the cylinder through the exhaust valve into the exhaust pipe, the exhaust valve is held open by a mechanical contrivance.

The simplest form of exhaust-valve mechanism is to be found in engines of the De Dion type, as illustrated in fig. 6.

a is the end of the crank-shaft (fly-wheel shaft) on to which the small gear wheel b is fixed. This wheel engages with another wheel c, which revolves on the shaft d. The gear wheel c is double the size of b, consequently c only revolves once for every two revolutions of the crank shaft. In the solid with wheel c is mounted an eccentric cam e, which raises the exhaust valve h against the pressure of the spiral spring g, thus allowing the exploded gases to escape from the cylinder. The mode of working