At this point the irrepressible Mabel was heard to whisper to Bedford, who sat next her: "Wasn't it curious that the same mind which grasped the immense capabilities of the steam-engine should have been able also to construct such a delicate lyric as

'How doth the little busy bee Improve each shining hour'?"

"Mabel," said Colonel Ingham, "you are absolutely unbearable. If you do not keep in better order I shall be sorry that I dissuaded you from descending to the kitchen. I see nothing incongruous myself in indulging in mechanical experiments, and in throwing one's thoughts into the form of verse,"--here the old gentleman colored slightly, as though he recollected something of the sort,--"but it may be well to counteract the impression your conversation may have made by stating that Isaac Watts did not invent the steam-engine, nor did James Watt write the beautiful words you have just quoted.--Now, Clem, I believe you have the floor."

"Well," said Clem, "I only want the floor for a short time in order to explain about Watt and the steam-engine, and how much he was the inventor of it, before we begin to read.

"There are various points about the steam-engine which are really Watt's invention,--the separate condenser, for instance,--but the idea of the steam-engine was not original with him; that is, when he saw the steam in the teakettle raise the lid and drop it again, he was not the first to speculate on the power of steam."

"Are you going to read us that part in the book, Clem?" asked Bedford, with some interest.

"Yes, if you like," said Clem. "I guess it tells about it in Mr.

Smiles's 'Life of Watt.'" So he began to overhaul the book he had brought, and shortly discovered the anecdote referred to by Mabel with such interest, and read it.

"On one occasion he [James Watt] was reproved by Mrs. Muirhead, his aunt, for his indolence at the tea-table. 'James Watt,' said the worthy lady, 'I never saw such an idle boy as you are. Take a book, or employ yourself usefully; for the last hour you have not spoken one word, but taken off the lid of that kettle and put it on again, holding now a cup and now a silver spoon over the steam, watching how it rises from the spout, catching and counting the drops it falls into.' In the view of M.

Arago, the little James before the teakettle, becomes the great engineer, preparing the discoveries which were soon to immortalize him.

In our opinion, the judgment of the aunt was the truest. There is no reason to suppose that the mind of the boy was occupied with philosophical theories on the condensation of steam, which he compa.s.sed with so much difficulty in his maturer years. This is more probably an afterthought borrowed from his subsequent discoveries. Nothing is commoner than for children to be amused with such phenomena in the same way that they will form air-bubbles in a cup of tea, and watch them sailing over the surface till they burst. The probability is that little James was quite as idle as he seemed."

"That is very interesting," remarked Mabel. "Don't you think now, Uncle Fritz, we had better go into the kitchen?" And she looked appealingly at the old gentleman, who merely held up his finger for silence as Clem continued his lecture.

"What I meant to say," Clem went on, "was that other people before Watt had found out the power of steam, and had used it too. There was one Hero of Alexandria, who lived about two thousand years ago, who used steam for many interesting purposes, notably for animating various figures that took part in the idolatrous worship of his time, and thus in deceiving the common people. But his contrivances, though engines which went by steam, would hardly be called steam-engines. Between Hero of Alexandria, of 160 B. C., and the Marquis of Worcester, of 1650 A.

D., there does not seem to have been much doing in the way of inventing the steam-engine. But the Marquis of Worcester in Charles II.'s time was a great philosopher, and did n.o.body knows exactly what with steam. But though he did great things, he did not produce a particularly capable engine, though he seems to have known more about steam than anybody else did at his time. After the Marquis of Worcester and before Watt, there were three men who did much towards inventing and improving the steam-engine. Their names were Savery, Papin, and Newcomen. I don't propose to tell you about the inventions of each one; but it's well enough to remember that each one did important service in getting the steam-engine to the point where Watt took hold of it. As it was on Newcomen's engine that Watt made his first serious experiments, I think we should all like to know something about it."

THE NEWCOMEN ENGINE.

