CHAPTER III

THE STEAM ENGINE The greatest of the world’s inventions appear to have had a very casual birth. So much an affair of chance has been their first manifestation, that science has not been called in aid; no law can be discerned which might govern the time and sequence of their coming; they seem to have been stumbled on, unpedigreed offspring of accident and time. A monk of Metz discovers gunpowder. “Surely,” says Fuller, “ingenuity may seem transposed, and to have crossed her hands, when about the same time a soldier found out printing.” “It should seem,” writes Lord Bacon, “that hitherto men are rather beholden to a wild goat for surgery, or to a nightingale for music, or to the ibis for some part of physic, or to the pot-lid that flew open for artillery, or generally to chance, or anything else, than to logic for the invention of the Arts and Sciences.” So it seemed. And in due time the legend of the pot-lid was woven round the unfortunate Marquis of Worcester, who, tradition had it, made the discovery of the steam engine by observation of the stew-pot in which, when confined a prisoner in the Tower, he was engaged in cooking his dinner. At a later date and in another form the story was connected with James Watt. In reality, the story of the discovery of the steam engine is far more inspiring. The history of the application of steam to human use is almost the history of science itself; the stages of its development are clearly marked for us; and the large succession of these stages, and the calibre of the minds which contributed to the achievement of the perfected steam engine, are some measure of the essential complexity of what is to-day regarded as a comparatively simple machine. For the steam engine was not the gift of any particular genius or generation; it did not leap from any one man’s brain. Some of the greatest names in the history of human knowledge can claim a share in its discovery. From philosopher to scientist, from scientist to engineer the grand idea was carried on, gradually taking more and more concrete form, until finally, in an age when by the diffusion of knowledge the labours of all three were for the first time co-ordinated, it was brought to maturity. A new force of nature was harnessed which wrought a revolution in the civilized world. An attempt is made in this chapter to chronicle the circumstances under which the successive developments of the steam engine took place. The progress of the scientific ideas which led up to the discovery of the power of steam is traced. The claims of the various inventors chiefly associated with the steam engine are set forth in some detail, not for the difficult and invidious task of assessing their relative merits, but because by the light of these claims and altercations it may be possible to discern, in each case, where the merit lay and to what stage each novelty of idea or detail properly belonged. From this point of view, it is thought, the recital of circumstances which hitherto have been thought so trivial as to be scarcely worthy of record, may be of some suggestive value. The result of the investigation is to make clear the scientific importance of the steam engine: the steam engine regarded, not as the familiar drudge and commonplace servant of to-day, but in all its dignity of a thermodynamic machine, that scientific device which embodied so much of the natural philosophy of the age which first unveiled it--the seventeenth century. § Before the Christian era steam had been used to do mechanical work. In a treatise, _Pneumatica_, written by Hero of Alexandria about 130 B.C., mention is made of a primitive reaction turbine, which functioned by the reactionary force of steam jets thrown off tangentially from the periphery of a wheel. In the same work another form of heat-engine is described: an apparatus in which, by the expansion from heating of air contained in a spherical vessel, water was expelled from the same vessel to a bucket, where by its weight it gave motion mysteriously to the doors of temples. And evidence exists that in these two forms heat engines were used in later centuries for such trivial purposes as the blowing of organs and the turning of spits. But except in these two primitive forms no progress is recorded for seventeen centuries after the date of Hero’s book. The story of the evolution of steam as a motive force really begins, with the story of modern science itself, at the end of the Middle Ages. With the great revival of learning which took place in Southern Europe in the latter part of the fifteenth century new light came to be thrown on the classical philosophies which still ruled men’s minds, and modern science was born. New views on natural phenomena began to irradiate, and, sweeping aside the myths and traditions which surrounded and stifled them, the votaries of the “new science” began to formulate opinions of the boldest and most unorthodox description.[70] The true laws of the equilibrium of fluids, discovered originally by Archimedes, were rediscovered by Stevinus. By the end of the sixteenth century the nature of the physical universe was become a pursuit of the wisest men. To Galileo himself was due, perhaps, the first distinct conception of the power of steam or any other gas to do mechanical work; for “he, the Archimedes of his age, first clearly grasped the idea of force as a mechanical agent, and extended to the external world the conception of the invariability of the relation between cause and effect.”[71] To his brilliant pupil Torricelli the questioning world was indebted for the experiments which showed the true nature of the atmosphere, and for the theory he proclaimed that the atmosphere by its own weight exerted its fluid pressure--a theory which Pascal soon confirmed by the famous ascent of his barometer up the Puy-de-Dôme, which demonstrated that the pressure supporting his column of mercury grew less as the ascent proceeded. Giovanni della Porta, in a treatise on pneumatics published in the year 1601, had already made two suggestions of the first importance. Discussing Hero’s door-opening apparatus, della Porta showed that steam might be substituted for air as the expanding medium, and that, by condensing steam in a closed vessel, water might be sucked up from a lower level by virtue of the vacuum so formed. And a few years later, in 1615, Solomon de Caus, a French engineer, had come to England with a scheme almost identical with della Porta’s, and actually constructed a plant which forced up water to a height by means of steam. Shortly afterwards the “new science” received an accession of interest from the invention, by Otto von Guericke of Magdeburg, of a suction pump by which the atmospheric air could be abstracted from a closed vessel. By the middle of this century the learned of all European countries had been attracted by the knowledge gained of the material universe. In England the secrets of science were attacked with enthusiasm under the new strategy of Lord Bacon, enunciated in his _Novum Organum_. The new philosophy was patronised by royalty itself, and studied by a company of brilliant men of whom the leading physicist was Robert Boyle, soon famous for his law connecting the volumes and the pressures of gases. In France, too, a great enthusiasm for science took birth. A group of men, of whom the most eminent was Christian Huyghens, banded themselves together to further scientific inquiry into the phenomena of nature and to demolish the reigning myths and fallacies: they also working admittedly by the experimental method of Bacon. The time was ripe, however, for wider recognition of these scientists and the grand object of their labours. Within a short time the two groups were both given the charter of their respective countries; in France they were enrolled as the Royal Academy of Sciences; in England, as the Royal Society for Improving Natural Knowledge. In other countries societies of a similar kind were formed, but their influence was not comparable with that exerted by the societies of London and Paris. Between these two a correspondence was started which afterwards developed into one of the most famous of publications: the _Philosophical Transactions_. In England, especially, the Royal Society served from its inception as a focus for all the great minds of the day, and in time brought together such men as Newton, Wren, Hooke, Wallis, Boyle--not to mention his majesty King Charles himself; who, with the best intentions, could not always take seriously the speculations of the savants. “Gresham College he mightily laughed at,” noted Mr. Pepys in his diary for the first of February, 1663, “for spending time only in weighing of ayre, and doing nothing else since they sat.” A year later Pepys was himself admitted a member of the distinguished company, and found it “a most acceptable thing to hear their discourse, and see their experiments, which were this day on fire, and how it goes out in a place where the air is not free, and sooner out in a place where the ayre is exhausted, which they showed by an engine on purpose.” § In the year 1663, just after the formation of the Royal Society, a small book was published by the Marquis of Worcester, _A Century of the Names and Scantlings of such Inventions as he had tried and perfected_. Of these inventions one, the sixty-eighth, is thus described: “An admirable and most forcible way to drive up water by fire, not by drawing or sucking it upwards, for that must be as the Philosopher calleth it, _Intra sphæram activitatis_, which is but at such a distance. But this way hath no bounder, if the vessels be strong enough; for I have taken a piece of a whole cannon, whereof the end was burst, and filled it three-quarters full of water, stopping and screwing up the broken end, as also the touch-hole; and making a constant fire under it, within twenty-four hours it burst and made a great crack. So that having a way to make my vessels, so that they are strengthened by the force within them, and the one to fill after the other; I have seen the water run like a constant fountain-stream forty foot high; one vessel of water rarified by fire driveth up forty of cold water. And a man that tends the work is but to turn two cocks, that one vessel of water being consumed, another begins to force and refill with cold water, and so successfully, the fire being tended and kept constant, which the selfsame person may likewise abundantly perform in the interim between the necessity of turning the said cocks.” On this evidence the claim is made that the marquis was the original inventor of the steam engine. Is he at all entitled to the honour? The whole affair is still surrounded with mystery. It is known that he was an enthusiastic student of physical science, and that for years he had working for him a Dutch mechanic, Caspar Kaltoff; it seems certain that he actually made a water-pumping engine worked by steam, of whose value he was so impressed that he promised to leave the drawings of it to Gresham College and intended to have a model of it buried with him.[72] But neither model nor drawings has ever yet been traced. And, considering the social influence of the inventor and the importance of the invention, the silence of his contemporaries on the discovery is strange and inexplicable. He received a patent for some form of water-pumping engine. Distinguished visitors came to Vauxhall to see his engine at work. He numbered among his acquaintances Sir Jonas Moore, Sir Samuel Morland, Flamstead and Evelyn: probably Mr. Pepys, Sir W. Petty, and others of the group of eminent men of his time who were interested in natural science. Yet no trace of his inventions has come down to us. His _Century_ was admittedly compiled from memory--“my former notes being lost”--and perhaps it was designedly obscure; science was at that time a hobby of the cultured few, and scientific men loved to mystify each other by the exhibition, without explanation, of paradoxes and toys of their own construction. The marquis, it will be agreed, left valuable hints to later investigators. Whether his claim to have invented the steam engine is sufficiently substantiated, we leave to the opinion of the interested reader, who will find most of the evidence on this subject in Dirck’s _Life of the Marquis of Worcester_. The power of steam to drive water from a lower to a higher level had been shown by Solomon de Caus,[73] who, in his work, _Les Raisons des Forces Mouvantes_, published in A.D. 1615, had described a hot-water fountain operated by heating water in a globe. In Van Etten’s _Récreation Mathematique_ of 1629 was an experiment, described fifty years later by Nathaniel Nye in his _Art of Gunnery_ as a “merry conceit,” showing how the force of steam could be used to discharge a cannon. As the century advanced the ornamental was gradually superseded by the utilitarian; the usefulness of steam for draining fens, pumping out mines, was realized; and applications for patents to cover the use of new and carefully guarded inventions began to appear. Gunpowder as a medium was a strong competitor of steam. In 1661 King Charles granted to Sir Samuel Morland, his master of mechanics, “for the space of fourteen years, to have the sole making and use of a new invention of a certain engine lately found out and devised by him, for the raising of water out of any mines, pits, or other places, to any reasonable height, and by the force of air and powder conjointly.” What form the engine took is not known; whether the gunpowder was used to produce a gaseous pressure by which the work was done, or whether its function was to displace air and thus cause a vacuum as its gases cooled. In France, too, efforts were made at this time to produce a gunpowder engine. In 1678 a Jean de Hautefeuille raised water by gunpowder, but authorities differ as to whether he employed a piston--which were then in use as applied to pumps--or whether he burned the powder so that the gases came in actual contact with the water. In the following year an important advance was made. Huyghens constructed an engine having a piston and cylinder, in which gunpowder was used to form a vacuum, the atmospheric pressure providing the positive force to produce motion; and in 1680 he communicated to the Academy of Sciences a paper entitled, “A new motive power by means of gunpowder and air.” But it was to his brilliant pupil, Denis Papin, that we are indebted for a further step in the materialization of the steam engine. Papin suggested the use of steam for gunpowder. In 1680 Papin, who like Solomon de Caus had brought his scientific conceptions to England in the hope of their furtherance, was admitted on the recommendation of Boyle to a fellowship of the Royal Society. After a short absence he returned to London in ’84 and filled for a time the post of curator to the society, meeting, doubtless, in that capacity the leading scientists of the day and coming in touch with all the practical efforts of English inventors. During his stay here he worked with enthusiasm at the production of a prime mover, and when he left in ’87 for a mathematical professorship in Germany he continued there his researches and experienced repeated failures. In a paper published in ’88 he showed a clear conception of a reciprocating engine actuated by atmospheric pressure, and in ’90 he suggested for the first time the use of steam for forming the vacuum required. As water, he wrote, has elasticity when fire has changed it into vapour, and as cold will condense it again, it should be possible to make engines in which, by the use of heat, water would provide the vacuum which gunpowder had failed to give. This memorable announcement gave a clear direction to the future development of the heat engine. Steam was the medium best suited for utilizing the expansive power of heat generated by the combustion of fuel; steam was the medium which, by its expansive and contractile properties, could be made to impart a movement _de va et vient_ to a piston. Though Papin did not succeed in putting his idea into practical form his conception was of great value, and he must be counted as one of the principal contributors to the early development of the steam engine. His life was an accumulation of apparent failures ending in abject poverty. To-day he is honoured by France as the inventor of the steam engine, and at Blois a statue has been erected and a street named to his memory. Before the end of the century an effective engine had been produced, in England. In 1698 Thomas Savery, a Devonshire man, obtained a patent for “a new invention for raising of water and occasioning motion to all sorts of millwork by the impellent force of fire.” Before the king at Hampton Court a model of this invention was displayed, and the importance of the new discovery was soon realized by the landed classes; for in the following year an act of parliament was passed for the encouragement of the inventor and for his protection in the development of what, it was recognized, was likely to prove of great use to the public. In the same year Savery published a pamphlet called _The Miner’s Friend_, and republished it, with additions, in 1702. This pamphlet contained a full and clear description of his engine; but significance has been attached to the omission from it of any claim that it embodied a new idea. The omission may be accidental. The steam engine, shown in the accompanying illustration, was simply a pump, whose cycle of operations was as follows. Steam, admitted into the top of a closed vessel containing water and acting directly against the water, forced it through a pipe to a level higher than the vessel itself. Then, the vessel being chilled and the steam in it thereby condensed, more water was sucked into the vessel from a lower level to fill the vacuum thus formed; this water was expelled by steam in the same way as before, cocks being manipulated, and, eventually, self-acting valves being placed, so as to prevent the water from returning by the way it came. Two chambers were used, operating alternately. For this achievement Savery is by many regarded as the first and true inventor. He certainly was the first to make the steam engine a commercial success, and up and down the country it was extensively used for pumping water and for draining mines. By others Savery was regarded as a copyist; and indeed it is difficult to say how far originality should be assigned him. The marquis too had claimed to raise water; his engine had evidently acted with a pair of displacement-chambers, from each of which alternately water was forced by steam while the other vessel was filling. And if he did not specify or appreciate the effect of the contractile force of the steam when condensed, yet in this respect both inventors had been anticipated by Giovanni della Porta. [Illustration: Steam from Boiler. SAVERY’S ENGINE] The marquis had a violent champion in Dr. Desaguliers, who in his _Experimental Philosophy_, published in 1743, imputed disreputable conduct to the later inventor. “Captain Savery,” said the doctor, “having read the Marquis of Worcester’s book, was the first who put into practice the raising of water by fire. His engine will easily appear to have been taken from the Marquis of Worcester; though Captain Savery denied it, and the better to conceal the matter, bought all the Marquis of Worcester’s books that he could purchase in Pater-Noster Row and elsewhere, and burned them in the presence of the gentleman his friend, who told me this. He said that he found out the power of steam by chance, and invented the following story to persuade people to believe it, viz. that having drunk a flask of Florence at a tavern, and thrown the empty flask upon the fire, he called for a bason of water to wash his hands, and perceiving that the little wine left in the flask had filled the flask with steam, he took the flask by the neck and plunged the mouth of it under the surface of the water in the bason, and the water in the bason was immediately driven up into the flask by the pressure of the air. Now, he never made such an experiment then, nor designedly afterwards, which I shall thus prove,” etc. etc. Other writers saw no good reason for depriving the captain of the title of inventor. With reference to the book-burning allegation, the only evidence tending to substantiate it lay in the fact that the book “on a sudden became very scarce, and but few copies of it were afterwards seen, and then only in the libraries of the curious.”[74] It has been remarked, also, that Desaguliers was himself to some extent a rival claimant, several improvements, such as the substitution of jet for the original surface condensation being due to him; and that this fact gave a palpable bias to his testimony on the work of others. In recent years the claims of Savery have been upheld, as against those of the marquis, by a writer who argued, not only that the engine of the marquis had never passed the experimental stage, but that no counter-claim was made by his successors at the time Savery produced his engine and obtained his patent. “Although a patent for ninety-nine years (from 1663 to 1762) was granted the marquis, yet Captain Savery and his successors under his patents which extended for thirty-five years (from 1698 to 1733) compelled every user of Newcomen’s and other steam engines to submit to the most grinding terms and no one attempted to plead that Savery’s patents were invalidated by the Marquis of Worcester’s prior patents.”[75] By the admirers of Papin it has been claimed that it was from him that Savery received his idea. “After having minutely compared Savery’s machine,” says a biographer of Papin, “one arrives at the conviction that _Savery discovered nothing_. He had borrowed from Solomon de Caus the use of steam as a motive force, perfected by the addition of a second chamber; from Papin, the condensation of the steam.... And as for the piston, borrowed ten years later by Newcomen, that was wholly Papin’s.”[76] Suppose it true; even so, his countrymen would always think great credit attaches to Savery for his achievement. His engine, though used extensively for lifting water through small distances, was exceedingly wasteful of fuel, nor could it be used conveniently for pumping out mines or for other purposes in which a large lift was required. The lift or “head” was directly proportional to the steam pressure. Efforts to improve the lift by augmenting the steam pressure resulted in endless accidents and discouragement; the solder of the engine melted when steam of a higher pressure was used, the joints blew open and the chambers burst. Living at Dartmouth, within some fifteen miles of Savery’s home, were two men, Newcomen, an ironmonger, and Cawley, a glazier. These two had, doubtless, every opportunity of seeing Savery’s engine at work. They appreciated its limitations and defects, and, undertaking the task of improving it, they so transformed the steam engine that within a short time their design had almost entirely superseded the more primitive form. Here, too, it might be said that they invented nothing. The merit of their new machine consisted in the achievement in practical form of ideas which hitherto had had scarcely more than an academic value. The labours of others gave them valuable aid. Newcomen, it is certain, could claim considerable knowledge of science, and though little is known of his personality there is evidence that he had pursued for years the object which he now achieved. He knew of the previous forms of piston engine which had been invented. He had probably read a translation, published in the _Philosophical Transactions_, of Papin’s proposal for an atmospheric engine with a vacuum produced by the condensation of steam. He obtained from Savery the idea of a separate boiler, and other details. And where Papin had failed, Newcomen and his partner succeeded. Their Atmospheric Steam Engine, as it was aptly called, was produced in the year 1705, and at once proved its superiority over the old “Miner’s Friend.” It had assumed an entirely new form. In a large-bore vertical cylinder a brass piston was fitted, with a leather flap round its edge and a layer of water standing on it to form a seal against the passage of steam or air. The top of the cylinder was open to the atmosphere, the bottom was connected by a pipe with a spherical boiler. The piston was suspended by a chain to one end of an overhanging timber beam, which was mounted on a brick structure so as to be capable of oscillating on a gudgeon or axis at its middle. One end of this beam was vertically over the piston; at the other end was the bucket of a water-pump, also attached to a crosspiece or “horse-head,” by means of a chain or rod. The whole machine formed a huge structure like a pair of scales, one of which (the water-pump) was loaded with weights so as to be slightly heavier than the other (the steam engine). [Illustration: NEWCOMEN’S ENGINE] To work it, steam was generated in the boiler at a pressure slightly greater than atmospheric. By the opening of a cock steam was admitted to the cylinder, below the piston, which was initially at rest in its highest position. The steam having filled the cylinder and expelled nearly all the air, the cock was shut and the cylinder was chilled by an external spray of cold water. Whereupon, as soon as the steam in the cylinder began to condense, the piston, forced down by the now unbalanced atmospheric pressure above it, began to descend. As soon as it had completed its downward stroke steam was again admitted beneath the piston, and, the pressure on the two sides of the piston becoming equal, the piston began to move up again to its original position. And so on. This was the original Newcomen engine. Even in this primitive form it far surpassed Savery’s in economy of fuel and in safety. It had, too, far greater flexibility in the manner in which its power could be applied; it could be used not only to lift a certain volume of water through a relatively small height, but a smaller volume through a greater height: which was a desideratum in the case of deep mines like those of Cornwall. In 1720 an engine was erected at Wheal Fortune mine having a cylinder nearly four feet in diameter and drawing water, at fifteen strokes a minute, from a depth of 180 feet. Yet it was apparent that the engine was in many respects inefficient. The cocks, for instance, which controlled the motion of the piston had to be opened and shut by a man. Sometimes he let the piston rise too far, in fact, right out of the cylinder; sometimes he let it down too fast, so as to damage the engine. Again, the external spraying of the cylinder at every stroke to induce condensation of the steam within was an obviously clumsy and primitive operation. It was not long before external spraying gave place to internal cooling of the steam by the injection of water; this method being discovered, it is said, as the result of a leaky piston allowing its sealing water to pass, yet giving unaccountably good results. The difficulties with the cocks were overcome by the laziness or initiative of a youth named Humphrey Potter, who attached some strings and catches to the cocks of an engine which he was employed to work at Wolverhampton.[77] With these improvements the engine remained practically without alteration for the next forty years. Its greatest sphere of usefulness was in the northern coalfields, where cheap and abundant fuel was close at hand. In Cornwall, until by special legislation the duty on seaborne coal was remitted when used for Newcomen’s engine, the cost of fuel proved a great obstacle to its use. § In 1764 James Watt, an instrument maker employed on work for Glasgow College, was given the task of repairing a working model of a Newcomen engine. A man of serious and philosophical mind, an intimate friend of Professor Robison, the physicist, and acquainted with the famous Dr. Black of Edinburgh, then in the thick of his researches on the phenomena of latent heat, Watt often discussed with these two scientists the possibility of improving the steam engine; which apparatus was still only employed for the purpose of pumping water, and which was so clumsy and so wasteful of fuel as to be comparatively little used. To this end he was induced to try some experiments on the production and condensation of steam. The results of these, and a knowledge of the newly discovered phenomenon of latent heat,[78] convinced him that the existing cycle of operations in the engine was fundamentally inefficient, and that improvement was to be sought in the engine itself rather than in the boiler, which was the element which was receiving most attention from contemporary investigators. In particular, he clearly discerned the thermal inefficiency of the Newcomen engine: the waste of heat involved in alternately heating and cooling the large metal cylinder, which absorbed such immense quantities of fuel. Watt’s first idea was, to lag the cylinder in wood so as to prevent all outward radiation. But the result of a trial of a lagged cylinder was disappointing. A gain was certainly obtained in that the steam, when admitted to the cylinder, did not require to raise by partial condensation the temperature of the walls; it exerted its expansive force at once and the piston rose. But on the other hand much greater difficulty was experienced in condensing it when a vacuum was required, for the down stroke. Moreover it was observed that an increase in the amount of injection water only made matters worse. Watt was faced with a dilemma, and he overcame it by a series of studies in the properties of steam which constitute, perhaps, the highest achievement of this workman-philosopher. Out of all his experiments two conclusions were drawn by him; first, that the lower the temperature of condensation of steam the more perfect the vacuum thereby formed; second, that the temperature of the cylinder should be as nearly as possible equal to that of the steam admitted to it. In Newcomen’s engine these two conditions were obviously incompatible, and the problem was,--how could they be reconciled? Early in 1765, while walking one Sunday afternoon in Glasgow Green the idea flashed upon him of condensing the steam in a separate vessel. The steam was generated in a separate vessel, why not produce the vacuum separately? With a view to trying this effect he placed a hollow air-tight chest beneath the steam cylinder, connected with it by a pipe having a stop-cock in it. This new or lower vessel was immersed in a cistern of cold water. Upon trial being made, it was found that by this simple contrivance as perfect a vacuum as desired was produced; the speed of the engine was greatly increased, the expenditure of fuel radically reduced, the walls of the steam cylinder were maintained at a high and constant temperature, and the whole arrangement promised great success. The new vessel Watt called a Condenser. Fresh difficulties now arose. As the engine worked, the condenser gradually filled with the condensed steam and had to be emptied periodically. The water in which it was immersed became so hot, by absorbing the heat of the steam, that it frequently required changing. Watt promptly called in aid two new auxiliaries, two organs whose motion was derived from the main beam of the