N the history of the development of prime movers or, in fact, of engineering machines generally, the path of development has rarely been more guided by the hand of physics than in the case of the internal combustion engine. Each important step forward has been undertaken only after an exploration by physical means. In the investigation of the heat processes in the cylinder, of the units going to useful work, and of those lost in cooling and exhaust, of the effect of variable specific heat, dissociation, after- burning, and other factors, many ingenious experiments with original apparatus have been carried out.

In later times when maximum pressures mounted up, heat flow increased and the speed of revolution also, research work in the characteristics of the materials used became important. Excep- tional accuracy in the engine parts themselves was then so necessary that special instruments of precision were called for to secure it.

As development went on, apart from the primary and secondary out-of-balance vibration of reciprocating masses, the high speed high pressure multi-cylinder engines developed dangerous tor- sional vibrations within the range of revolution speed, when useful research work again shewed the way to security. Similarly with each progressive step, difficulties arose, were elucidated, and the laws governing the new phase clearly set out.

At the present time a vast amount of research work is being carried out in connection with the injection and combustion pro— blems of the quick running compression ignition engine. The true determination of the various factors affecting this problem has assumed such importance as to overshadow much of the work done before the War on mixture engines, which has now, perhaps, lost some of its earlier consequence.

The internal combustion engine in its earlier years may be considered just a mechanical contrivance and those engaged in its construction were more concerned in their own designs, with the immediate practical difficulties of burning a gas and air mixture