Newcomen's engine may be thus briefly described: The steam was generated in a separate boiler, as in Savery's engine, from which it was conveyed into a vertical cylinder underneath a piston fitting it closely, but movable upwards and downwards through its whole length. The piston was fixed to a rod, which was attached by a joint or chain to the end of a lever vibrating upon an axis, the other end being attached to a rod working a pump. When the piston in the cylinder was raised, steam was let into the vacated s.p.a.ce through a tube fitted into the top of the boiler, and mounted with a stopc.o.c.k. The pump-rod at the further end of the lever being thus depressed, cold water was applied to the sides of the cylinder, on which the steam within it was condensed, a vacuum was produced, and the external air, pressing upon the top of the piston, forced it down into the empty cylinder. The pump-rod was thereby raised; and, the operation of depressing it being repeated, a power was thus produced which kept the pump continuously at work. Such, in a few words, was the construction and action of Newcomen's first engine.[8]

While the engine was still in its trial state, a curious accident occurred which led to a change in the mode of condensation, and proved of essential importance in establishing Newcomen's engine as a practical working power. The accident was this: in order to keep the cylinder as free from air as possible, great pains were taken to prevent it pa.s.sing down by the side of the piston, which was carefully wrapped with cloth or leather; and, still further to keep the cylinder air-tight, a quant.i.ty of water was kept constantly on the upper side of the piston.

At one of the early trials the inventors were surprised to see the engine make several strokes in unusually quick succession; and on searching for the cause, they found it to consist in _a hole in the piston_, which had let the cold water in a jet into the inside of the cylinder, and thereby produced a rapid vacuum by the condensation of the continued steam. A new light suddenly broke upon Newcomen. The idea of condensing by injection of cold water directly into the cylinder, instead of applying it on the outside, at once occurred to him; and he proceeded to embody the expedient which had thus been accidentally suggested as part of his machine. The result was the addition of the injection pipe, through which, when the piston was raised and the cylinder full of steam, a jet of cold water was thrown in, and, the steam being suddenly condensed, the piston was at once driven down by the pressure of the atmosphere.

An accident of a different kind shortly after led to the improvement of Newcomen's engine in another respect. To keep it at work, one man was required to attend the fire, and another to turn alternately the two c.o.c.ks, one admitting the steam into the cylinder, the other admitting the jet of cold water to condense it. The turning of these c.o.c.ks was easy work, usually performed by a boy. It was, however, a very monotonous duty, though requiring constant attention. To escape the drudgery and obtain an interval for rest or perhaps for play, a boy named Humphrey Potter, who turned the c.o.c.ks, set himself to discover some method of evading his task. He must have been an ingenious boy, as is clear from the arrangement he contrived with this object. Observing the alternate ascent and descent of the beam above his head, he bethought him of applying the movement to the alternate raising and lowering of the levers which governed the c.o.c.ks. The result was the contrivance of what he called the _scoggan_ (meaning presumably the loafer or lazy boy), consisting of a catch worked by strings from the beam of the engine. This arrangement, when tried, was found to answer the purpose intended. The action of the engine was thus made automatic; and the arrangement, though rude, not only enabled Potter to enjoy his play, but it had the effect of improving the working power of the engine itself; the number of strokes which it made being increased from six or eight to fifteen or sixteen in the minute. This invention was afterward greatly improved by Mr. Henry Beighton, of Newcastle-on-Tyne, who added the plug-rod and hand-gear. He did away with the catches and strings of the boy Potter's rude apparatus, and subst.i.tuted a rod suspended from the beam, which alternately opened and shut the tappets attached to the steam and injection c.o.c.ks.

Thus, step by step, Newcomen's engine grew in power and efficiency, and became more and more complete as a self-acting machine. It will be observed that, like all other inventions, it was not the product of any one man's ingenuity, but of many. One contributed one improvement, and another another. The essential features of the atmospheric engine were not new. The piston and cylinder had been known as long ago as the time of Hero. The expansive force of steam and the creation of a vacuum by its condensation had been known to the Marquis of Worcester, Savery, Papin, and many more. Newcomen merely combined in his machine the result of their varied experience; and, a.s.sisted by the persons who worked with him, down to the engine-boy Potter, he advanced the invention several important stages; so that the steam-engine was no longer a toy or a scientific curiosity, but had become a powerful machine capable of doing useful work.

