The fourth subdivision of steam engines ac cording to the method of using the steam com prises condensing and non-condensing engines. In a non-condensing engine the waste steam from the cylinder is exhausted into the air at air pressure, or 14.7 pounds per square inch. In the condens ing engine it exhausts into a vacuum and is con densed into water. Thus, in a non-condensing engine the steam has to force the piston against a back pressure of 14.7 pounds per square inch, while in a condensing engine this hack pressure is obliterated, with a consequent gain of 14.7 pounds per square inch in the effective working pressure of the steam. The practical advantage of the condensing engine may, therefore, be ex pressed by saying that it will receive the same power with a smaller cylinder and greater power with the same cylinder, as compared with a non condensing engine. (See CONDENSER.) A fifth subdivision of steam engines according to the method in which steam is used comprises simple engines, compound engines. and multiple expan sion engines. In a simple engine the steam, after having done its work in forcing the piston through its stroke, is exhausted into the air or into a condenser. In a compound or multiple-expansion engine the steam, after having done its work in one cylinder, is exhausted into a succeeding cylin der of larger size, where it continues to perform work. The operation may be explained by the diagram Fig. 5. Steam enters the cylinder HP, which is connected with cylinder LP by two passages a and b closed by valves, and here per forms the work of driving piston P, to the front end of the cylinder. This steam, instead of being exhausted into the air or into a condenser to allow the return stroke of the piston is ex hausted through the passage a into cylinder 1.1', where it acts to drive forward piston Were the size of the cylinder LP the same as that of the cylinder HP, it is evident that the propulsive effect of the steam on piston P, would be exactly counterbalanced by the back pressure on piston and, therefore, that there would be no in crease, but rather a loss, in the work done. If,
however, cylinder LP is made twice the diameter of cylinder HP, then the forward pressure on piston P, is greater than the back pressure on piston P, and a position or working effect is ob tained by the second use of the steam. A two cylinder engine like Fig. 5 using steam as de scribed is called a compound engine. Were the exhaust steam from the second cylinder to enter a third and larger cylinder and continue working we should have a triple expansion engine. If a fourth cylinder were added to the set we should have a quadruple expansion engine. The expan sive working of steam has not been successfully carried beyond four expansions. The cylinders of compound engines are generally arranged parallel to each other or side by side, or else one behind the other, or tandem. A tandem com pound with the cylinders set vertical is called a steeple engine. When the cylinders are placed side by side, but some distance apart. so as to allow space for a fly wheel between them, the engine is commonly called a cross-eompound en gine. Modern marine engines for screw vessels are nearly always inverted vertical compound or multiple expansion engines. (See Plates un der STEAM NAVIGATION.) The arrangement. of the cylinders of triple and quadruple expansion engines varies considerably. The great practical advantage of the compound or multiple expansion engine is the economy resulting from the expan sive use of steam, as indicated in discussing ex pansive and non-expansive working engines. It is obviously possible to use the steam according to any of the methods discussed with any of the arrangements of mechanism previously dis cussed and thus make a great number of com binations.