Up to the medium sizes, the cylinders are cast in one piece, the largest cylinders being made in two parts of cast steel with air furnace iron bushings. The central part of the cylinder is open as will be seen from the cuts, and is covered with a cast iron split band bolted at the center line. The valve chambers are located directly opposite one another on a vertical center line, the inlet valve being at the top and the exhaust valve at the bottom. This arrangement gives a better distribution of the mixture, increases the output with given size of cylinder and equalizes the stresses occasioned by the explosions. As the engine is double acting in all cases there is one inlet and one exhaust at each end of the cylinder. Both the inlet valve and the corresponding exhaust valve on each end of the cylinder are operated by a single eccentric on the horizontal lay-shaft shown running below and parallel to the cylinders. The regulating valves which are controlled by the action of the governor are perfectly balanced against the pressure in the cylinder which results in a very small resistance to the governor action, therefore no oil relay nor similar complications are required. Any of these valves are easily removed for clearing, a point of great importance when running on a gas that is laden with tar or other impurities. [Illustration: Fig. 27. End View of Mesta Engine. ] The chrome-vanadium piston rod carries the pistons floating free from the cylinder walls reducing the wear on the bore, while the piston rings maintain a gas tight contact with the cylinder walls. Each piston rod is made in two halves, the joint between the sections being made between the cylinders at which point the rods are supported by an intermediate cross-head and guide. Both parts of the rod are interchangeable. The pistons are made in one casting. As will be seen from the accompanying cuts the front end of the piston rod is carried by a cross-head which relieves the pressure on the piston and packing glands. Speed regulation is performed by the governor by controlling both the quantity and the quality of the mixture. Independent valves in the gas and air passages are actuated by the governor according to changes in the load. This method of control combines all of the good features of quantity and quality regulation. Make and break ignition is used, with the igniter trip gear so designed as to allow all of the igniters to be timed from one lever, or adjusted independently as the case may require. Each combustion chamber is supplied with two igniters, one at the top and one at the bottom, which insures regular and rapid combustion and therefore gives a maximum of efficiency and reliability.