measured by imperfect vacua to maxima determined by the final temperatures of the steam produced. At the freezing point, this pressure is about 0.006 atmosphere; at the boiling point at the level of the sea, under one atmosphere pressure, the temperature becomes 212° F., 100° C. At pressures employed in the modern steamengine, 6 to 10 and 15 atmospheres, the temperatures rise to from 320° to 356° and 390° F., 160° to 180° and 199° C. Meantime, the volumes of the vapor decrease, relatively to unit volume of water of maximum density, from 1646 at one atmosphere to 300 at 6, to 188 at 10, and to 125 at 15 atmospheres. This change demands the expenditure of energy sufficient to increase the rate of molecular vibration, storing the sensible heat producing the change of temperature and measured by the product of the range of temperature into the specific heat of the fluid, and an amount of energy measured by the product of the change of volume into the external and internal resistances to that expansion, measuring the external and internal, so-called "latent» heats. Sensible and total heats are usually measured from the freezing point. At ten atmospheres, for example, the heat measured, respectively, as sensible, as internal latent and as

Newcomen's Engine (1705).-B, boiler; a, steamcylinder; sr, piston and rod; K, pump rod. external latent and as total latent heats, have the relation, very nearly, of one to two and a half, to one fourth, to three and three fourths.

In the production of steam from water at temperatures below the boiling point, three stages may be observed. In the first, the water rises in temperature without sensible change of volume, and substantially all of the heat supplied remains in the form of sensible heat; in the second, the process is one of conversion of the water at the boiling point under the observed maximum pressure, from the liquid to the vaporous state at unchanging temperature, and all heat supplied is converted into the mechanical work of expanding the fluid against internal and external resistances from the volume of the liquid to that of the vapor; in the third, heat added produces "superheat" in raising the temperature above that of the water and steam at the temperature of saturation, converting the vapor into a gas and performing work of expansion if the volume is permitted to increase,

or simply raising temperature, and without change from the form of sensible heat, if at constant volume.

The total heat is, in all cases, the sum of that supplied in enlarging the stock of sensible heat and that furnished to perform the work of expansion and thus becoming "latent." Latent heats have the measures: 11091.7-0.695 (t-32)--0.000000103 (t-39.1); m = 606.5

0.695 tm-0.000000333 (tm-4)3; for British and metric measures respectively. The last term may usually be omitted. Total heats have the values, from the freezing point, h=1091.7 +0.305 (t-32) hm = 606.5 +0.305tm, in the two systems of measurement, respectively. The equivalents of these quantities of heat measure the amounts of mechanical energy expended in steam-making. Superheated steam has a specific heat at customary pressures of 0.4805, the pressure being constant, as is usual in superheating, and this quantity is added with each degree rise in temperature above that of saturation at the same pressure.

In all cases, the heat and the equivalent energy required are measured by the sum of that needed to produce the observed change of temperature and that required to perform the work of expansion against internal and external resistances as measured by the molecular cohesion and the pressure on the confining walls of the chamber in which the process takes place, whether the steam be saturated, moist, or superheated.

Algebraically, H=H1+H2+ pdv; where H is the total heat, H1 that present at the initiation of the change observed, H. that required to increase temperature and p and v the mean pressure and resultant change of volume; all energy being here measured in dynamic terms, foot-pounds or kilogrammetres.

Where the steam is wet, the heat and energy demanded in such changes are measured by the sum of that absorbed by the water present and that taken up by the steam. If r be the proportion of steam in the mixture, the latent heat becomes, per unit weight of mixture lx=xl; the total heat will be hx=h+xl; and the total volume will be very nearly rv, that of the fraction of steam present, v, h and I being the specific volume, total heat of water and latent heat of steam, per unit of weight, at the observed temperature.

For superheated steam, pv=aT; p, v and T being respectively, the pressure, specific volume and absolute temperature.

