STEAM. 521 STEAM ENGINE. following are the principal items that are found in the tables: ( 1 ) The absolute pressure in pounds per square inch ; it is equal to the gauge pressure plus the atmospheric pressure of 14.7 pounds. (2) The temperature of the steam, or boiling water, at the corresponding pressure. (3) The heat of the liquid; or the nund)er of B. T. U. necessary to raise one pound of water from 32° F. to the boiling point corresponding to the given pressure. (4) The heat of vaporization, or the latent heat ; this is the number of B. T. U. necessary to change one pound of water, at the boiling point, into dry satur.ated steam at the same tem- perature and pressure. ( 5 ) The total heat ; or the number of B. T. U. necessarj' to change one pound of water from 32° F. into steam at the given temperature or pres- sure. The total heat is evidently equal to the sum of the heat of the liquid and the heat of vaporization. ( G ) The density of the steam ; that is, the weight in pounds of one cubic foot of steam at the given temperature or pressure. (7) The specific volume; or volume in cubic feet of one pound of steam at the required tem- perature or pressure. Evidently the specific vol- 1 ume is equal to ^^^^ All these properties have been calculated by means of various formulas which have been de- duced from the results of actual experiment. We have seen that a saturated vapor contains' just enough heat to keep it in the form of a vapor; if it loses heat it will condense. A super- heated vapor is one that has been heated after vaporization ; it can lose this extra heat before any condensation will take^place. A vapor in contact with its liquid is saturated; one heated after removal from the liquid is superheated. For saturated steam there is a fixed tempera- ture for every pressure. If we know either the pressure or the temperature, we can find the other in the steam tables. For instance, if the gauge pressure of a boiler is 60.3 pounds and we wish to know the temperature, we simply add atmospberic pressure and turn to our tables and find it to be 307° (approximately). With superheated steam the case is entirely difTerent, for there is no longer the same direct relation between the temperature and pressure. In fact, the relation between temperature and pressure of superheated steam depends upon the amount of superheating. Superheated steam at 60.3 poimds gauge pressure may have a tempera- ture considerably above 307° F. At a given pressure the temperature and volume of ii given weight of superheated steam are always greater than the temperature and volume of the same weight of saturated steam. The properties of superheated steam at given pressure are not con- stant as is the case with saturated steam. If superheated steam were a perfect gas, we could determine the relation of p. v. and f by the equation pv^ct; but superheated steam is not a perfect gas, hence we must modify our equa- tion. By experiment it has been determined that the following equation is nearly correct: pv = 93.5 < — 97Ip %, in which p = absolute pressure in pounds per square inch, t — absolute temperature, and f = volume of 1 pound in cubic feet. STEAM CARRIAGE. See Avtomobile. STEAM ENGINE. A motor in which the ex- |)ansive force of steam is employed as the medium for transforming the energy of heat into useful work. Ordinarily the steam acts u])on a piston inclosed within a cylinder in such a manner as to be capable only of icciprocotiiiy moticm. Imt in certain rare forms of engines it acts u]ion an in- closed piston or vane, which rotutcn around an axis. The rotary engine has the advantages of compactness and of being capable of ap])lying its power directly, while the reciprocating engine has to have the rectilinear motion of the piston transformed into the rotary motion of a fly wheel or shaft bj' a cumbersome intermediate mechan- ism. These advantages of the rotary engine are, however, more than counterl)alanced liv its de- cided lack of economy in steam consumjition, which prevents it from successfully competing commercially with the reciprocating engine. Only the latter is here treated. Early History. The first in.stance of the use of steam as a motive power is generally as- sumed to have been the seolipile of Hero of Alexandria (q.v.). As early as 1543 a Span- ish captain, named Blasco de Garay, is reputed to have shown in the harbor of Barcelona a steamboat of his own invention. A French en- gineer, Salomon de Cans, describes in Ifil.i a steam nuichine, which was merely .a contrivance for forcing the water contained in a co])per ball tlirough a tube by applying heat. An Italian engineer, Giovanni Branca, invented, in 1020, a .sort of steam windmill ; the steam, being gener- ated in a boiler, was directed by a spout against the flat vanes of a wheel, which was thus set in motion. See Steiam Turbine. In England the first successful efftu-t was that of the ilarquis of Worcester, who. in l(it>3. de- scribes a steam apparatus by which he raised a column of water to the height of 40 feet. The first patent for the ap)ilication of steam power to various kinds of machines was taken out in 1608 by Thomas Savery. His engines* were the first used and seem to have been employed for some years in the drainage of mines in Cornwall and Devonshire. The essential improvement in them over the older ones was the use of a boiler sep- arate from the vessel in which the steam did its work. To Denis Papin, a celebrated Frenchman, is due the idea of the piston. The next great step in advance was made about 1705 in the 'atmospheric' engine, conjointly invented by Xewcomen, Calley, and Savery. In this engine, which is shown in Fig. 1, the pre- vious inventions of the separate boiler and of the cylinder with its movable steam-tight pis- ton are utilized, although in a new form. The "beam,' which has ever since been in use in pump- ing engines, was used for the first time, and for the first time also the condensation of the steam was made an instantaneous process, instead of a slow and gradual one. To one end of a beam moving on an axis, I, was attached the rod, N, of the pump to be worked; to the other, the rod, M, of a piston moving in a cylinder, C, below.
