considerable interest as precursors, not of the first commercial gas engines (e.g. Lenoir engine in 1860, which used the two-stroke cycle without compression, or the Otto and Langen atmospheric engines), but rather of the later generation of engines including Otto's engines operating on the four-stroke cycle and Dugald Clerk's engine operating on the two-stroke cycle, both with compression of the gas/air mixture in the cylinder. Characteristics of the engines Note that all three engines operate on the two-stroke cycle, as did most early engines (the competition was steam engines, which also operate on the two-stroke cycle, and are usually double acting so every stroke is a power stroke). It was a remarkable leap to discover that, for internal combustion engines, the four-stroke cycle with three non-power strokes could deliver both economy and performance, see Alphonse Beau de Rochas who patented the idea in 1861, and Otto who made commercial engines using the four-stroke cycle after his earlier work with atmospheric engines. All three of Barnett's engines were vertical engines with a layout similar to steam table engines. The first engine is single acting using the two-stroke cycle, with separate pumps for gas and air, and a power cylinder. This cylinder is connected directly to a receiver vessel in which the combustion is initiated, the expansion flows into the power cylinder driving the piston. Timing of inlet and exhaust is via a piston valve, and an exhausting pump may be added to remove exhaust gases from the receiver prior to the new charge being forced in by the pumps. This is described as optional for smaller engines, as the exhaust gases are not so much a problem for them. The second engine is a double acting version of the first, i.e. combustion is alternately one side and then the other side of the piston. The operating principles are otherwise the same. The third engine is also double acting using the two-stroke cycle, and although it has the gas and air pumps of the others, it has an exhaust port uncovered by the piston towards the end of the power stroke, allowing the exhaust gases to exit and the fresh charge to enter the power cylinder and then be compressed by the piston prior to ignition at the end of the compression stroke. The displayed image of the third engine reveals a double acting engine in the style of a steam table