"(c) Under the impulse of the explosion, the piston shoots upward nearly to the top of the cylinder, the pressure in which falls by expansion to about 4 lbs. absolute, while absorbing the energy of the piston. "(d) The piston descends by its own weight and the atmospheric pressure, and in doing so causes a roller-clutch on a spur-wheel gearing with a rack on the piston-rod to engage, so that the fly-wheel shaft shall be driven by the piston; during this down-stroke the pressure increases from 4 lbs. absolute to that of the atmosphere, and averages 7 lbs. per square inch effective throughout the stroke. "(e) When the piston is near the bottom of the cylinder, the pressure rises above atmospheric, and the stroke is completed by the weight of the piston and rack, and the products of combustion are expelled. "(f) The fly-wheel now continues running freely till its speed, as determined by a centrifugal governor, falls below a certain limit when a trip gear causes the piston to be lifted the short distance required to recommence the cycle. "Ignition is performed by an external gas jet, near a pocket in the slide valve by which the charge is admitted; this pocket carries flame to the charge, thus igniting it without allowing any escape. The valve also connects the interior of the cylinder with the exhaust pipe, and a valve in the latter controlled by the governor throttles the discharge, and so defers the next stroke until the speed has fallen below normal. To run the engine empty about four explosions per minute are necessary, and at full power 30 to 35 are made, so that about 28 explosions per minute are available for useful work under the control of the governor." The definitive improvement in this gas engine was introduced in 1876 by Dr. N. A. Otto, when he compressed the explosive mixture in the working cylinder before igniting it. This expedient--so all-important in its results--had been suggested by William Barnett in 1838, but at that time gas engines were not sufficiently developed to make use of the idea. Now, however, Dr. Otto demonstrated that by compressing the gas before exploding it a much more diluted mixture can be fired, and that this gives a quieter explosion, and a more sustained pressure during the working stroke, while as the engine runs at a high speed the fly-wheel action is generally sufficient to correct the fluctuations arising from there being but one explosion for four strokes of the piston. In this perfected engine, then, the method of operation is as follows: