1

edges or sides of the passages it moves through, is considerably raised. But if, in so expanding, it must meanwhile perform the work of lifting or pushing a piston or other movable load, the theory requires that in so doing it shall lose a corresponding portion of heat, and that, if it were before at the point of saturation, by the cooling that results a portion of the steam shall return to the liquid form; and it is believed that practical observations and tests confirm this result of the thermo-dynamic theory. From the known rate of expansion of perfect gases, it is inferred that were a given volume of a gas, as air, for example, continually cooled, its volume would uniformly diminish; and that at -461.2° F., or 493.2° below the freezing point of water, it must wholly collapse, its elasticity and volume becoming 0. This point, then, -461.2°, is considered as the "absolute zero" of heat; and temperature reckoned from it is "absolute temperature." If a given volume of air, under a constant pressure, be heated from 0° F. through 461.2°, its volume is doubled; heated through 2× 461.2, its volume is tripled; and universally, the volume is augmented part for every addition of 1° of 481 heat. This expression also gives the rate of expansion or contraction for superheated steam. For the permanent gases and for superheated vapors, then, the laws respecting volume and pressure may thus be briefly stated: 1, the temperature remaining constant, the tension or pressure varies inversely as the volume; 2, the pressure constant, the volume varies directly as the absolute temperature; 3, the volume constant, the pressure varies directly as the absolute temperature. Now, omitting for the moment any effect of specific heat, the amount of heat that must have entered, in vaporizing it, a pound of water vapor, will be found to consist always of two readily distinguishable parts: first, the whole amount of the heat required to raise the liquid, before evaporation takes place, from some fixed temperature to that of the evaporation, i. e., the "sensible heat;" secondly, the whole amount of the heat which disappears in the process or work of converting the pound of water already at its evaporating point into vapor, i. e., the latent heat of evaporation. In reckoning the sensible heat, it is not necessary to start from the absolute zero; and as the most convenient point, and one sufficiently low to underlie all ordinary calculations about steam, 32° is adopted as, for these cases, the 0 of the sensible temperatures considered. The sum of these two parts of the heat contained in steam, the sensible reckoned from 32°, and the latent of evaporation, is termed the total heat of evaporation, or total heat of steam. As the sensible heat is very readily and nearly determined in all cases, it has been, ever since the complete development of the capacities of steam mechanism by the inventions of Watt, an important problem to ascertain precisely the amount and rate of variation of the latent heat of the vapor. The estimates arrived at, for the

latent heat of 1 lb. of water at 212°, have varied from that of Dr. Black, at 810°, to those of Count Rumford, 1050.5°, and of the committee of the Franklin institute, Philadelphia, 1037°. (For details of apparatus and methods in these, as well as in the experiments of Regnault, see the works of Tredgold and Bourne on the steam engine.) It was during such researches, also, that the supposed laws of Watt and of Southern were arrived at; the former that "the total quantity of heat necessary to vaporize a given weight of water was the same at all sensible temperatures," the latent heat diminishing as the sensible heat was raised; the latter, conflicting with that of Watt, and still more erroneous, that "the latent heat of vaporization was the same under every degree of pressure." In Regnault's experiments and calculations, probably conducted with the utmost attainable precaution and accuracy, the total heat of evaporation at 0° C., = 32° F., was determined as equal to 606.5° C., =1091.7° F. He also found that, between 0° and 230° C., the total heat of saturated steam increases (a slight change in specific heat being here disregarded) uniformly by .305° for each added degree of heat. This result also determines the specific heat of ordinary steam as .305, that of water being 1. Letting t represent the indicated temperature in any case, and conforming the expression to the observed total heat at 212°, Regnault's formula for total heat at all temperatures, in degrees F., is H-(1113.4-32)+.305t; or, H=1081.4+ .305t; so that the total heat of saturated steam at 212° is 1146° F. This is the total consumption of heat if the water be supplied at 32°. When the water is supplied at temperatures above this, for 32 in the formula substitute the given temperature. Thus, taking cold water at average temperature, H=(1113.4—62)+.305t; or, H=1051.4+.305t. If, as in condensing engines, the water be at 100°, then H= 1013.4+.305t. If the water be supplied at boiling point, allowing .9° for specific heat, then H=(1113.4-212.9)+.305t=900.5+.305ť. And converting the formula given by Clausius for the latent heat of steam, we obtain in Fahrenheit's scale, L-1115.2-.708t. Let it further be borne in mind that the same figures which above express in degrees the relations of the constituent heat of steam, in form of ratios merely, and not as absolute quantities, will also express positive values, in units of heat, if we assume the quantity of steam as 1 lb. weight, so as to accord with the requirements of the thermal unit. The appropriation of all the heat contributing to the formation of 1 lb. of saturated steam at 212°, and given both in units of heat and of work, will now be understood from the following tabular statement:

It becomes evident at the same time, that the total latent heat of steam cannot be taken as in any way the measure of the energy or work in, or that can practically be obtained from, the steam. Much the larger part of such heat is expended in merely overcoming the cohesion of the liquid; and at all temperatures, it is but a small fraction of the latent heat that can be made available in performing work.-Water at 212° and under one atmosphere, becoming steam, is ordinarily said to expand into a volume 1,700 times greater than that occupied by the water itself. The increase of volume is, however, always less than this, being differently stated at from 1,670 down to 1,642 times the original volume. It is remarkable that the uncombined oxygen and hydrogen gases composing the same weight of steam, at the same temperature and pressure, would occupy 2,500 times the volume of the water. Thus, the density and pressure of actual steam always exceed those which the ideal steam, or that on the supposition of a perfect gas, would exhibit. By means of recorded observations of experiments on steam, and finding the mean of the most trustworthy results, with the further aid of formulas and calculations, some of which are in this article intimated rather than detailed, very full tables of the properties of saturated and of superheated steam have been prepared. Of such a table for saturated steam, a brief abstract only can here be introduced.

TABLE OF PROPERTIES OF SATURATED STEAM.

As in case of all gaseous bodies observable near their point of liquefaction, steam diminishes in volume and tension, and increases in density, more rapidly as it approaches near to condensation, than under like variations of pressure when it is heated much above that point; but the amount of such irregularity is not ascertained. The density of steam is expressed by the weight of a given constant volume, usually that of a cubic foot; its relative volume is the ratio of its actual volume to the volume of the water producing it. The density and relative volume of saturated steam have been determined with tolerable accuracy, by comparison of the quantities which experiments have furnished us, in connection with the elements of pressure, temperature, and latent heat.-STEAM ENGINE. I. History. The history of the steam engine is not the history, in full, of the discovery and application of the force of steam. The steam engine proper, first produced and patented by Watt in 1768-'9, is not yet (1862) 100 years old; but the more obvious properties of steam, and among them its expansive force, were understood and treated of, and mechanical effects by its agency produced, before the Christian era. In respect to the earlier experiments with steam, comparatively little is now known. Heron, in his "Pneumatics," about 230 B. C., describes three different, but simple contrivances showing mechanical effects of steam. No further advance is known to have been made until the 16th or 17th century of our era, when, through the impulse given by the new art of printing, the works of Heron and Archimedes were disseminated and much read, and an age scarcely second to our own for the great number and variety of its mechanical contrivances was entered upon. Blasco de Garay of Barcelona, in 1543, is said to have propelled a vessel of 200 tons by paddles, with "a water boiler, liable to burst;" a statement to be received with much hesitation. In 1601 Giambattista della Porta described in his "Pneumatics" an apparatus of his for raising water by a tube into a close vessel, in which a vacuum had been obtained by condensation of steam. In 1615 Solomon de Caus, a French engineer, published in his Raisons des forces mouvantes the statement that by fire water is dissolved into an air with such violence as to burst a closed copper ball containing a small quantity of it, and highly heated; and he described the propelling of a jet of water, by pressure on its surface in the vessel, of steam generated from it. In 1629 Branca, an Italian engineer, described a machine in which a wheel was driven round by the impulse of steam against vanes. The first engine in which steam was applied to the performance of useful work, seems to have been that invented by the marquis of Worcester, and described by him in his " Century of Inventions (1663); his description, so far as it is intelligible, indicating that steam was generated alternately in two vessels, and by pipes