JAMES WATT AND THE STEAM-ENGINE.

It was in the year 1759 that Robison[9] first called the attention of his friend Watt to the subject of the steam-engine. Robison was then only in his twentieth, and Watt in his twenty-third year. Robison's idea was that the power of steam might be advantageously applied to the driving of wheel-carriages; and he suggested that it would be the most convenient for the purpose to place the cylinder with its open end downwards to avoid the necessity of using a working-beam. Watt admits that he was very ignorant of the steam-engine at the time; nevertheless, he began making a model with two cylinders of tin plate, intending that the pistons and their connecting-rods should act alternately on two pinions attached to the axles of the carriage-wheels. But the model, being slightly and inaccurately made, did not answer his expectations.

Other difficulties presented themselves, and the scheme was laid aside because Robison left Glasgow to go to sea. Indeed, mechanical science was not yet ripe for the locomotive. Robison's idea had, however, dropped silently into the mind of his friend, where it grew from day to day, slowly and at length fruitfully.

At his intervals of leisure and in the quiet of his evenings, Watt continued to prosecute his various studies. He was shortly attracted by the science of chemistry, then in its infancy. Dr. Black was at that time occupied with the investigations which led to his discovery of the theory of latent heat, and it is probable that his familiar conversations with Watt on the subject induced the latter to enter upon a series of experiments with the view of giving the theory some practical direction. His attention again and again reverted to the steam-engine, though he had not yet seen even a model of one. Steam was as yet almost unknown in Scotland as a working power. The first engine was erected at Elphinstone Colliery, in Stirlingshire, about the year 1750; and the second more than ten years later, at Govan Colliery, near Glasgow, where it was known by the startling name of "The Firework."

This had not, however, been set up at the time Watt had begun to inquire into the subject. But he found that the college possessed the model of a Newcomen engine for the use of the Natural Philosophy cla.s.s, which had been sent to London for repair. On hearing of its existence, he suggested to his friend Dr. Anderson, Professor of Natural Philosophy, the propriety of getting back the model; and a sum of money was placed by the Senatus at the professor's disposal, "to recover the steam-engine from Mr. Sisson, instrument-maker in London."

In the mean time Watt sought to learn all that had been written on the subject of the steam-engine. He ascertained from Desaguliers, Switzer, and other writers, what had been accomplished by Savery, Newcomen, Beighton, and others; and he went on with his own independent experiments. His first apparatus was of the simplest possible kind. He used common apothecaries' phials for his steam reservoirs, and canes hollowed out for his steam-pipes. In 1761 he proceeded to experiment on the force of steam by means of a small Papin's digester and a syringe.

The syringe was only the third of an inch in diameter, fitted with a solid piston; and it was connected with the digester by a pipe furnished with a stopc.o.c.k, by which the steam was admitted or shut off at will. It was also itself provided with a stopc.o.c.k, enabling a communication to be opened between the syringe and the outer air to permit the steam in the syringe to escape. The apparatus, though rude, enabled the experimenter to ascertain some important facts. When the steam in the digester was raised and the c.o.c.k turned, enabling it to rush against the lower side of the piston, he found that the expansive force of the steam raised a weight of fifteen pounds, with which the piston was loaded. Then on turning on the c.o.c.k and shutting off the connection with the digester at the same time that a pa.s.sage was opened to the air, the steam was allowed to escape, when the weight upon the piston, being no longer counteracted, immediately forced it to descend.

