_Various._—We have described about thirty different sorts of motors, selected from the best-known and most original types. About one hundred other motors exist in Europe, which are similar in one way or another to those already described, such as the engines of Dürkopp, Forward, Brouhot, Debry de Soissons, Narjot, Archat, Wertenbruch, the “Acmé” motor, and many others. But we are obliged to limit our descriptions, and to conclude the chapter by a couple of examples of motors performing one cycle of operations per three revolutions. _Griffin motor._—In this engine we have only two explosions over three revolutions, but as it is double-acting this number is reduced to one explosion per revolution and a half. The different operations are as follows:—(1) gases drawn into the cylinder, (2) compression of gases, (3) ignition and expansion, (4) products of combustion driven out of the cylinder, (5) a volume drawn into it to completely sweep away any residue of the exhaust gases, (6) this volume of air drawn out. Admission and ignition are obtained by the action of a sliding valve and eccentric. The governor causes the gas admission valve to remain open for a shorter or longer time, so as to ensure constancy of speed. The exhaust gases escape by two valves actuated by a pair of cams, opening them at every turn and a half, so that the gases are alternately discharged from the back and front parts of the cylinder. The consumption of fuel for a 12 horse-power motor was about 792 litres of coal gas per horse-power hour in an official trial. The speed is very constant in spite of the long cycle. _Rollason gas engine._—This is also an engine using a long cycle of operations, the arrangement of the parts being copied off the Otto motor. The governor is electric, and acts on the admission valve, varying the amount of gas admitted to the cylinder in proportion to the demand for power. In the larger size of from 20 to 100 horse-power a self-starting arrangement has been added. This engine, like the Griffin motor, has proved that it is possible, by completely getting rid of the products of combustion, to use a very dilute explosive mixture, which would be impossible in the Otto motor. The Rollason engine is constructed by Messrs. Beck and Co. of Newcastle-on-Tyne. CHAPTER IV CARBURETTED AIR ENGINE If cold air be saturated with the vapour of volatile spirits such as gasoline, or distillates of petroleum of about ·65 mean specific gravity, an explosive mixture is formed with similar properties to that produced by coal gas mixed with air. This carburetted air can be used as fuel for heat engines of the explosive type. About twenty different methods of carburating air are in existence, some of which are more practical than others. We shall proceed to describe the best known of these.