The heat stored in steam and available in the production of work by expansion, as in a steam-boiler explosion, was first computed_by Airy, later more accurately by Rankine. The latter gave approximate expressions thus: J(T-212)2 J(T-100)2

; Um =

U = T+1134.4 T+648 for British and metric measures, respectively; energy being expressed in foot-pounds and kilogrammetres and temperatures in Fahrenheit and centigrade. J is the mechanical equivalent of heat, in foot-pounds or in kilogrammetres.

The quantity of this stored energy is thus found to be enormous. At 10 atmospheres pressure, the energy thus liberated by one pound of water released from under that pressure would be above 10,000 foot-pounds, and one

pound of steam would give over 125,000 footpounds. The total energy stored in the steamboiler is contained in a large weight of water and a comparatively insignificant quantity of steam; thus it happens that the danger to life and property when a boiler explodes is greatest where the boiler contains most water. In the common cylindrical fire-tube boiler, of 1,000 square feet heating surface, these quantities may be, respectively, 60,000,000 foot-pounds and 1,200,000, sufficient to raise the boiler itself a mile high, in the one case, and about 1,000 feet in the other. The locomotive often stores twice these amounts of energy in destructive form, and the larger water-tube boiler about two thirds as much as the standard fire-tube boiler, per unit of rated power.

At usual pressures, the quantity of heat stored in steam, available and unavailable, is about the equivalent of two and a quarter pounds per horse-power-hour, or very nearly a kilogram. This would be the consumption of steam by an ideally perfect engine, operating with an efficiency of unity. The most economical steamengines the world has produced approximate 25 per cent thermodynamic efficiency and demand about 10 pounds of steam per horse-power-hour, or nearly five kilograms.

Steam-engines and Boilers constitute the apparatus by means of which the stored heatenergy of fuel, transferred to water and steam, is transformed into mechanical work. This transformation of thermal into dynamic energy, this thermodynamic change, requires for its successful and economical conduct special forms of mechanism and is subject to a variety of wastes of serious aggregate amount, even with the most perfect of modern engines. The series of processes in the train between the fuel and the point of application of the useful energy with statement of the corresponding wastes and efficiencies are as follow; it being understood that an efficiency is the quotient of useful result divided by outgo producing it, the two being expressed in similar terms: These efficiencies are those of

1. Combustion of fuel; ratio of heat set free to total heat latent in the fuel. This efficiency is usually not far from 0.90. Wastes due to incomplete combustion.

2. Heat-transfer from furnace to boiler; efficiency, as a rule, about 0.75, as measured by heat stored in the steam supplied. Wastes occurring mainly at the chimney.

3. Heat-transfer from boiler to engine with loss by conduction and radiation, en route. Efficiency of operation about 0.90 in small boilers and increasing to 0.95 or 0.98 in large sizes.

4. Heat transformation into work at the engine with wastes by defective thermodynamic change and rejec tion of heat at the lower limit by conduction and radiation within and without the cylinder, variable with size, with mean temperature of steam and other conditions. Efficiencies for the ideal case usually approximate 0.25 with only thermodynamic wastes, and attain to 0.20 with successful constructions in the real case; the wastes including the thermodynamic and in evitable losses and the partly controllable extra-thermodynamic wastes.

5. The transfer of mechanical energy from cylinder to point of application. The wastes occur by friction and usually amount to about 0.10, as a minimum in condensing, and to 0.05 in non-condensing engines.

Efficiency, 0.90 to 0.95.

The thermodynamic efficiency of the best steam-engines may be thus taken to be 0.25; the thermal efficiency at the engine, involving other wastes than thermodynamic, about 0.20; the total efficiency between steam-valve and fly

wheel about 0.18 and the efficiency of engine and boiler combined not far from 0.14; while the total efficiency of engine, boiler, and furnace, from coal-pile to engine-belt, may be about 0.125. In common constructions these efficiencies are much reduced and in many cases may be divided, by from two to four, the demand for fuel of good quality ranging from about one pound or half a kilogram in the best work to several times that amount per horse-power-hour, and for steam from ten pounds, about four and a half kilograms, to a multiple of that quantity. In some instances, as with many small boiler feed-pumps, 10 or even 20 times the minimum figures just given are reached, the wastes becoming enormous and the utilized energy of the fuel insignificant.