Watt saw that it would be easy to contrive that the c.o.c.ks should be turned by the machinery itself with perfect regularity. But there was an objection to this method. Water is converted into vapor as soon as its elasticity is sufficient to overcome the weight of the air which keeps it down. Under the ordinary pressure of the atmosphere water acquires this necessary elasticity at 212; but as the steam in the digester was prevented from escaping, it acquired increased heat, and by consequence increased elasticity. Hence it was that the steam which issued from the digester was not only able to support the piston and the air which pressed upon its upper surface, but the additional load with which the piston was weighted. With the imperfect mechanical construction, however, of those days, there was a risk lest the boiler should be burst by the steam, which was apt to force its way through the ill-made joints of the machine. This, conjoined with the great expenditure of steam on the high-pressure system, led Watt to abandon the plan; and the exigencies of his business for a time prevented him from pursuing his experiments.

At length the Newcomen model arrived from London; and in 1763 the little engine, which was destined to become so famous, was put into the hands of Watt. The boiler was somewhat smaller than an ordinary teakettle. The cylinder of the engine was only of two inches diameter and six inches stroke. Watt at first regarded it as merely "a fine plaything." It was, however, enough to set him upon a track of thinking which led to the most important results. When he had repaired the model and set it to work, he found that the boiler, though apparently large enough, could not supply steam in sufficient quant.i.ty, and only a few strokes of the piston could be obtained, when the engine stopped. The fire was urged by blowing, and more steam was produced; but still it would not work properly. Exactly at the point at which another man would have abandoned the task in despair, the mind of Watt became thoroughly roused.

"Everything," says Professor Robison, "was to him the beginning of a new and serious study; and I knew that he would not quit it till he had either discovered its insignificance or had made something of it." Thus it happened with the phenomena presented by the model of the steam-engine. Watt referred to his books, and endeavored to ascertain from them by what means he might remedy the defects which he found in the model; but they could tell him nothing. He then proceeded with an independent course of experiments, resolved to work out the problem for himself. In the course of his inquiries he came upon a fact which, more than any other, led his mind into the train of thought which at last conducted him to the invention of which the results were destined to prove so stupendous. This fact was the existence of latent heat.

In order to follow the track of investigation pursued by Watt, it is necessary for a moment to revert to the action of the Newcomen pumping-engine. A beam, moving upon a centre, had affixed to one end of it a chain attached to the piston of the pump, and at the other a chain attached to a piston that fitted into the steam-cylinder. It was by driving this latter piston up and down the cylinder that the pump was worked. To communicate the necessary movement to the piston, the steam generated in a boiler was admitted to the bottom of the cylinder, forcing out the air through a valve, where its pressure on the under side of the piston counterbalanced the pressure of the atmosphere on its upper side. The piston, thus placed between two equal forces, was drawn up to the top of the cylinder by the greater weight of the pump-gear at the opposite extremity of the beam. The steam, so far, only discharged the office of the air it displaced; but if the air had been allowed to remain, the piston once at the top of the cylinder could not have returned, being pressed as much by the atmosphere underneath as by the atmosphere above it. The steam, on the contrary, which was admitted by the exclusion of air, _could be condensed_, and a vacuum created, by injecting cold water through the bottom of the cylinder. The piston, being now unsupported, was forced down by the pressure of the atmosphere on its upper surface. When the piston reached the bottom, the steam was again let in, and the process was repeated. Such was the engine in ordinary use for pumping water at the time that Watt began his investigations.

Among his other experiments, he constructed a boiler which showed by inspection the quant.i.ty of water evaporated in any given time, and the quant.i.ty of steam used in every stroke of the engine. He was astonished to discover that a _small_ quant.i.ty of water in the form of steam heated a large quant.i.ty of cold water injected into the cylinder for the purpose of cooling it; and upon further examination he ascertained that steam heated six times its weight of cold water to 212, which was the temperature of the steam itself. "Being struck with this remarkable fact," says Watt, "and not understanding the reason of it, I mentioned it to my friend Dr. Black, who then explained to me his doctrine of latent heat, which he had taught for some time before this period (the summer of 1764); but having myself been occupied by the pursuits of business, if I had heard of it I had not attended to it, when I thus stumbled upon one of the material facts by which that beautiful theory is supported."