engine: the Air Pump and the Circulating Pump. By these expedients the action of the condenser was rendered satisfactory, and an engine resulted which had a fuel-consumption less than half that of Newcomen’s engine. Much, he saw, yet remained to be done to obtain economical expenditure of steam. In particular the open-topped cylinder, whose walls were chilled at every descent of the piston by contact with atmospheric air, was an obvious source of inefficiency. He therefore determined not to expose the walls to the atmosphere at all, but to enclose all the space above the piston; and, thinking thus, he conceived the idea of replacing the air above the piston by steam, an equally powerful agent. The cylinder he proposed to maintain at a constant high temperature by means of a layer of hot steam with which he encased it, which he called a steam jacket. And so the atmospheric engine as left by Newcomen evolved into the _single-acting steam engine_ of Watt;--an engine in which steam was still used below the piston, only to displace air and provide a vacuized space for the downward motion of the piston; but in which steam now acted positively above the piston, in lieu of atmospheric air, to drive it down. It was still a sufficiently primitive form of prime mover. The piston was still lifted by the counterweight at the other end of the timber cross-beam; the engine had not yet developed the organs necessary for producing a satisfactory rotary motion. This step was shortly to follow. In 1769 Watt obtained his patent for the “double impulse,” as it was called; and by this step, by the transition from a single- to a double-acting engine, the possibilities of such machines for every variety of application first came into general view. This stage of the development showed to the full the ingenuity of Watt’s mechanical mind. By the invention of the slide-valve he distributed steam to the top and to the bottom of the cylinder, and in appropriate phase with these actions opened the two ends to the condenser; so that the piston was actuated positively and by an equal force on both up and down strokes. The chain by which the piston had been suspended was no longer adequate; it was replaced by a rod. A straight-line motion was required for the top end of the rod; so he formed a rack, to gear with the circular end or horse-head of the beam. But this noisy mechanism was soon superseded by another contrivance, the beautifully simple “parallel motion,” in which two circular motions are combined to produce one which is rectilinear. This was patented in ’84. Four years before this, that ancient mechanism the crank and connecting rod had been applied, together with a flywheel, to transform the reciprocating motion of a steam engine into a rotary motion; and the non-possession of this invention of James Pickard’s proved for a time a stumbling-block to Watt in his further development of his engine. Watt would have nothing to do with it. By now he had joined his fortunes with those of Mr. Boulton, of Soho, Birmingham, a man of great business ability, in conjunction with whom he was engaged in constructing engines in large numbers to suit the varying conditions of the mines in Cornwall and the North. Considerable ingenuity was expended by him in trying to circumvent the troublesome crank of Pickard, and many devices were produced, the most noteworthy being the “sun-and-planet wheels,” which enabled him with some sacrifice of simplicity to obtain the rotary motion desired. Watt seemed to be borne along by the momentum of his own discoveries; every inquiry yielded him valuable reward. For some time he had studied the possibility of reducing the violence with which the piston, now positively steam-driven on both sides, came to the end of its stroke. This problem led him to the discovery of the advantage of using steam expansively: of cutting off the inflow of steam before the piston had travelled more than a fraction of its stroke, and letting its inherent elastic force impel it through the remainder of its journey, the steam meanwhile expanding and thus exerting a continuously decreasing force. Later came the throttle valve, and the centrifugal governor for controlling the speed of rotating engines; there was no end to his ingenuity. And so complete was his inquiry into the possible sources of improvement of the steam engine, that he even considered means of regulating the force which the piston exerted on the crank throughout its working stroke, a force which was compounded of the steam pressure itself and of the mass-acceleration of the piston and other moving parts. Another cardinal invention followed: the Indicator. The principle of the indicator is now applied to every form and kind of piston engine. It is a reproduction on a small scale of the essential part of the engine itself; a small piston, held by a spring and moving in a cylinder connected by a pipe with the cylinder of the engine itself, shows by the degree of compression imparted to the spring the gaseous pressure actually present at any moment in the engine cylinder. By recording the position of the indicator piston on a paper wrapped round a rotating drum whose motion represents the motion of the engine’s piston, a diagram