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In 1759, Mr. James Brindley, whose name is so celebrated as the engineer of the Duke of Bridgewater's canal, obtained a patent for some improvements in the atmospheric engine. He proposed that the boiler should be made of wood and stone, with a stove or fire-place of cast iron within it, so that the fire should be surrounded on every side by water. The chimney was to be an iron pipe or tube, conducted through the boiler; so that the heated air, in passing from the fire, should impart a portion of its heat to the water. He also proposed a method of feeding the boiler, which, by self-acting machinery, would keep the water in the boiler at a fixed level, independently of any attention on the part of the engine-man. This was to be accomplished by a buoy or float upon the surface of the water in the boiler, which should communicate with a valve in the feed-pipe, so that when the level of the water in the boiler fell, the float or buoy, falling with it, would open the valve and supply the feed. It is stated, in the Biographia Britannica, that Mr. Brindley, in 1756, undertook to erect an engine at Newcastle-under-Lyne; but he is said to have been discouraged by the obstacles which were thrown in his way, and to have abandoned the steam engine.

The interval between the invention of the atmospheric engine, and the amelioration it received at the hands of Smeaton, has been rendered memorable by the advent of one who was destined to work a mighty change in the condition [Pg077] of the human race by the application of his vast genius to the adaptation of steam power to the uses of life.

The great-grandfather of Watt, a farmer in Aberdeenshire, was killed in one of the battles of Montrose. The victorious party, not thinking death a sufficient expiation for the political opinions in support of which he had fought and bled, punished him in the person of his son, by confiscating his little property. Thomas Watt, the son, thus deprived of support, was received by distant relations, and, for a time, applied himself to study, by which he was enabled, after the restoration of tranquillity, to establish himself at Greenock as a teacher of practical mathematics and navigation. He resided in the burgh or barony of Crawford's Dyke, and attained a position of sufficient respectability to be elected to the office of baron-baillie, or chief magistrate, and died in 1734, at the advanced age of ninety-two years.

Thomas Watt had two sons. The elder, John, adopted the profession of his father, and was a teacher of mathematics and navigation at Glasgow: he died in 1737, at the age of fifty years. The second son, James, the father of the celebrated engineer, was, during a quarter of a century, treasurer of the town council of Greenock, and a local magistrate. He was remarked for the ardent zeal and enlightened spirit with which he discharged his public duties. His business was that of a ship-chandler, builder, and general merchant; but, unhappily, notwithstanding his active industry, he lost, in the decline of his life, by unsuccessful commercial speculations, a part of the property which he had so honourably acquired. He died in 1782, at the age of eighty-four years.

James Watt, to whom the world is so largely indebted for the extension and improvement of steam power, had from his birth an extremely delicate constitution. From his mother, [Pg078] whose family name was Muirhead, he received his first lessons in reading, and he learned from his father writing and arithmetic. Although he was entered as a pupil in the grammar school of Greenock, yet such was his delicate state of health, that his attendance there was so interrupted by constant indisposition that he could derive but little benefit from the opportunities of instruction which it afforded. For a great period of the year he was confined to his room, where he devoted himself to study without the aid of instruction. It was in the retirement of the sick chamber that the high intellectual faculties of Watt, which were destined to produce such precious fruits, began to unfold themselves. He was too sickly to be subjected to the restraints which the business of education usually imposes on children. His parents, therefore, found it necessary to leave him at liberty to choose his occupations and amusements. The following anecdotes will show the use he made of this freedom.

A friend of his father found the boy one day stretched upon the hearth tracing with chalk various lines and angles. "Why do you permit this child," said he, "to waste his time so; why not send him to school?" Mr. Watt replied, "You judge him hastily; before you condemn us, ascertain how he is employed." On examining the boy, then six years of age, it was found that he was engaged in the solution of a problem of Euclid!

Having observed the tendency of his son's mind, Mr. Watt placed at his disposal a collection of tools. These he soon learned to use with the greatest skill. He took to pieces and put together, again and again, all the children's toys which he could procure; and he was constantly employed in making new ones. Subsequently he used his tools in constructing a little electrical machine, the sparks proceeding from which became a great subject of amusement to all the playfellows of the poor invalid.

