The mode of operation of one of Otto’s most successful engines is as follows: The large fly wheel is started by hand or other means, and as the piston moves forward it draws into the cylinder a light charge of mixed coal gas and air, and the gas inlet is then cut off. As the piston returns it compresses this mixture. At the moment the down stroke is completed the compressed mixture is ignited, and, expanding, drives the piston before it. In the second return stroke the burnt gases are expelled from the cylinder and the whole made ready to start afresh. Work is actually done in the piston only during one-quarter of the time it is in motion. The fly-wheel carries forward the work at the outset and the gearing the rest of the time. Otto was associated with Langen in producing his first machine, and its introduction at the Centennial Exposition at Philadelphia in 1876 excited great attention. Otto and E. W. and W. J. Crossley jointly, and then Otto singly, subsequently patented notable improvements. Simon Bischof and Clark, Hurd and Clayton in England; Daimler of Deutz on the Rhine, Riker and Wiegand of the United States, and others, have made improvements in the Otto system. Ammoniacal gas engines have been successfully invented. _Aqua ammonia_ is placed in a generator in which it is heated. The heat separates the ammonia gas from the water, and the gas is then used to operate a suitable engine. The exhaust gas is cooled, passed into the previously weakened solution, reabsorbed and returned to the generator. In 1890 Charles Tellier of France patented an ammoniacal engine, also means for utilising solar heat and exhaust steam for the same purpose; and in the same year De Susini, also of France, patented an engine operated by the vapour of ether; A. Nobel, another Frenchman, in 1894, patented a machine for propelling torpedoes and other explosive missiles, and for controlling the course of balloons, the motive power of which is a gas developed in a closed reservoir by the chemical reaction of metallic sodium or potassium in a solution of ammonia. These vapour engines are used for vapour launches, bicycles and automobiles. In 1851 the ideas of Huygens and Papin of two hundred years before were revived by W. M. Storm, who in that year took out a gunpowder engine patent in the United States, in which the air was compressed by the explosions of small charges of gunpowder. About fifteen other patents have been taken out in America since that time for such engines. In some the engines are fed by cartridges which are exploded by pulling a trigger.