The "ideal case" is understood to be that purely thermodynamic operation which illustrates the conversion of thermal into dynamic energy where no other energies than thermal and

Watt's Engine (1774).-a, cylinder; b, piston; x, rod; y, beam; ij, air-pump and condenser; m, valve-gear. dynamic are concerned, and where the change is effected in a machine which is not subject to wastes by conduction or radiation; an apparatus composed of perfectly non-conducting materials and perfectly constructed. In the real case," the materials of construction are necessarily good conductors and good radiators of heat, and the wastes by conduction and radiation are often supplemented by leakage of steam as well, as of heat. In the real case, the details of construction, adjustment and operation affect very greatly the resultant efficiency and the commercial rating of the engine. The study of the steam-engine thus comprehends the ideal, the purely thermodynamic, case and the real case with its various wastes, thermodynamic and extra-thermodynamic, as well as an investigation of the principles and practice in the design and construction of the real engine.

Engines.- The power of steam and the employment of that fluid in various sorts of en

gines have been familiar to mankind from an unknown and possibly prehistoric period. The earliest known record is that of Hero, who, in his Pneumatica,' of which the manuscript was produced at Alexandria, about 120 B.C., described a steam-turbine and several forms of steam-fountains and steam-boilers. So far as known, none of them had any useful application and they were simply toys or impracticable schemes. It is unknown, in fact, whether any

Watt's Double-acting Engine (1784).-C, cylinder; b,

beam; O, connecting rod; Qs, governor and valve. of them were constructed; although the drawings appear in some cases to be those of actual constructions.

Through the later centuries, up to the 17th, but little progress was made either in the acquirement of a knowledge of the properties of steam or in its application to useful purposes. Some forms of "eolipile," furnished a steamjet for improving the draft of the chimney, apparatus for turning the spit and even more ambitious uses were either attempted or suggested; but, until Da Porta's treatise on pneumatics appeared in 1601, in which a steam-fountain was described, and the description in 1629 of an impulse steam-turbine, by Branca, no development took place of any real importance. It was not until the second Marquis of Worcester, Edward Somerset, constructed a steam-fountain (1650) and employed it in raising water from the moat to the top of the tower of Raglan Castle, and later erected another for similar purposes at Vauxhall, that the story of the evolution of the steam-engine really begins. Meantime the scientific men of the later centuries were acquiring some exact knowledge of the nature of steam, earlier confounded with other gases, and some familiarity with its latent powers.

Steam power first became an acknowledged industrial agent and useful as a prime mover when Savery, at the beginning of the 18th century (1698), made Worcester's steam-fountain practically applicable to the drainage of mines and the elevation of water for watersupply generally. This apparatus, which could not be properly called an engine, consisted of a pair of cylindrical or ellipsoidal "forcing vessels"

which were alternately filled with water, by the production of a vacuum within the vessel, and emptied by the introduction of high-pressure steam from an adjacent boiler; the one being emptied while the other was filling and vice versa. This apparatus, introduced by Savery, improved and further made known by Desaguliers and by Smeaton, was known and in use before the year 1775 throughout the world where mining at considerable depths and in presence of water was carried on. The steam-fountain. is still in use and is known as the "pulsometer."

Newcomen's steam-engine, the first steamengine properly so termed, the first which consisted of a train of mechanism as distinguished from the Hero steam-fountain, which was a piece of apparatus without moving parts, was patented in 1705. It consisted of a steam-cylinder and piston, actuating a beam, above, from the opposite end of which was pendant the pump-rod operating the pumps in the shaft of the mine; it was always used as a steam pumping engine. Thomas Newcomen and his partner, John Calley, are thus to be credited with the invention and introduction of the modern steam-engine with all its essential elements as a pumping engine. It was the improvement of this engine by the addition of various valuable devices which gave James Watt his fame and fortune.