When Watt found that water in its conversion into vapor became such a reservoir of heat, he was more than ever bent on economizing it; for the great waste of heat involving so heavy a consumption of fuel was felt to be the princ.i.p.al obstacle to the extended employment of steam as a motive power. He accordingly endeavored, with the same quant.i.ty of fuel, at once to increase the production of steam and to diminish its waste.

He increased the heating surface of the boiler by making flues through it; he even made his boiler of wood, as being a worse conductor of heat than the brickwork which surrounds common furnaces; and he cased the cylinders and all the conducting pipes in materials which conducted heat very slowly. But none of these contrivances were effectual; for it turned out that the chief expenditure of steam, and consequently of fuel, in the Newcomen engine, was occasioned by the reheating of the cylinder after the steam had been condensed, and the cylinder was consequently cooled by the injection into it of the cold water. Nearly four fifths of the whole steam employed was condensed on its first admission, before the surplus could act upon the piston. Watt therefore came to the conclusion that to make a perfect steam-engine it was necessary that _the cylinder should be always as hot as the steam that entered it_; but it was equally necessary that the steam should be condensed when the piston descended, nay, that it should be cooled down below 100, or a considerable amount of vapor would be given off, which would resist the descent of the piston, and diminish the power of the engine. Thus the cylinder was never to be at a less temperature than 212, and yet at each descent of the piston it was to be less than 100,--conditions which, on the very face of them, seemed to be wholly incompatible.

Though still occupied with his inquiries and experiments as to steam, Watt did not neglect his proper business, but was constantly on the look-out for improvements in instrument-making. A machine which he invented for drawing in perspective proved a success; and he made a considerable number of them to order, for customers in London as well as abroad. He was also an indefatigable reader, and continued to extend his knowledge of chemistry and mechanics by perusal of the best books on these sciences.

Above all subjects, however, the improvement of the steam-engine continued to keep the fastest hold upon his mind. He still brooded over his experiments with the Newcomen model, but did not seem to make much way in introducing any practical improvement in its mode of working. His friend Robison says he struggled long to condense with sufficient rapidity without injection, trying one experiment after another, finding out what would _not_ do, and exhibiting many beautiful specimens of ingenuity and fertility of resource. He continued, to use his own words, "to grope in the dark, misled by many an _ignis fatuus_." It was a favorite saying of his that "Nature has a weak side, if we can only find it out;" and he went on groping and feeling for it, but as yet in vain.

At length light burst upon him, and all at once the problem over which he had been brooding was solved.

THE SEPARATE CONDENSER.

One Sunday afternoon, in the spring of 1765, he went to take an afternoon walk on the Green, then a quiet gra.s.sy meadow used as a bleaching and grazing ground. On week days the Glasgow la.s.ses came thither with their largest kail-pots to boil their clothes in; and st.u.r.dy queans might be seen, with coats kilted, trampling blankets in their tubs. On Sundays the place was comparatively deserted; and hence Watt, who lived close at hand, went there to take a quiet afternoon stroll. His thoughts were as usual running on the subject of his unsatisfactory experiments with the Newcomen engine, when the first idea of the separate condenser suddenly flashed upon his mind. But the notable discovery is best told in his own words, as related to Mr.

Robert Hart, many years after:--

"I had gone to take a walk on a fine Sabbath afternoon. I had entered the Green by the gate at the foot of Charlotte Street, and had pa.s.sed the old washing-house. I was thinking upon the engine at the time, and had gone as far as the herd's house, when the idea came into my mind that as the steam was an elastic body, it would rush into a vacuum, and if a communication were made between the cylinder and an exhausted vessel, it would rush into it and might be then condensed without cooling the cylinder. I then saw that I must get rid of the condensed steam and the injection water if I used a jet, as in Newcomen's engine.

Two ways of doing this occurred to me. First, the water might be run off by a descending pipe, if an off-let could be got at the depth of 35 or 36 feet, and any air might be extracted by a small pump. The second was to make the pump large enough to extract both water and air." He continued: "I had not walked farther than the Golf-house when the whole thing was arranged in my mind."

