The table on page 20, which we have borrowed from M. Witz’s very complete work on gas engines, shows at a glance the cycle of operations in the cylinders of the different types: they are arranged in parallel columns, in order to make it more easy for the reader to compare the operations undergone by the gases before and after their combustion. It is necessary to subdivide the motors of the second group into three, according as the cycle of operations is completed in one, two, or three complete revolutions of the fly-wheel. Perhaps this subdivision is somewhat unnecessary, because the employment of a second cylinder for compressing the gases does not alter the character of the cycle, but we think that it will make the classification clearer if we proceed in this manner. +----------------+----------------+----------------+-----------------+ | Group I. | Group II. | Group III. | Group IV. | | Without | With | Combustion | Atmospheric. | | compression. | compression. | and | | | | | compression. | | +----------------+----------------+----------------+-----------------+ | 1. Explosive | 1. Explosive | 1. Explosive | 1. Explosive | | mixture enters | gases enter | mixture enters | mixture enters | | the cylinder | the cylinder | the cylinder | the cylinder at | | at atmospheric | at atmospheric | at atmospheric | atmospheric | | pressure | pressure | pressure | pressure | +----------------+----------------+----------------+-----------------+ | | 2. Compression | 2. Compression | | | | of the gaseous | of the gaseous | | | | mixture | mixture | | +----------------+----------------+----------------+-----------------+ | 2. Explosion | 3. Explosion at| 3. Combustion | 2. Explosion at | | at constant | constant | at constant | constant volume | | volume | volume | pressure | | +----------------+----------------+----------------+-----------------+ | 3. Expansion | 4. Expansion | | 3. Piston | | of gases in | of gases | | driven back by | | cylinder | | | the pressure of | | | | | the atmosphere | +----------------+----------------+----------------+-----------------+ | 4. Products of | 5. Products | 4. Products of | 4. Products of | | combustion | of combustion | combustion | combustion | | expelled from | expelled from | expelled from | expelled from | | the cylinder | the cylinder | the cylinder | the cylinder | +----------------+----------------+----------------+-----------------+ Group I. Explosion without compression.