Popular Science Monthly/Volume 12/December 1877/The Growth of the Steam-Engine II
By Professor B. H. THURSTON,
OF THE STEVENS INSTITUTE OF TECHNOLOGY.
THE STEAM-ENGINE AS A TRAIN OF MECHANISM.
SECTION III. The Period of Development. Newcomen and Watt, a. d. 1700 to a. d. 1800.—22. The evident defects of Savery's engine, its extravagant consumption of fuel, the inconvenient necessity of placing it near the bottom of the mine to be drained, and of putting in several for successive lifts where the depth was considerable, and, especially, the risk which its use with high pressures involved even in its best form, considerably retarded its introduction, and it came into use very slowly, notwithstanding its superiority in economic efficiency over horse-power.
23. The first important step taken toward remedying these defects was by Thomas Newcomen and John Cawley, or Calley, two mechanics of the town of Dartmouth, Devonshire, England, who produced what has been known as the Atmospheric or Newcomen Engine.
Newcomen was a blacksmith, and Cawley a glazier and plumber.
It has been stated that a visit to Cornwall, where they witnessed the working of a Savery engine, first turned their attention. to the subject; but a friend of Savery has stated that Newcomen was as early with his general plans as Savery.
After some discussion with Cawley, Newcomen entered into correspondence with Dr. Hooke, proposing a steam-engine, to consist of a steam-cylinder containing a piston similar to those of Hugghens's and Papin's engines, and driving a separate pump, similar to those generally in use where water was raised by horse or wind power.
Dr. Hooke advised and argued strongly against their plan; but, fortunately, the obstinate belief of the unlearned mechanics was not
Fig. 10.—The Newcomen Engine, as improved by Smeaton, 1775.
overpowered by the disquisitions of their distinguished correspondent, and Newcomen and Cawley attempted an engine on their peculiar plan.
This succeeded so well as to induce them to continue their labors, and in 1705 to patent—in combination with Savery, who held the right of surface condensation, and who induced them to allow him an interest with them—an engine combining a steam-cylinder and piston, surface condensation, and a separate boiler and separate pumps.
24. In the atmospheric engine as first designed, the slow process of condensation by the application of the condensing water to the exterior of the cylinder to produce the vacuum caused the strokes of the engine to take place at very long intervals. An improvement was, however, soon effected, which immensely increased this rapidity of condensation. A jet of water was thrown directly into the cylinder, thus effecting for the Newcomen engine just what Desaguliers had previously done for the Savery engine. As thus improved, the Newcomen engine is shown in Fig. 11.
Here d is the boiler. Steam passes from it through the cock d, and up into the cylinder a, equilibrating the pressure of the atmosphere, and allowing the heavy pump-rod k to fall, and, by its greater weight, acting through the beam i i, to raise the piston s to the position shown.
The cock d being shut, f is then opened, and a jet of water from the reservoir g enters the cylinder, producing a vacuum by the condensation of the steam. The pressure of the air above the piston now forces it down, again raising the pump-rods, and thus the engine works on indefinitely.
The pipe h is used for the purpose of keeping the upper side of the piston covered with water, to prevent air-leaks—a device of Newcomen.
Two gauge-cocks, c, c, and a safety-valve, N, are represented in the figure, but it will be noticed that the latter is quite different from the now usual form. Here, the pressure used was hardly greater than that of the atmosphere, and the weight of the valve itself was ordinarily sufficient to keep it down. The rod m was intended to carry a counter-weight when needed.
The condensing water, together with the water of condensation, flows off through the open pipe p. Newcomen's first engine made six or eight strokes a minute; the later and improved engines made ten or twelve.
25. The steam-engine has now assumed a form that somewhat resembles the modern machine.
An important defect still existed in the necessity of keeping an attendant by the engine to open and shut the cocks. A bright boy, however, Humphrey Potter, to whom was assigned this duty on a Newcomen engine in 1713, contrived what he called a scoggan—a catch rigged with a cord from the beam overhead—which performed the work for him.
The boy, thus making the operation of the valve-gear automatic, increased the speed of the engine to fifteen or sixteen strokes a minute, and gave it a regularity and certainty of action that could only be obtained by such an adjustment of its valves.
This ingenious young mechanic afterward became a skillful work-man, and an excellent engineer, and went abroad on the Continent, where he erected several fine engines.