Great and prolific ideas are almost always simple. What seems impossible at the outset appears so obvious when it is effected, that we are p.r.o.ne to marvel that it did not force itself at once upon the mind. Late in life Watt, with his accustomed modesty, declared his belief that if he had excelled, it had been by chance, and the neglect of others. To Professor Jardine he said that when it was a.n.a.lyzed the invention would not appear so great as it seemed to be. "In the state," said he, "in which I found the steam-engine, it was no great effort of mind to observe that the quant.i.ty of fuel necessary to make it work would forever prevent its extensive utility. The next step in my progress was equally easy,--to inquire what was the cause of the great consumption of fuel: this, too, was readily suggested, viz., the waste of fuel which was necessary to bring the whole cylinder, piston, and adjacent parts from the coldness of water to the heat of steam, no fewer than from fifteen to twenty times in a minute." The question then occurred, How was this to be avoided or remedied? It was at this stage that the idea of carrying on the condensation in a separate vessel flashed upon his mind, and solved the difficulty.

Mankind has been more just to Watt than he was to himself. There was no accident in the discovery. It had been the result of close and continuous study; and the idea of the separate condenser was merely the last step of a long journey, a step which could not have been taken unless the road which led to it had been traversed. Dr. Black says, "This capital improvement flashed upon his mind at once, and filled him with rapture,"--a statement which, in spite of the unimpa.s.sioned nature of Watt, we can readily believe.

On the morning following his Sunday afternoon's walk on Glasgow Green, Watt was up betimes, making arrangements for a speedy trial of his new plan. He borrowed from a college friend a large bra.s.s syringe, an inch and a third in diameter, and ten inches long, of the kind used by anatomists for injecting arteries with wax previous to dissection. The body of the syringe served for a cylinder, the piston-rod pa.s.sing through a collar of leather in its cover. A pipe connected with the boiler was inserted at both ends for the admission of steam, and at the upper end was another pipe to convey the steam to the condenser. The axis of the stem of the piston was drilled with a hole, fitted with a valve at its lower end, to permit the water produced by the condensed steam on first filling the cylinder to escape. The first condenser made use of was an improvised cistern of tinned plate, provided with a pump to get rid of the water formed by the condensation of the steam, both the condensing-pipes and the air-pump being placed in a reservoir of cold water.

"The steam-pipe," says Watt, "was adjusted to a small boiler. When the steam was produced, it was admitted into the cylinder, and soon issued through the perforation of the rod and at the valve of the condenser; when it was judged that the air was expelled, the steam-c.o.c.k was shut, and the air-pump piston-rod was drawn up, which leaving the small pipes of the condenser in a state of vacuum, the steam entered them, and was condensed. The piston of the cylinder immediately rose, and lifted a weight of about eighteen pounds, which was hung to the lower end of the piston-rod. The exhaustion-c.o.c.k was shut, the steam was re-admitted into the cylinder, and the operation was repeated. The quant.i.ty of steam consumed and the weights it could raise were observed, and, excepting the non-application of the steam-case and external covering, the invention was complete in so far as regarded the savings of steam and fuel."

COMPLETING THE INVENTION.

But although the invention was complete in Watt's mind, it took him many long and laborious years to work out the details of the engine. His friend Robison, with whom his intimacy was maintained during these interesting experiments, has given a graphic account of the difficulties which he successively encountered and overcame. He relates that on his return from the country, after the college vacation in 1765, he went to have a chat with Watt and communicate to him some observations he had made on Desaguliers' and Belidor's account of the steam-engine. He went straight into the parlor, without ceremony, and found Watt sitting before the fire looking at a little tin cistern which he had on his knee. Robison immediately started the conversation about steam; his mind, like Watt's, being occupied with the means of avoiding the excessive waste of heat in the Newcomen engine. Watt all the while kept looking into the fire, and after a time laid down the cistern at the foot of his chair, saying nothing. It seems that Watt felt rather nettled that Robison had communicated to a mechanic of the town a contrivance which he had hit upon for turning the c.o.c.ks of his engine.

