All heat engines at present in use take in heat from a source at a high temperature and discharge most of it at a lower temperature, the disappearance of heat in the process being the equivalent of the work done by the engine. In all cases at the present time the source of heat is from fuel of some kind, and after working the engine the residue is discharged in the case of the steam engine either to the condenser or in the exhaust steam when non-condensing. In the gas engine it is discharged in the waste gases and into the water jacket around the cylinder. The earliest records of heat engines are found in the Pneumatics of Hero of Alexandria, about 200 B. C. He describes a reaction steam turbine, a spherical vessel mounted on axes supplied with steam through one of the trunnions from a boiler beneath; the steam escaping through two nozzles diametrically opposite to each other and tangential to the sphere, causing the sphere to rotate by the reaction or momentum of the issuing steam, and analogous to a Barker’s water wheel. Thus, the first engine deriving its motive power from fuel was a crude form of steam turbine, and though it could have been applied to useful work, and could easily have been made sufficiently economical to replace manual and horse power in many instances, yet it lay dormant till 1629 A. D., when Bianca suggested the same principle in a different form. Bianca’s steam turbine consisted simply of a steam jet fed from a boiler impinging against vanes or paddles attached to the rim of a wheel which was blown round by the momentum of the steam issuing from the jet. The piston engine is, however, of comparatively modern origin, and dates from about the year 1700 A. D. Engines of this class are so well known that it suffices to say that they have been practically the sole motive-power engines from fuel in use from 1700 up to 1845, and have constituted one of the most important factors in the development of modern engineering enterprise. Air engines were introduced about the year 1845, and although the larger engines of the Stirling type were very economical in fuel, yet, on account of the inherent difficulty of heating large volumes of air within metal chambers or pipes--a difficulty arising from the low conductibility of air and consequently the overheating and burning of the metal--they have only come into commercial use for very small powers. [Illustration: FIG. 1.--LONGITUDINAL DIAGRAM OF PARSONS’S STEAM TURBINE, WITH SIDE OF CONTAINING CASE REMOVED. The construction of the blades and guide vanes is more clearly shown in Fig. 2. The steam enters at _J_ and exhausts after leaving the low-pressure cylinder _C_.]