Right, class! Please pay special attention! (Steam enthusiasts among you can look away now, or prepare to e-mail MFUK about any howlers I commit. Or both.) THIS (see picture) is a triple expansion, steam reciprocating engine. The kind of engine that built the nineteenth century empire. Shieldhall‘s steam engine system was very unusual by the time she was built (1954), because most ships were diesel. Still are. Anyway –“triple expansion” means the steam is used 3 times – squashed into a small, then a larger, then the largest of the round cylinders whose tops you can see. The pressure of the steam drives the engine. Then it’s recycled back as water and heated into steam again. The “reciprocating” bit means that the pistons round the propeller shaft are pushed up and pushed down – reciprocation. Phew. Now I need to lie down!