When Robison therefore pressed his inquiry, Watt at length looked at him and said briskly, "You need not fash yourself any more about that, man.

I have now made an engine that shall not waste a particle of steam. It shall all be boiling hot,--ay, and hot water injected, if I please." He then pushed the little tin cistern with his foot under the table.

Robison could learn no more of the new contrivance from Watt at that time; but on the same evening he accidentally met a mutual acquaintance, who, supposing he knew as usual the progress of Watt's experiments, observed to him, "Well, have you seen Jamie Watt?" "Yes." "He'll be in fine spirits now with his engine?" "Yes," said Robison, "very fine spirits." "Gad!" said the other, "the separate condenser's the thing; keep it but cold enough, and you may have a perfect vacuum, whatever be the heat of the cylinder." This was Watt's secret, and the nature of the contrivance was clear to Robison at once.

It will be observed that Watt had not made a secret of it to his other friends. Indeed, Robison himself admitted that one of Watt's greatest delights was to communicate the results of his experiments to others, and set them upon the same road to knowledge with himself; and that no one could display less of the small jealousy of the tradesman than he did. To his intimate friend Dr. Black he communicated the progress made by him at every stage. The Doctor kindly encouraged him in his struggles, cheered him in his encounter with difficulty, and, what was of still more practical value at the time, helped him with money to enable him to prosecute his invention. Communicative though Watt was disposed to be, he learnt reticence when he found himself exposed to the depredations of the smaller fry of inventors. Robison says that had he lived in Birmingham or London at the time, the probability is that some one or other of the numerous harpies who live by sucking other people's brains would have secured patents for his more important inventions, and thereby deprived him of the benefits of his skill, science, and labor.

As yet, however, there were but few mechanics in Glasgow capable of understanding or appreciating the steam-engine; and the intimate friends to whom he freely spoke of his discovery were too honorable to take advantage of his confidence. Shortly after Watt communicated to Robison the different stages of his invention, and the results at which he had arrived, much to the delight of his friend.

It will be remembered that in the Newcomen engine the steam was only employed for the purpose of producing a vacuum, and that its working power was in the down stroke, which was effected by the pressure of the air upon the piston; hence it is now usual to call it the atmospheric engine. Watt perceived that the air which followed the piston down the cylinder would cool the latter, and that steam would be wasted by reheating it. In order, therefore, to avoid this loss of heat, he resolved to put an air-tight cover upon the cylinder, with a hole and stuffing-box for the piston-rod to slide through, and to admit steam above the piston, to act upon it instead of the atmosphere. When the steam had done its duty in driving down the piston, a communication was opened between the upper and lower part of the cylinder; and the same steam, distributing itself equally in both compartments, sufficed to restore equilibrium. The piston was now drawn up by the weight of the pump-gear; the steam beneath it was then condensed in the separate vessel so as to produce a vacuum, and a fresh jet of steam from the boiler was let in above the piston, which forced it again to the bottom of the cylinder. From an atmospheric engine it had thus become a true steam-engine, and with much greater economy of steam than when the air did half the duty. But it was not only important to keep the air from flowing down the inside of the cylinder; the air which circulated within cooled the metal and condensed a portion of the steam within; and this Watt proposed to remedy by a second cylinder, surrounding the first, with an interval between the two which was to be kept full of steam.

One by one these various contrivances were struck out, modified, settled, and reduced to definite plans,--the separate condenser, the air and water pumps, the use of fat and oil (instead of water, as in the Newcomen engine) to keep the piston working in the cylinder air-tight, and the enclosing of the cylinder itself within another to prevent the loss of heat. These were all emanations from the first idea of inventing an engine working by a piston, in which the cylinder should be continually hot and perfectly dry. "When once," says Watt, "the idea of separate condensation was started, all these improvements followed as corollaries in quick succession, so that in the course of one or two days the invention was thus far complete in my mind."