cylinder, much of the high-temperature steam is condensed as water droplets onto the metal surfaces, significantly reducing the steam available for expansive work. When the expanding steam reaches low pressure (especially during the exhaust stroke), the previously deposited water droplets that had just been formed within the cylinder/ports now boil away (re-evaporation) and this steam does no further work in the cylinder. There are practical limits on the expansion ratio of a steam engine cylinder, as increasing cylinder surface area tends to exacerbate the cylinder condensation and re-evaporation issues. This negates the theoretical advantages associated with a high ratio of expansion in an individual cylinder. Compound engines A method to lessen the magnitude of energy loss to a very long cylinder was invented in 1804 by British engineer Arthur Woolf, who patented his Woolf high-pressure compound engine in 1805. In the compound engine, high-pressure steam from the boiler expands in a high-pressure (HP) cylinder and then enters one or more subsequent lower-pressure (LP) cylinders. The complete expansion of the steam now occurs across multiple cylinders, with the overall temperature drop within each cylinder reduced considerably. By expanding the steam in steps with smaller temperature range (within each cylinder) the condensation and re-evaporation efficiency issue (described above) is reduced. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine. By staging the expansion in multiple cylinders, variations of torque can be reduced. To derive equal work from lower-pressure cylinder requires a larger cylinder volume as this steam occupies a greater volume. Therefore, the bore, and in rare cases the stroke, are increased in low-pressure cylinders, resulting in larger cylinders. Double-expansion (usually known as compound) engines expanded the steam in two stages. The pairs may be duplicated or the work of the large low-pressure cylinder can be split with one high-pressure cylinder exhausting into one or the other, giving a three-cylinder layout where cylinder and piston diameter are about the same, making the reciprocating masses easier to balance. Two-cylinder compounds can be arranged as: Cross compounds: The cylinders are side by side. Tandem compounds: The cylinders are end to end, driving a common connecting rod Angle compounds: The cylinders are arranged in a V (usually at a 90° angle) and drive a common crank. With two-cylinder compounds used in railway work, the pistons are connected to the cranks as with a two-cylinder simple at 90° out of