This earliest type was a condensing engine in which condensation was effected by means of a jet of water directed into the steam cylinder when the pressure on the under side of the piston was to be removed. The upper side was open to the air, there being no upper cylinderhead. The engine was thus operated by the atmospheric pressure, steam being held at about atmospheric pressure and only employed to secure a vacuum below the piston. The pressure of the atmosphere depressing the piston, the pump-rod on the opposite end of the beam was raised and the pump filled. With the fall of the pump-rod the water was forced out of the pump and raised to the upper level. The weight on the outer end of the beam always overbalanced the weight of the piston and attachments suffi

ciently to do the required work. This type of engine remained in use for a century, and old engines of Newcomen's time are still in existence. The type became known, later, as the Cornish engine, Watt's improvements having been meantime added. After Newcomen's death, the machine was improved in details by Desaguliers and by Smeaton, who considerably increased its economy by attaching wood to the piston and cylinder-head to prevent what has been called "cylinder condensation" by action of alternately heated and cooled metal in contact with the steam. This was probably the first rec

ognition in construction of this important phe

nomenon.

The valves of this engine were at first worked by hand; but a boy, Humphrey Potter, is credited with having devised an automatic system, which, later in 1718, carefully designed and constructed in a workmanlike manner by Henry Beighton, a well-known engineer of that period, became the first automatic valve-motion.

James Watt, introducing the needed improvements in the Newcomen engine, finally produced the modern types of "reciprocating" steam-engine. His first great improvement was the separate condenser, which permitted condensation to be effected without the introduction of water into the working cylinder and thus reduced very greatly the waste of steam by initial condensation. Watt first enunciated the principle: "Keep the cylinder, if possible, as hot as the steam that enters it." The first step was this of removing the primary cause of refrigeration. The next was to surround the cylinder with a chamber containing steam at boiler pressure; thus introducing his second great invention, the "steam-jacket." He next covered the upper end of the cylinder, excluding the cold air and supplying the place of the atmosphere and its pressure on the upper side of the piston by steam from the boiler, completing his scheme of keeping the cylinder as far as was practicable as hot as the entering steam.

The "double-acting engine" constituted the next and an easy step. With steam admitted at both ends of the cylinder, it was immediately evident that each might be utilized, alternately, in the performance of work and Watt soon adjusted his valve-gear and connections in such manner as to permit this alternation and produce a push and a pull on the piston-rod. This compelled a rigid connection between the piston and overhead beam, on the one end, and between the outer end of the beam and its work, now become that of rotating a shaft with crank and fly-wheel. Thus one improvement led to another and Watt's steam-engine ultimately became capable of supplying power to every imaginable kind of machine or work. The singleacting engine was, for many years after Watt's death, used in raising water and the doubleacting engine continues to turn the shaft of mill, locomotive, steamship and factory.

Watt invented and introduced many accessory inventions and devices, as the attachment of the governor - already a well-known apparatus the steam-engine "indicator," the expansion of steam, the compound engine, the non-condensing engine, practically all that distinguishes the modern engine from that of Newcomen. These improvements raised the "duty" of the pumping engine, in the course of 25 years, from about 7,000,000 foot-pounds to 30,000,000, and, in the latest forms of Cornish engines, about 1850, to twice the last figure or more, reducing cost of steam-power enormously, and at the same time adapting the steam-engine to every requirement in the industries, giving to the world, in fact, its contemporary civilization, This cost in coal per horse-power-hour is reduced from the 35 pounds of Smeaton's time to one pound, as a minimum to-day, and the work of the world is performed by steam-engines, mainly, probably amounting to 150,000,000 horsepower and equivalent to the working power of

several times the population of the globe, if employed in manual labor.

At the commencement of the 19th century, Trevethick and other able mechanics and inventors were seeking to construct locomotives, and complete success was achieved by George Stephenson in engines built from 1814 to 1833. The steamboat had been suggested by numerous writers and engineers, and, after many attempts, was made a practical success by John Fitch in the United States about 1785, by John Stevens in 1804-9 and commercially by Fulton, 1807-15. In Great Britain, after many early failures, Miller and Symmington and Bell, step by step, attained permanent success and by 1830, the date of the first transatlantic steamship voyages, those of the Cirius and the Great Western, all civilized countries were employing the steamboat. See STEAM VESSELS.