26. Potter's rude valve-gear was soon improved by Henry Beighton, and the new device was applied to an engine which that talented engineer erected at Newcastle-on-Tyne in 1718, in which engine he substituted substantial materials for Potter's unmechanical arrangement of cords, as seen in Fig. 12.
In this sketch, r is a plug-tree, plug-rod, or plug-frame, as it is variously called, suspended from the great beam with which it rises and falls, bringing the pins p and k, at the proper moment, in contact with the handles k k and n n of the valves, moving them in the proper direction and to the proper extent. A lever safety-valve is here used, at the suggestion (it is said) of Desaguliers.
The piston was packed with leather or with rope, and lubricated with tallow.
27. Further improvements were effected in the Newcomen engine by several engineers, and particularly by Smeaton, and it soon came into quite extensive use in all of the mining districts of Great Britain, and it also became generally known upon the Continent of Europe.
Its greater economy of fuel as compared with the Savery engine in its best form, its greater safety—a consequence of the low steam-pressure adopted—and its greater working capacity, gave it such manifest superiority that its adoption took place quite rapidly, and it continued in general use in some districts where fuel was cheap up to a very recent date. Some of these engines are even now in existence.
From about 1758 to the time of the introduction of the Watt engine, this was the machine in almost universal use for raising large quantities of water.
28. The success of the Newcomen engine naturally attracted the attention of mechanics, and of scientific men as well, to the possibility of making other applications of steam-power.
The greatest men of the time gave much attention to the subject; but, until James Watt began the work that has made him famous, nothing more was done than to improve the proportions and to slightly alter the details of the Newcomen and Cawley engine, even by such skillful engineers as Brindley and Smeaton.
Of the personal history of the earlier inventors and improvers of the steam-engine very little is known; but that of Watt has been fully traced.
29. This great man was born at Greenock, then a little Scotch fishing-village, but now a considerable and a busy town, which annually
launches upon the waters of the Clyde a fleet of steamships whose engines are probably, in the aggregate, far more powerful than were all the engines in the world at the date of Watt's birth—January 19, 1736.
He was a bright boy, but exceedingly delicate in health, and quite unable to attend school regularly, or to apply himself closely to either study or play.
His early education was given by his parents, who were respectable and intelligent people, and the tools borrowed from his father's carpenter's-bench served at once to amuse him and to give him a dexterity and familiarity with their use that must undoubtedly have been of inestimable value to him in after-life.
M. Arago, the eminent French philosopher, who wrote one of the earliest and most interesting biographies of Watt, relates anecdotes of him which, if correct, illustrate well the thoughtfulness and the intelligence, as well as the mechanical bent, of the boy's mind.
He is said, at the age of six years, to have occupied himself during leisure hours with the solution of geometrical problems, and Arago discovers in a story, in which he is described as experimenting with the tea-kettle, his earliest investigations of the nature and properties of steam.
When finally sent to the village-school, his ill-health prevented his making rapid progress, and it was only when more than fourteen years of age that he began to show that he was capable of taking the lead in his class, and to exhibit his ability in the study particularly of mathematics. His spare time was principally spent in sketching with his pencil, in carving, and in working at the bench, both in wood and metal. His favorite work seemed to be the repairing of nautical instruments.
In boyhood, as in after-life, he was a diligent reader, and he seemed to find something to interest him in every book that came into his hands.
At the age of eighteen Watt was sent to Glasgow, there to reside with his mother's relatives, and to learn the trade of a mathematical instrument maker. The mechanic with whom he was placed was soon found too indolent, or was otherwise incapable of giving much aid in the project; and Dr. Dick, of the University of Glasgow, with whom Watt became acquainted, advised him to go to London.
Accordingly, he set out in June, 1755, for the metropolis, where, on his arrival, he arranged with Mr. John Morgan, in Cornhill, to work for a year at his chosen business, receiving as compensation twenty guineas. At the end of the year he was compelled by serious ill-health to return home.
30. Having become restored to health, he went again to Glasgow, in 1756, with the intention of pursuing his calling there. But not being the son of a burgess, and not having served his apprenticeship in the town, he was forbidden by the guilds, or trades-unions, to open a shop in Glasgow. Dr. Dick came to his aid, and employed him to repair some apparatus which had been bequeathed to the college; and he was finally allowed the use of three rooms in the university-building, its authorities not being under the municipal rule.
He remained here until 1760, when, the trades no longer objecting, he took a shop in the city, and in 1761 moved again into a shop on the north side of the Trongate, where he earned a scanty living without molestation, still keeping up his connection with the college.