is obtained which by its area measures the work done by the steam during the stroke of the engine. This instrument was designed by Watt to give his firm some standard of work which would serve as a basis for the power of each engine, on which to charge their customers; their engines being sold by the horse-power. But its usefulness far exceeded the immediate purpose for which it was produced. Its diagram, to the eye of an expert, gave valuable information in respect of the setting of the valves, the tightness of the piston, the dryness of the steam, the degree of vacuum in the condenser, and, generally, of the state of efficiency of the engine. “It would be difficult to exaggerate the part which this little instrument has played in the evolution of the steam engine. The eminently philosophic notion of an indicator diagram is fundamental in the theory of thermodynamics; the instrument itself is to the steam engineer what the stethoscope is to the physician, and more, for with it he not only diagnoses the ailments of a faulty machine, whether in one or another of its organs, but gauges its power in health.”[79] § We have now traced the evolution of the steam engine up to the time when it was first adapted to the propulsion of war-vessels. There we must leave it. In a later chapter we shall consider the evolution of the propelling machinery in its relation, especially, to the military qualities of ships. A few observations will be sufficient to illustrate the conditions, as to design, practice, and material, under which the steam engine made its appearance in the royal navy. After the death of Watt all improvement of steam machinery was strenuously opposed by the combined force of prejudice and vested interest. The great Watt himself had set his face against the use of high-pressure steam, and, such was the lingering force of his authority, years passed before the general public gave assent to the advances made by his talented successors--Hornblower, Woolf, Evans, and Trevithick. Before the end of the eighteenth century the first steps had been made to use the force of steam for driving ships. Before Trafalgar was fought steam engines had made their appearance in the royal dockyards. Then there was a pause; and many years passed by before steam propulsion was admitted to be a necessity for certain classes of war-vessels. An interesting account of the state of design and practice as it existed on ship-board in the year of Queen Victoria’s accession is given by Commander Robert Otway, R.N., in his treatise on _Steam Navigation_. Low-pressure principles are still in vogue; steam is generated still, at a pressure not exceeding three pounds per square inch, in rectangular boilers of various forms according to the fancy of the maker, scarcely two being alike. The engines are also of varying forms, every size, variety, and power being deemed suitable for similar vessels. They are amazingly ponderous: weigh about twelve hundredweight, and the boilers eight hundredweight, to the horsepower. The engines of all makers exhibit the greatest variations in the relative dimensions of their various parts: one firm embodies a massive frame and light moving rods and shafts, another adopts massive rods and shafts, and supports them within the lightest framework. The author advocates a correct design and a “total dispensation of all superfluous ornament.” [Illustration: CONNECTING ROD From Otway] Already, however, following the example of the Cornish mines, the builders of steam vessels were at this time beginning to adopt high-pressure steam, generated at a pressure of ten to fifteen pounds per square inch in cylindrical boilers, and working expansively--“doing work in the cylinder by its elasticity alone”--before returning to the jet condenser. This improvement, strenuously opposed by orthodox engineers as being unsafe for ship practice, was introduced first into the Packet Establishment at Falmouth, and then, tardily, into Government steamers. It gave a gain in economy measured by the saving of “thousands of bushels of coal per month.” Steam engines working on the low-pressure system used from nine to twelve pounds of coal per hour, for each horse-power. These engines were carried in vessels “built on the scantling of 10-ton brigs,” of great draught and of such small coal capacity--about 35 tons, on an average--that when proceeding out of home waters “they were burthened with, at the least, four days’ more fuel, _on their decks_ (top hamper), in addition to that which already filled up their coal-boxes below.” Boilers emitted black clouds of smoke at sea. In harbour the paddle-wheels had to be turned daily, if but a few float-boards only, by the united force of the crew. “Coaling ship” was carried out with the help of convicts from the hulks:--“pampered delinquents,” observes the author, “whose very movements are characteristic of their moral dispositions--being thieves of time; for their whole day’s duty is not worth an hour’s purchase.” In these unattractive circumstances the steam engine, most wonderful contrivance of the brain and hand of man, presented itself for embodiment in the navy, by the personnel of which it was regarded, not without reason, as an unmitigated evil.