Practically the only way of obtaining mechanical effort from an expanding gas is to enclose it in a cylinder (c) fitted with a freely sliding plunger or piston (p) as shown in Fig. 1. Two positions of the piston are shown, one at M indicated by the dotted lines, and one at N indicated by the full lines. It will be assumed that the space between the cylinder head P and the piston at M represents the volume of the gas before it is heated and expanded, and that the volume between O and N represents the volume after heating and expansion have occurred. The vessel B represents a chamber containing air that is periodically heated by the lamp L, and which is connected to the working cylinder C by the pipe O. [Illustration: =Figs. 1–2–3.= Showing Expansion in an External Combustion Engine, the Cycle of Operations in an Internal Combustion Engine, and the Pressure Diagram of the Latter Engine Giving the Pressures at Various Points in the Stroke. ] With the piston at M, the lamp L is lighted and placed under the retort B which results in the immediate expansion of the air in B. The expanded air passes through O into the cylinder, and if sufficient heat is supplied, exerts pressure against the piston since it occupies much more than its original volume. Providing that the friction of the device and the load on the shaft S are low enough the pressure on the piston will, move it to the position N in the direction of the arrow, thus accomplishing mechanical work. The motion of the piston revolves the crank to which it is connected by the rod X from D to E. During the trip from M to N the volume of gas has greatly increased being supplied continuously with heat from the lamp. As a considerable amount of heat has been radiated from the cylinder during the piston travel, and a considerable portion of the mechanical work lost through the friction of the piston on the cylinder walls, and by the crank, not all of the heat units are represented at the crank as mechanical effort.