He spent much of his leisure time, of which he had more than was desirable, in making philosophical experiments, and in the manufacture of musical instruments, making himself familiar with the sciences, and devising improvements in the construction of organs.
His reading was still very desultory; but the introduction of the Newcomen engine in the neighborhood of Glasgow, and the presence of a model in the college collections, which model was placed in his hands in 1763 for repairs, led him to study the history of the steam-engine, and to conduct for himself an experimental research into the properties of steam, using a set of improvised apparatus.
31. The Newcomen model, as it happened, had a boiler, which, although made to a scale from engines in actual use, was quite incapable of furnishing steam enough to work the engine.
It was about nine inches in diameter, and the steam-cylinder was two inches in diameter, and of six inches stroke of piston, arranged as in Fig. 13.
This is a picture of the most carefully-preserved treasure in the collections of the University of Glasgow. Watt at once noticed the defect referred to, and immediately sought first the cause and then the remedy.
32. He soon concluded that the sources of loss of heat in the Newcomen engine—which loss would be greatly exaggerated in a small model—were: first, the dissipation of heat by the cylinder itself, which was of brass, and was both a good conductor and a good radiator; secondly, the loss of heat consequent upon the necessity of cooling down the cylinder at every stroke in producing the vacuum; and, finally, a loss of power was due to the existence of vapor beneath the piston, the presence of which vapor was a consequence of the imperfect method of condensation which characterizes the Newcomen engine.
He first made a cylinder of non-conducting material—wood soaked in oil and then baked—and found a decided advantage in the economy of steam thus secured.
He then conducted a series of experiments upon the temperature and pressure of steam at such points in the scale as he could readily reach, and, constructing a curve with his results, the abscissas representing temperatures, and the pressures being represented by the ordinates, he ran the curve backward until he had obtained approximate measures of temperatures less than 212°, and of pressures less than atmospheric.
He thus discovered that, with the amount of injection-water used in the Newcomen engine, bringing the temperature of the interior, as he found, down to from 140° to 175° Fahr., a very considerable back-pressure would be met with.
Continuing his research still further, he measured the amount of steam used at each stroke; and, comparing it with the quantity that would just fill the cylinder, he found that at least three-fourths was wasted.
The quantity of cold water necessary to produce condensation of a given weight of steam was next determined, and he found that one pound of steam contained enough heat to raise about six pounds of cold water, as used for condensation, from the temperature of 52 Fahr. to the boiling-point; and, going still further, he found that he was compelled to use, at each stroke of the Newcomen engine, four times as much injection-water as should suffice to condense a cylinder full of steam. Thus was confirmed his previous conclusion that three-fourths of the heat supplied to the engine was wasted.
His experiments having revealed to him the now well-known fact of the existence of latent heat, he went to his friend Dr. Black, of the university, with this intelligence; and the latter then informed him of the Theory of Latent Heat which had but a short time earlier been discovered by Dr. Black himself.
33. Watt had now, therefore, determined by his own researches, as he himself enumerates them, the following facts:
(1.) The capacities for heat of iron, copper, and of some sorts of wood, as compared with water.
(2.) The bulk of steam compared with that of water.
(3.) The quantity of water evaporated in a certain boiler by a pound of coal.
(4.) The elasticities of steam, at various temperatures greater than that of boiling water, and an approximation to the law which it follows at other temperatures.
(5.) How much water, in the form of steam, was required, at every stroke, by a small Newcomen engine, with a wooden cylinder six inches in diameter and twelve inches stroke.
(6.) The quantity of cold water required, at every stroke, to condense the steam in that cylinder, so as to give it a working power of about seven pounds on the square inch.
34. After these well-devised and truly scientific investigations, Watt was enabled to enter upon his work of improving the steam-engine with an intelligent understanding of its existing defects, and with a knowledge of their cause.
It was on a Sunday afternoon, in the spring of 1765, that he devised his first and his greatest invention—the separate condenser. His object in using it was, as be says himself, to keep the cylinder as hot as the steam that entered it. He was therefore the first to apprehend and to state a problem which the modern engineer is still vainly endeavoring completely to solve.
Watt was, at this time, twenty-nine years of age. Having taken this first step and made such a radical improvement, the success of the invention was no sooner determined than others followed in rapid succession as consequences of the exigencies arising from the first radical change in the old Newcomen engine.
