English: A trunk engine, a type of naval engine developed in the mid-19th century, designed to have a profile low enough to fit entirely below the waterline of a ship, thus ensuring its safety from enemy fire.

The accompanying text for this diagram in the original source is as follows:

"Each engine is generally worked by two cylinders. The cylinder A B is laid on its side; and down the centre, passing through both ends of the cylinder, goes a large trunk a b, on which (all in one piece) is cast the piston c d, so that the effective working part of the piston is an annulus or ring. The trunk is fitted steam tight by means of stuffing boxes. The connecting rod is attached to a pin at o, fixed in the middle of the trunk, while the other end engages and works the crank c s, where s is the main shaft."

"C D, the rectangular figure to the right, the condenser, is divided into the condenser proper, the hot well, and the pump barrel. The large pipe E P is called the eduction pipe; its purpose is to bring the exhaust steam into the condenser, where it is condensed at the bottom, after which the double acting pump p delivers the water into the hot well H W."