Though endowed with great retentive powers, Watt would probably never have figured among the prodigies of a common school: he would have been slow to commit his lessons to memory, from the repugnance which he would feel to repeat like a parrot anything which he did not perfectly [Pg079] understand. The natural tendency of his mind to meditate on whatever came before it, would give him, to superficial observers, the appearance of dullness. Happily, however, he had a parent who was sufficiently clear-sighted, and who entertained high hopes of the growing faculties of his son. More distant and less sagacious relations were not so sanguine. One day Mrs. Muirhead, the aunt of the boy, reproaching him for what she conceived to be listless idleness, desired him to take a book and occupy himself usefully. "More than an hour has now passed away," said she, "and you have not uttered a single word. Do you know what you have been doing all this time? You have taken off, and put on, repeatedly, the lid of the tea-pot; you have been holding the saucers and the spoons over the steam, and you have been endeavouring to catch the drops of water formed on them by the vapour. Is it not a shame for you to waste your time so?"

Mrs. Muirhead was little aware that this was the first experiment in the splendid career of discovery which was subsequently to immortalise her little nephew. She did not see, as we now can, in the little boy playing with the tea-pot, the great engineer preluding to those discoveries which were destined to confer on mankind benefits so inestimable.

One of the social qualities of mind which was remarkable throughout his life, was the singular felicity and grace with which he related anecdotes. This power was manifested even in his earliest childhood. The following is an extract from a letter written by Mrs. Marion Campbell, his cousin, and the playfellow of his childhood:—

"He was not fourteen when his mother brought him to Glasgow to visit a friend of hers; his brother John accompanied him. On Mrs. Watt's return to Glasgow, some weeks after, her friend said, 'You must take your son James home; I cannot stand the degree of excitement he keeps me in; I am worn out for want of sleep. Every evening before ten o'clock, our usual hour of retiring to rest, he contrives to engage me in conversation, then begins some striking tale, and, whether humorous or pathetic, the interest is so overpowering that the family all listen to him with breathless attention, and hour after hour strikes unheeded.'" [Pg080]

Watt had a younger brother, John, who was subsequently lost by shipwreck, in a voyage from Scotland to the United States. This lad, having determined on following the business of his father, left James more completely at liberty to choose his own occupation. But such a choice was difficult for a student who commanded equal success in every thing to which he directed his attention.

The excursions which he was in the habit of making on the Scottish mountains surrounding Loch Lomond, naturally directed his attention to botany and mineralogy, in each of which he attained considerable knowledge. His love of anecdote and romance was likewise gratified by the scenery which he enjoyed in these walks; and the traditions and popular songs with which they made him acquainted. When from ill-health, as constantly happened, he was confined to the house, he devoted himself to chemistry, natural philosophy, and even to medicine and surgery. In chemistry he acquired some experimental skill, and studied with eager zeal the elements of natural philosophy by S'. Gravesande. His own unhappy maladies prompted him to read works on surgery and medicine; and to such an extent did the activity of his mind impel him on these subjects, that he was found one day dissecting, in his room, the head of a child, who had died of some unknown disease, with a view to ascertain the cause of its death.

In 1775, at the age of nineteen, at the recommendation of Dr. Dick, professor of natural philosophy in the university of Glasgow, he went to London, where he employed himself in the house of Mr. John Morgan, a mathematical instrument maker, in Finch Lane, Cornhill, to whom he apprenticed himself for three years. He remained, however, only a year, at the expiration of which (probably owing to his delicate state of health) he was released from his apprenticeship, and returned to Glasgow, with the intention of establishing himself in business as an optician and mathematical instrument maker. In the fulfilment of this intention, however, he was obstructed by the interposition of the Corporation of Trades in that town, who regarded him as an intruder, not qualified by the necessary apprenticeship to carry on business. All means of conciliation being [Pg081] exhausted, the Professors of the University interfered, and gave him the use of three apartments within the college, for carrying on his business, and likewise appointed him mathematical instrument maker to the University. Soon afterwards the opposition of the local trades seems to have given way, and he opened a shop in Glasgow for the sale of mathematical instruments.