But in the working out of the forms and proportions of details in the new engine, even Watt's powerful mind, with its stores of happily-combined scientific and practical information, was occupied for years.
35. In attaching the separate condenser, he first tried surface condensation, as in Fig. 14, which is a sketch of his first model; but this not succeeding well, he substituted the jet. Some provision became at once necessary for preventing the filling of the condenser with water.
Watt at first intended adopting the same expedient which worked satisfactorily with the less effective condensation of Newcomen's engine, i. e., leading a pipe from the condenser to a depth greater than the height of the column of water which could be counterbalanced by the pressure of the atmosphere; but he subsequently employed the air-pump, which relieves the condenser, not only of the water, but of the air which also usually collects in considerable volume, and vitiates the vacuum.
He next substituted oil and tallow for the water previously used in lubrication of the piston and keeping it steam-tight, in order to avoid the cooling of the cylinder incident to the use of water.
Still another cause of refrigeration of the cylinder, and consequent waste of power in its operation, was seen to be the entrance of the atmosphere, which came in at the top and followed the piston down the cylinder at each stroke.
This the inventor concluded to prevent by covering the top of the cylinder, and allowing the piston-rod to play through a "stuffing-box," which device had long been known to mechanics. He accordingly not only covered the top, but surrounded the whole cylinder with an external casing or "steam-jacket," and allowed the steam from the boiler to pass around the steam-cylinder and to press upon the upper surface of the piston where its pressure was readily variable, and therefore more manageable than that of the atmosphere. It also, besides keeping the cylinder hot, could do comparatively little harm should it leak by the piston, as it might be condensed and readily disposed of.
36. This completed the change of the "atmospheric engine" of Newcomen into the steam-engine of James Watt. The engine as improved is shown in Fig. 15, which represents the engine as patented in April, 1769. Watt's first engine was erected with the pecuniary aid of Dr. Roebuck, the lessor of a coal-mine on the estate of the Duke of Hamilton, at Kinneil, near Borrowstounness. This engine, which was put up at the mine, had a steam-cylinder eighteen inches in diameter.
In the figure, the steam passes from the boiler through the pipe d and the valve c to the cylinder casing, or steam-jacket, Y Y, and above the piston b, which it follows in its descent in the cylinder a, the valve f being at this time open to allow the exhaust to pass into the condenser h.
The piston now being at the lower end of the cylinder, and the pump-rods at the opposite end of the beam y thus raised, and the pumps filled with water, the valves c and f close, while e opens, allowing the steam which remains above the piston to flow beneath it, until, the pressure becoming equal above and below by the weight of the pump, it is rapidly drawn to the top of the cylinder, while the steam is displaced above, passing to the underside of the piston.
Now the valve e is closed, and c and f are again opened, and the down-stroke is repeated as before. The water and air entering the condenser are removed, at each stroke, by the air-pump i, which communicates with the condenser by the passage s. The pump q supplies condensing-water, and the pump A takes away a part of the water of condensation, which is thrown by the air-pump into the "hot well" k, and with it supplies the boiler. The valves are moved by valve-gear very similar to Beighton's, by the pins m m in the "plug-frame" or "tappet-rod" n n.
The engine is mounted upon a substantial foundation, B B. F is an opening, out of which, before starting the engine, the air is driven from the cylinder and condenser.
37. In the building and erection of his engines, Watt had the greatest difficulty in finding skillful workmen to make the parts with accuracy, to fit them with skill, and to erect them properly when once finished.
The fact that both Newcomen and Watt found such serious trouble indicates that, even had the engine been designed earlier, it is quite unlikely that the world would have seen the steam-engine a success until this period, when mechanics were just acquiring the skill requisite for its construction. But, on the other hand, it is not at all certain that, had the mechanics of an earlier period been as skillful and as well educated in the manual niceties of their business, the steam-engine might not have been much earlier brought into use.
In the time of the Marquis of Worcester, it would have probably been found impossible to obtain workmen to construct the steam-engine of Watt, had it been then invented. Indeed, Watt, upon one occasion, congratulated himself that one of his steam-cylinders only lacked three-eighths of an inch of being truly cylindrical.
38. Pecuniary misfortunes soon deprived Watt of the assistance
of his friend and partner Dr. Roebuck, but in 1773 he became connected with an intelligent, energetic, and wealthy manufacturer of Birmingham, Matthew Boulton. Thenceforward, the establishment of Boulton & Watt, at Soho, near Birmingham, for a long time furnished the greater proportion of all the steam-engines made in the world.
