ignition and this system is standard on aircraft engines, although in this case improved performance is only a secondary consideration to safety. With dual ignition, one set of plugs is connected to the first magneto and the other set to the second magneto, the sparks occurring simultaneously at the two plugs. As a result explosion is propagated from two points in the cylinder. The sparking plugs are usually placed as apart as possible and it was Swaine who suggested that better results might be obtained with sparking plugs placed very close together. As far as the author is aware, no experimental work has been carried out along these lines.