previous diagram. After the explanation of the first diagram it is unnecessary to go into the details of this figure, as the lettering and numbering correspond. The sequence in this case is:
| First Cylinder | Second Cylinder |
| Suction | Firing |
| Compression | Exhaust |
| Firing | Suction |
| Exhaust | Compression |
In this case half a revolution occurs between the two working strokes, or, as previously mentioned, a working stroke is obtained every revolution.
WATER CIRCULATION
In the large motors, water is utilised to cool the engine, and it is essential that this water should be made to circulate so that the boiling water in the jacket will be displaced by cooler water, and the former cooled in radiators, before it is again used. There are two systems, viz. natural circulation and forced circulation, which we shall now proceed to describe.
In natural circulation the fact that cold water is heavier than hot water is availed of. A head of water is obtained by fitting a tank above the level of the water-jacketed cylinder, and as the water in the jacket is heated by the explosions, the colder water from the tank flows in, forcing the heated water in the tank to take its place, and thus an automatic circulation is set up. The connecting pipes must be so arranged that they offer every facility for the free circulation of the water, the cold leaving through a pipe at the bottom of the tank and entering at the lowest point of the cylinder, while the hot leaves the top of the cylinder and enters the tank at the top. The circulation, though automatic and certain, is slow, and for this reason requires a larger body of water to produce a given cooling effect than is the case with forced circulation.