In the new firm, Boulton took charge of the general business, and Watt superintended the design, construction, and erection, of their engines. Boulton's business capacity, with Watt's wonderful mechanical ability; Boulton's physical health, and his vigor and courage, offsetting Watt's feeble health and depression of spirits; and, more than all, Boulton's pecuniary resources, both in his own purse and in the wealth of his friends, enabled the firm to conquer all difficulties, whether in finance, in litigation, or in engineering.
39. Watt had, before meeting Boulton, conceived the idea of economizing some of that power the loss of which was so plainly indicated by the violent rush of the exhaust steam into the condenser, and had described the advantages that would follow the use of steam expansively, by means of a "cut-off," in a letter to Dr. Small, of Birmingham, dated Glasgow, May, 1769. He had also planned a "compound engine."
This invention of the expansion of steam, which, in importance, was hardly exceeded by any other improvement of the steam-engine, was adopted at Soho in 1776, but the patent was not obtained until 1782.
Fig. 16.—Watt's Steam-Engine, a. d. 1780.
During this interval, Watt invented the crank and fly-wheel, but, as the former had been first patented by Wasborough, who is supposed to have obtained a knowledge of it from workmen employed by Watt, the latter patented several other methods of producing rotary motions, and temporarily adopted that known as the "sun-and-planet wheels," subsequently using the crank.
The adaptation of the steam-engine to the production of rotary motion was soon succeeded by the introduction of the Double-Acting Engine, the Fly-ball Governor, the Counter, the Steam-Engine Indicator, and other minor but valuable improvements, which were the final steps by which the Watt steam-engine became applicable to driving mills, to use on railroads, to steam-navigation, and to the countless purposes by which it has become, as it has already been denominated, the great material agent of civilization.
40. Fig. 16 represents the Watt Double-Acting engine. It will be noticed that it differs from the Single-Acting engine in having steam-valves, B B, and exhaust-valves, E E, at each end of the cylinder, thus enabling the steam to act on each side of the piston alternately, and practically doubling the power of the engine.
The end of the beam opposite to the cylinder is usually connected with a crank-shaft.
Watt's Workshop at Home.
41. At this point, the history of the steam-engine becomes the story of its applications in several different directions, the most important of which are the raising of water, which has hitherto been its only application; the propulsion of carriages, as in the locomotive the driving of mills and machinery; and steam-navigation.
Here we take leave of James Watt, of whom a French author says, "The part which he played, in the mechanical application of the power of steam, can only be compared to that of Newton in astronomy, and of Shakespeare in poetry."
Retiring from the firm in the first year of the present century, Watt remained quietly on his estate at Heathfield. He fitted up a little workshop in his house, and there spent nearly all his time, inventing, designing, and constructing ingenious machines for special purposes. He died peacefully, full of years and great in fame, August 25, 1819,
Since the time of Watt, improvements have been principally in matters of mere detail, and in the extension of the range of application of the steam-engine.
42. To complete the history of its application to raising water, the succeeding figures are given as exhibiting the principal forms of pumping-engine as now constructed.
Fig. 17.—The Cornish Pumping-Engine, 1877.
Fig. 17 represents the Cornish pumping-engine, which, in spite of its great weight and high cost, is still much used.
It will be seen that it is the engine of James Watt in all its general features.
It is single-acting, and has a steam-jacket and a plug-rod valve-gear, J K. The improvements are principally in the form and proportions of its parts, and in its adaptation to high steam and "short 'cut-off.' "
A is the steam-cylinder, B C the piston and rod, D the beam, and E the pump-rod. The condenser is seen at G, and the air-pump at H. The steam-cylinder is "steam-jacketed," and is surrounded by a casing, O, composed of brickwork or other non-conducting material. Steam is first admitted above the piston, driving it rapidly downward and raising. the pump-rod. At an early point in the stroke the admission of steam is checked by the sudden closing of the induction-valve, and the stroke is completed under the action of expanding steam assisted by the inertia of the heavy parts already in motion. The necessary weight and inertia are afforded in many cases, where the engine is applied to the pumping of deep mines, by the immensely long and heavy pump-rods. Where this weight is too great, it is counterbalanced; and where, as when used for the water-supply of cities, too small, weights are added. When the stroke is completed, the "equilibrium-valve" is opened, and the steam passes from above to the space below the piston, and, an equilibrium of pressure being thus produced, the pump-rods descend, forcing the water from the pumps and raising the steam-piston. The absence of the crank or other device which might determine absolutely the length of stroke compels a very careful adjustment of steam admission to the amount of load. Should the stroke be allowed to exceed the proper length, and should danger thus arise of the piston striking the cylinder-heads, the movement is checked by buffer-beams. The regulation is effected by a "cataract," a kind of hydraulic governor, consisting of a plunger-pump with a reservoir attached. The plunger is raised by the engine, and then automatically detached. It falls with greater or less rapidity, its velocity being determined by the size of the eduction orifice, which is adjustable by hand. When the plunger reaches the bottom of the pump-barrel, it disengages a catch, a weight is allowed to act upon the steam-valve, opening it, and the engine is caused to make a stroke. When the outlet of the cataract is nearly closed, the engine stands still a considerable time while the plunger is descending, and the strokes succeed each other at long intervals. When the opening is greater, the cataract acts more rapidly, and the engine works faster. This has been regarded until recently as the most economical of pumping-engines, and it is still generally used in Europe in freeing mines of water.