Meanwhile the elements of economy became recognized and steam-pressures rose from the two to seven pounds above the atmosphere of Watt's time to 25 or 30, about 1850, and to 100 and upward to occasionally 200 at the end of the 19th century; the ratio of expansion of the steam increasing in similar ratio. The speeds of engine-piston also gradually increased from about 100 feet per minute, at the beginning, to

Stephenson's Engine (1825).

600 and often to 1,000 at its end. The weights of engine and sizes for the usual powers meantime fell from 1,000 pounds or more per horsepower developed at the time of Watt, 500 about 1850 and to 250 in 1900 where weights were comparatively unimportant and, in special cases, where weight and volume required to be reduced to the smallest possible figures, as for torpedoboats, to a fourth or a fifth, the last named quantity; while, in aeronautic work, ten pounds per actual horse-power has been reached and still lower figures are considered probable in the near future.

The compound, the triple and the quadruple expansion engine have largely displaced the simple engine of Watt; the first of these types having been introduced in Watt's time by Hornblower, 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 complications are mainly the outcome of the endeavor to follow Watt in repressing the waste by cylinder condensation, reducing the proportion of heat-absorbing surface and the temperature-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 working 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 end of the 19th century finally culminated in a retrogression to the simple form of the Hero engine, the steam-turbine, in which all the complication 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. Meantime, 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 types 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 nonutilization of the vacuum. In the former class, condensation may be effected by surface or by jet-condensation; this distinction indicating a subordinate method of identification of a variation within the type. The usual classifications are based upon the essential features of structure, and these are ordinarily as follows:

1. According to the number of cylinders.
(1) Single cylinder, simple engines.
(2) Multiple-cylinder engines,

compound," etc. 2. With reference to the construction of cylinders: (1) Fixed cylinder.

(2) Movable cylinder.

In the first case, the engines are:

(a) Vertical.

(b) Horizontal.

(c) Inclined.

In the second case, they are:

(a) Oscillating, vibrating, etc.

(b) Rotary, steam-turbines.

3. With reference to the action of the steam:

(1) Single-acting.

(2) Double-acting.

4. With reference to the transmission of the steampower:

(1) Direct-acting.

(2) Indirect-acting.

And in the latter case either

(a) With balance lever, or beam.
(b) Without lever or working beam,
geared, etc.

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 differences in engines is largely a difference in grouping. Since Watt, but few advances have been made in real invention, and the progress observed 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 Porter and Al

len, and others, successfully established the "high-speed" engine as a motor where rapid rotation of the prime mover facilitated transmission of power, as with electric generators and in rolling mills.

Similarly, the locomotive proposed by a number of earlier inventors, particularly by Trevethick, 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 features 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 "bogie," devised by John B. Jervis, which, by permitting the forward wheels to swivel and the engine to rock upon the truck, accommodates the locomotive 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 navigation and of that of the rivers of the United States. The substitution of surface condensation for condensation by the jet has been compelled in seagoing ships by the use of high-pressure steam and the impracticability of using sea-water in the boilers. The later 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 economically 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 to five and even ten thousand horsepower, 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 50,000 horse-power may be needed for the latest and largest transatlantic steamers. The locomotive, in the time of Stephenson weighing, in the case of his first successful machines, four to six tons is now built of above 100 tons weight and capable of hauling loads of 5,000 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 20,000,000 and 30,000,000; while its duty has risen from the comparatively insignificant figures of the times of the inventors to 150,000,000 and 160,000,000 foot-pounds per hundred 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 1,000 in the largest sizes, to 10,000 or more in the small, and its smooth rotation, make its use distinctively advantageous in electric services and its small weight and volume are peculiarly helpful to the marine engineer and naval constructor.

The Thermodynamics of the Steam-engine, the science of its ideal case, involves the funda