The compound, the triple and the quadruple expansion engine have largely displaced the sim ple engine of Watt ; the first of these types having been introduced in Watt's time by Horn blower, Woolf and Falk and the second by Kirk about 1874; while the last-mentioned became standard with the rise of steam-pressures to about 15 atmospheres, about 1890. These com plications are mainly the outcome of the en deavor to follow Watt in repressing the waste by cylinder condensation, reducing the propor tion of heat-absorbing surface and the temper ature-head producing flow of heat into the metal of the cylinder. Incidentally, the multiple cylinder engine gives a steadier rotation of the crank-shaft and a smoother action of the steam than the simple engine, and also reduces weight by lessening the maximum load upon the work ing parts, the range of pressure in each cylinder being reduced with this reduction of tempera ture-range.
This steady progression from the days of Watt to the present finally culminated in a re trogression to the simple form of the Hero en gine, the steam-turbine, in which all the compli cation of the Watt-Newcomen engine is done away with and but one moving part performs every essential office, apart from condensation, and yet secures, in its best constructions, the economical results of the whole series of changes distinguishing the 19th century, with the added gain of reduced volume, weight and cost, both initial and operative. The turbine promises thus to provide power with maximum ultimate result in financial efficiency. Mean time, the gas-engine, after a similar period of development, is now rivaling the reciprocating steam-engine in many of its fields. The best steam-engines of both the standard and the gas-engine are now capable of deriving large powers from substantially the same quantity of energy potential in fuel.
The Structure of the Steam-engine differs in detail according to place and purpose. The familiar forms may be thus classed : A primary classification as condensing and non-condensing distinguishes engines by their utilization or non utilization of the vacuum. In the former class, condensation may be effected by sur face or by jet-condensation; this distinction indicating a subordinate method of identi fication of a variation within the type. The usual classifications are based upon the essen tial features of structure, and these are ordi narily as follows: The essential details of these engines are usually the same in all the forms in which the individual piece is found. A rod or a crank, a shaft or a valve, will commonly be found to have assumed a standard form, and the differ ence in engines is largely a difference in group ing. Since Watt, but few advances have been made in real invention, and the progress ob served has been mainly one of refinement and adaptation. Frederick E. Sickels introduced a successful form of "drop cut-off'; Corliss, Greene and others invented improved valve gears embodying the same general principles, and Potter and Allen, and others, successfully established the Thigh-speed)) engine as a motor where rapid rotation of the prime mover facili tated transmission of power, as with electric generators and in rolling mills.
Similarly, the locomotive proposed by a num ber of earlier inventors, particularly by Treve thick, who constructed several, was successfully brought into use by George Stephenson and, to-day, in its many forms and uses, the engine in its essential details and distinguishing feat ures is that of Stephenson, refined and adapted to high and to low speeds, to heavy and to light loads. A very noticeable feature of the later
engines is the forward °truck° or de vised by John B. Jervis, which, by permitting the forward wheels to swivel and the engine to rock upon the truck, accommodates the locomo tive to sharp curves and irregular track.
In marine construction, a similar adaptation of the form and proportions of the engine to the special purpose in view gives rise to the types employed with side-wheel and screw, high powers and low, to the essential requirements in lightness and small bulk of torpedo-boat practice and the needs of transatlantic naviga tion and of that of the rivers of the United States. The substitution of surface condensa tion for condensation by the jet has been com pelled in sea-going ships by the use of high pressure steam and the impracticability of using sea-water in the boilers. The latter forms of engine are thus refinements and adaptations of the earlier.
Meantime, in all directions, the steam-engine has 'come to be utilized in the production of very large powers, and its construction in very large units is found to be very frequently eco nomically desirable. Stationary engines for mills, and especially for large power-stations supplying the energy applied in electric lighting or power distribution for electric railways, are built in sizes ranging from a few hundred horse power up to 13,000 horse power, and sometimes grouped into systems rating as high as 100,000. Marine engines are also constructed in these large sizes and powers, and as high as 70,000 horse power may be used for the latest and largest transatlantic steamers, as in the Mauretania. The locomotive, in the time of Stephenson weighing, in the case of his first successful machines, four to six tons is now built of nearly 120 tons weight and capable of hauling loads of 30,000 tons at good speeds, on level rails. The steam pumping engine of the time of Newcomen and Watt had a capacity of a few hundred thousand gallons per day; it is now furnished in sizes up to 75,000,000 gal lons per day for reciprocating engines and up to 320,000,000 for rotary engines, while its duty has risen from the comparatively insignificant figures of the times of the inventors to 150,000, 000 and even 180,000,000 foot-pounds per hun dred pounds of fuel. The steam-turbine, for all these uses, may now be obtained in as large powers as the reciprocating engine and with substantially the same guaranteed duty. Its relatively high speed of rotation, ranging from 600 to 3,000 in the largest sizes, to 30,000 in the small, and its smooth rotation, make its use distinctively advantageous in electric serv ices and its small weight and volume are pe culiarly helpful to the marine engineer and naval constructor, The Thermodynamics of the Steam-engine, the science of its ideal case, involves the mental principles of Energetics and in lar the laws governing the transformation of energy from the form of heat to that of mechan ical energy and vice-versa. An all-comprehend ing law, of which the laws of Energetics are in fact corollaries, the law of Existence, or of whether matter or force or their resultant, en ergy, and in whatever form, is indestructible by finite power.