43. Fig. 18 represents a lighter, cheaper, and almost equally effective machine, known as the Bull Cornish or Direct-Acting Cornish engine. It was first designed by the competitor of Watt, by whose name it is known. As is seen by reference to the engraving, its cylinder a is directly above the pump-rods c, d, g, and is carried on cross-beams, b b. The air-pump m l o p, the tank n, and valve-gear q r s, are quite similar to those of the beam Cornish engine. The balance-beam is seen at h i.
Fig. 18.—Bull's Cornish Engine, 1798.
Fig. 19 represents another form of pumping-engine which belongs to the class known as the "compound" or "two-cylinder" engine. This class of engines, in which the steam exhausted from one cylinder is further expanded in the second, was first introduced by Hornblower, in 1781, and was patented, in combination with the Watt condenser, by Woolf, at a later date (1804), with a view to adopting high steam and considerable expansion.
The Woolf engine was to some extent adopted, but was not successful in competing with Watt engines where the latter were well built, and, like Hornblower's engine, was soon given up.
The compound engine has come up again within a few years, and, with what is now considered high steam and considerable expansion, and designed with more intelligent reference to the requirements of economy in working steam in this manner, it seems gradually displacing all other forms of engine.
Fig. 19.—Hornblower's Compound Engine, 1781.
44. An example of this form of pumping-engine, and one which is a favorite with many engineers, is the beam and crank engine (Fig. 20),
Fig. 20.—Compound Pumping-Engine, 1800.
C D, E F, with double cylinder, A, B, working the "combined bucket and plunger," or double-acting pump, J. In its cylinders steam is usually expanded from four to eight times. The Leavitt compound engseine is shown in Fig. 21.
Fig. 21.—Leavitt's Pumping-Engine, 1875.
In this engine the lower ends, A, B, of the two cylinders are brought close together under the centre of the beam, thus shortening
Fig. 22.—The Worthington Compound Pumping-Engine, 1870.
Still another recent form of steam pumping-engine, noted for its cheapness combined with efficiency, is that of Worthington (Fig. 22), in which two pairs of steam cylinders, A, B, are placed side by side, each pair driving a pump-plunger, F, attached to its piston-rod, and each having its valve-gear, H L, M N, actuated by the movement of the piston of the other. The cylinders together form a compound engine; the steam exhausted from the smaller, A, passing into the
Tomb of James Watt, Handsworth Church, 1819.
larger, B, where it is further expanded. The valve-gear of this engine is peculiarly well adapted to this type of engine. There is no fly-wheel, and the motion of each of the two independent engines, which together form the pair, is controlled by its neighbor, the valve-gear of the one being moved by the piston of the other. This ingenious combination permits each piston to move from end to end of its cylinder, holds it stationary an instant while the pump-cylinders come completely filled and their valves closed, and then sets it in motion on the return-stroke. Thus the pistons move alternately. These engines have given a very high duty. The condenser is seen at C, and the air-pump is at D, the latter being worked from the bell-crank lever H by means of links, I, K. The steam-valves, Q, R, are balanced. V V are the water-induction valves, and T T those on the eduction-side.
Here we leave the steam-engine as applied to raising water. We have invariably noticed, in the forms of engines so used, that a condenser forms a part of the apparatus.
We will next briefly trace the history of that now familiar form of engine in which the steam, having done its work, is discharged directly into the atmosphere.