of the two-stroke cycle. Clerks engine used compression and a novel system of ignition", one of these was exhibited in July 1879. However it was not until the end of 1880 that he succeeded in producing the Clerk engine operating on the two-stroke cycle, which became the commercial product. Clerk states "The Clerk engine at present in the market was the first to succeed in introducing compression of this type, combined with ignition at every revolution ; many attempts had previously been made by other inventors, including Mr. Otto and the Messrs. Crossley, but all had failed in producing a marketable engine. It is only recently that the Messrs. Crossley have made the Otto engine in its twin form and so succeeded in getting impulse at every turn." Dugald Clerk was the author of three comprehensive books covering the development of the oil and gas engine from its early inception, and including details of his own work in this area. The first edition was produced in 1886, and the notes here are taken from the 7th edition, revised and updated up to 1896. In "Gas and Oil Engines", Clerk refers to the significant earlier gas engine patents of Barnett in 1838 and Wright in 1833. Clerk cycle In 1878 Clerk obtained a Brayton "Ready Motor" engine made from 1872 to 1876 by George Brayton in Philadelphia, Pennsylvania, US. Clerk wondered if he could improve the performance of the engine. He soon outfitted the engine with a spark plug and an improved fuel system. Initially Clerk used one cylinder for compression and the other for expansion. At one point an explosion occurred which broke the engine in two pieces. The engine was repaired and displayed in 1879. Later Clerk decided to abandon use of the pumping cylinder for compression and use it only to transfer the air / fuel mixture into the power cylinder. The Otto cycle was patented in 1876, immediately recognised to have a significant practical value. Clerk quickly followed with his concept of a two-stroke engine of 1880, that would not infringe the Otto's patent (being a four-stroke engine). Clerk describes a Cambell engine as using his cycle, as follows: "It has two cylinders, respectively pump and motor, driven from cranks placed at almost right angles to each other, the pump crank leading. The pump takes in a charge of gas and air, and the motor piston overruns