The reversing engine
In order to run ahead and astern, a marine triple expansion engine requires a means of reversing. On SS Explorer, the reversing is powered by a Brown’s patent combined steam and hydraulic direct-acting reversing engine. Reversing the engine is achieved by changing the timing of the steam inlet valve on each cylinder. The engine crankshaft has two eccentrics for each cylinder. One eccentric is designed to open the valve to let steam into the cylinder to drive ahead, and the other astern.. This done by Stephenson valve gear which allows dynamic changing of valve opening. For a description of the Stephenson reversing mechanism see here. https://en.wikipedia.org/wiki/Stephenson_valve_gear
The two nearest verticals rods are connected to eccentrics at the lower end and to the HP inlet valve at the upper end. Similar assemblies are attached to the IP and LP cylinders. To go from ahead to astern, these rods are be moved by the expansion links, one of which can be seen at the top right of this photo which is operated by …
The weighbar shaft is the large rather rusty shaft pictured above. The HP, IP and LP expansion links are keyed on to this shaft. Behind the two vertical rods can be seen the steam operating cylinder on the lower part of the reversing engine.
The Brown’s patent combined steam and hydraulic direct-acting reversing engine. As this description would suggest, it can be operated by steam or hydraulically. Hydraulic would be used when no steam is available and is operated by a manual pump on the manoeuvring platform. The hydraulic cylinder is at the top of the assembly with the red valve attached. As mentioned in the previous paragraph, the steam actuating cylinder is underneath and not visible in this photo.