transferred and made to exert a pressure upon the surface of water in a third; the water was raised, as he tells us, continuously to a height of 40 feet, one vessel of the vaporized water being sufficient thus to elevate 40 times its volume of cold water. Cosmo, grand duke of Tuscany, relates that he saw one of these machines in use at Vauxhall in 1656. The separate boiler seems to have been the original part of this invention. Dr. Denis Papin, well known as the originator of "Papin's digester, experimented much on the production and force of steam from 1695 to 1707, and at the former date first devised and employed a piston within a cylinder, and under which a vacuum was produced by condensation of steam. His plan was not practically realized in any form. He had previously invented the safety valve for boilers. In 1698 Capt. Thomas Savery secured letters patent for a machine for raising water by steam. It consisted of two boilers and two receivers for the steam, with valves, and the needful tubes. One of the receivers being filled with steam, its communication with the boiler was then cut off, and the steam condensed by cold water outside it; into the vacuum thus formed the atmosphere forced the water from below, when the steam was again caused to press upon the surface of the water and drive it higher. This engine was extensively used for draining mines; and the water raised was, in some instances, made to turn a water wheel, by which lathes and other machinery were driven. Some of the earliest cotton mills of Lancashire were thus supplied with power. As Papin's machine involved great waste of time, so did Savery's a very considerable waste of steam in reheating the cooled receivers, and in heating at the first the surface of the water to be raised. In 1705 Thomas Newcomen, a smith, and John Cawley, plumber, of Dartmouth, along with Savery, patented an engine combining for the first time the cylinder and piston and separate boiler, doubtless deriving the former from Papin's plan. Steam was admitted beneath the piston, and condensation at first secured by application of cold water without the cylinder; the pressure of the atmosphere forced down the piston, and in so doing worked a pump rod for raising water attached at the opposite end of a rude working beam, actuated by the piston rod. Thus, this was really an atmospheric pumping engine. Through the accident of a hole in the piston, letting the cold water directly into the cylinder, they discovered the superior rapidity and effectiveness of such condensation, and substituted the cold jet into the cylinder accordingly. The valves were opened and closed by hand, until a boy named Humphrey Potter, to gain time for play, substituted an apparatus of catches and strings, thus making the engine automatic. The contrivance first used was very clumsy; and it was wholly removed by Henry Beighton, who in 1788 first worked the valves by a rod direct

from the beam. In the mean time, many attempts at producing other forms of steam mechanism were being made in England and on the continent. The most important of these in principle, because containing the germ of the non-condensing or high pressure engine, was that of Jacob Leupold, a German, who in 1725 fitted directly upon a large boiler two cylinders, the steam and the escape passages of these being alternately opened and closed by a fourway steam cock; and who introduced beneath the pistons steam of comparatively high pressure, which was made to act against the atmosphere, plus the work of pumping water by means of a beam and pump rod actuated by each of the pistons. As Newcomen's atmospheric engine became extensively introduced for draining collieries and mines, and was found to effect a great saving of expense, its capabilities were soon fully tested, and indeed exceeded, by the demand for its use. The cylinders were enlarged from 12 to 60 inches diameter, and the other parts in proportion; so that the engines became gigantic, and required for their construction a degree of skill then rarely possessed. At this juncture, 1765– "74, John Smeaton devised a succession of improvements in the atmospheric engine, and carried it to its utmost perfection. This was still called the "fire engine," being so named even in Watt's earlier memoranda and patents, and the improvements in it had thus far been empirical only; the force was exerted in but one direction; the total pressure could not exceed that which air exerts, and in fact always fell short of it, because the vacuum beneath the piston was far from complete; while the purposes to which such an engine could be applied were necessarily very limited. The improvement of the engine on scientific principles commenced with the labors of Watt, who, while repairing a model of Newcomen's engine, discovered its various defects, and began to devise the methods of remedying them. He reflected that, "in order to make the best use of the steam, it was necessary, first, that the cylinder should be maintained always as hot as the steam which entered it; and secondly, that when the steam was condensed, the water of which it was composed, and the injection itself, should be cooled down to 100°, or lower where that was possible." The means to these ends occurred to his mind in 1765; namely, the separate condenser, in which condensation of steam, effecting its removal from the cylinder, was to be secured by cold water surrounding or injected into the reservoir for the purpose, the accumulating water to be removed by a pump. It next occurred to him that the open mouth of the cylinder must allow the latter to be cooled by the air following the piston, and that consequently some of the steam next admitted must be condensed. He therefore proposed to "put an air-tight cover upon the cylinder, with a hole and stuffing box for the piston rod to slide

