When the steam after being utilized in the cylinder makes its exit directly to the open air, the engine is called _single expansion_ for the reason that the action of the steam takes place in one cylinder during a single stroke, and what expansion takes place must be during one half of a revolution. When the steam from one cylinder instead of exhausting into the open air, is passed to a second cylinder, of larger area, and by expanding exerts a pressure on a second piston to aid in the completion of the revolution, the engine is called _double expansion or compound_, because the steam instead of completing its work in a single operation is afforded a double opportunity for expansion and an increased range of action. _In the single cylinder the temperature of the walls is reduced in each revolution to correspond with that of the steam at the exhaust pressure._ This temperature must be restored by incoming steam at the beginning of a new stroke which means a reduction of power. With a double cylinder owing to the greater range of expansion, a higher temperature can be maintained in the first cylinder and a large amount of initial condensation is prevented. _A still greater use of expansion may be obtained by the introduction of a condenser_ which allows the final exhaust to be carried below the atmospheric pressure to the extent of the vacuum formed. In stationary and marine practice triple and quadruple expansion engines are common. These are used in large units to give the greatest possible economy in fuel. _Properties of Steam._—Before taking up in detail the valve and other mechanism of the steam pump it may not be out of place to consider briefly the action of steam and its expansive properties. _Heat is identical with mechanical force and the one can be converted into the other._ Aside from the means used in converting or developing the action _a certain quantity of heat always produces a certain quantity of work_. [Illustration: FIG. 246. Relative Volume of Steam at 200 pounds Pressure from One Cubic Inch of Water Relative Volume of Steam at Atmospheric Pressure from One Cubic Inch of Water] The temperature of steam at atmospheric pressure (14.7 lbs. absolute) is 212° Fahr. As the pressure increases the temperature rises, but is always the same for a given pressure. The sensible heat required to raise the temperature of water from 32° to 212° is 180° and the heat absorbed by the water or latent heat at 212° is 996° making the total amount of heat expended 1176°. As the temperature rises the latent heat decreases in nearly the same proportion as the sensible heat increases. This number may therefore be taken as a constant to express the unit of heat in one pound of steam from 32° up to the temperature at which evaporation takes place. Then 1176 × 772 = 907,872 pounds raised one foot which represents the mechanical equivalent or maximum theoretical duty of the quantity of heat contained in one pound of steam.