Watt's general patent, however, was judged to exclude this invention, and it did not at that time come into use. In the single-acting engine, one half the motion was still unaccompanied by useful effect; and the application of the force was ill adapted to impart any other than a simple reciprocating movement. Since the time of Savery, it had been an object of importance with inventors to convert this movement into one of revolution, as adapted to machinery; and Hulls, Fitzgerald, Stewart, and others had contrived various means of effecting this change. Watt early conceived of the use of the common winch or crank for this purpose; it was patented, however, by Wasbrough, and then by Steed, in 1781. As Watt was at this time engaged in his invention by which the engine was to be made "double-acting"-the steam being admitted to press alternately, and in turn condensed, both above and below the pistonthus fitting it to impart revolution to a shaft, to wheel work, &c., he was obliged to resort to other methods to secure this part of his purpose, among them to the “ sun and planet” wheel. The adaptation of the engine to the production of a rotary motion prepared the way for its employment as the prime mover of every kind of machinery. The specification of the double-acting steam engine, with several kindred improvements, was enrolled July 4, 1782; and in 1784 patents were secured for the "par

through, and to admit steam above the piston to act upon it instead of the atmosphere. To prevent cooling of the cylinder by the external air, he would surround this by a larger one, the "jacket," the interspace to be kept filled with steam, and would cover or clothe the whole with wood or other substance conducting heat poorly. Thus, we must credit to the marquis of Worcester the first successful application of steam pressure to use; to Savery the application of the vacuum due to condensation, though he did not foresee the true method, or the full value of its application; to Papin the piston for receiving and transmitting the force of air or steam within a cylinder; to Newcomen and Cawley the cylinder and piston independent of the boiler, as also the working beam, and the plan of internal condensation; to Beighton the successful introduction of automatic apparatus for the valves; and to Watt the separate condenser, saving the cooling of the cylinder and consequent waste of steam in reheating it, and the exclusion of air from the cylinder, with introduction of steam above the piston, changes which, with those that followed and grew out of them, rendered the engine at length practical, economical, and complete. This was still a "single-acting" engine; the steam pressure during the pushing down of the piston being that alone which took effect on the mechanism to be driven, and the only object of the subsequent admission of steam below being to return the piston to the top of the cylinder. This engine was also chiefly used for pumping and draining. It seems to have occurred to Watt, as early as 1769, that additional economy would be secured, especially, as he thought, in working light loads, by closing the steam pipe before the piston had descended the full length of stroke, thus saving the filling of the cylinder completely with steam of the initial density, and allowing the stroke to be completed by expansion of that already admitted, aided of course by momentum of the beam and piston. This principle he first applied in 1776 in an engine erected at Soho, but he published no account of it until on patenting this and certain other improvements in 1782; the variety he named the expansive engine. The first public announcement of benefit from expansive working of steam, however, was by Jonathan Hornblower, who in 1781 employed two cylinders, one larger than the other; the steam, of the boiler pressure, having first driven the smaller piston, was immediately transferred, and allow ed to act during its expansion (of course with diminishing pressure) upon the increased area of the larger piston, the two cylinders being thus approximatively equal in power. VOL. XV.-4

FIG. 1.

allel motion," the throttle-valve, the governor, the indicator, &c.; as also for a form of locomotive engine, which however proved impracticable. Among the earliest of the double-acting or rotative engines produced for sale was the Albion mill engine, fig. 1; an inspection