After the celebrity at which he has arrived, it will be easily believed that every trace of his earlier connection with Glasgow college is carefully cherished. There are accordingly preserved at that place little instruments and pieces of apparatus of exquisite workmanship, which were executed entirely by the hand of Watt, at a time when he was not in a condition to command the aid of workmen under him.

At the time of obtaining this appointment in the University, Watt was in his twenty-first year. His natural talents and winning manners were speedily the means of gaining for him the esteem and friendship of all those eminent persons connected at the time with that university whose regard was most valued. Among these the earliest of his friends and patrons were—Adam Smith, the author of "The Wealth of Nations;" Black, afterwards celebrated for his chemical discoveries, and more especially for his theory of latent heat; and Robert Simson, rendered illustrious by his works on ancient geometry. In releasing Watt from the persecution of the Glasgow corporation, these distinguished persons first imagined that they were conferring a benefit merely on an industrious and clever artisan, whose engaging manners won their regard; but a short acquaintance with him was sufficient to convince them how superior his mind was to his position, and they conceived towards him the most lively friendship. His shop became the common rendezvous, the afternoon lounge, of all who were most distinguished for literary and scientific attainments among the professors and students. There they met to discuss the topics of the day in art, science, and literature. Among these students, the name which afterwards attained the highest distinctions, and among these distinctions, not the least, the lasting personal friendship and esteem of Watt himself, was Robison, [Pg082] the author of a well known work on Mechanics, and one of the contributors to the Encyclopoedia Britannica.

The following extract from an unpublished manuscript by Robison himself will show at once the estimation in which Watt was held, and will illustrate one of the most interesting traits of his personal character:—

"I had always, from my earliest youth, a great relish for the natural sciences, and particularly for mathematical and mechanical philosophy, when I was introduced by Drs. Simson, Dick, and Moor, gentlemen eminent for their mathematical abilities, to Mr. Watt. I saw a workman, and expected no more; but was surprised to find a philosopher as young as myself, and always ready to instruct me. I had the vanity to think myself a pretty good proficient in my favourite study, and was rather mortified at finding Mr. Watt so much my superior.. .. Whenever any puzzle came in the way of any of the young students, we went to Mr. Watt. He needed only to be prompted, for every thing became to him the beginning of a new and serious study, and we knew that he would not quit it till he had either discovered its insignificancy, or had made something of it. He learnt the German language in order to peruse Leupold's 'Theatrum Machinarum;' so did I, to know what he was about. Similar reasons made us both learn the Italian language. *** When to his superiority of knowledge is added the naÏve simplicity and candour of Mr. Watt's character, it is no wonder that the attachment of his acquaintances was strong. I have seen something of the world, and am obliged to say I never saw such another instance of general and cordial attachment to a person whom all acknowledged to be their superior. But that superiority was concealed under the most amiable candour, and a liberal allowance of merit to every man. Mr. Watt was the first to ascribe to the ingenuity of a friend things which were nothing but his own surmises, followed out and embodied by another. I am the more entitled to say this, as I have often experienced it in my own case."

Watt never permitted the inquiries which arose out of these reunions to interfere with the discharge of the duties of his workshop. There he passed the day, devoting the [Pg083] night to study. Every inquiry appeared to him to be attractive in proportion to its difficulty, and to have charms in proportion as it was removed from the common routine of his business. As an example of this may be mentioned the fact, that, being himself so insensible to the charms of music that he could not distinguish one note from another, he was actually induced to undertake the construction of an organ, in which he was nevertheless completely successful. The instrument he constructed, as might have been expected, contained many improvements in its mechanism; but what is much more remarkable, its tone and its musical qualities commanded the admiration of all the professional musicians who heard it. In the construction of this instrument Watt showed that vigorous spirit of investigation which characterised all the subsequent labours of his life. He made out the scale of temperament by the aid of the phenomena of beats, of which he could only obtain a knowledge by a profound but obscure work published by Dr. Robert Smith of Cambridge.

The earliest occasion on which the attention of Watt is said to have been called to the agency of steam, was in the year 1759, when his friend Robison entertained some speculations for applying that agent as a means of propelling wheel carriages; and he consulted Watt on the subject. No record, however, has been preserved of any experiments which were tried on this occasion; nor does it appear that the inquiry was carried farther than a verbal discussion, such as habitually took place on other subjects of science between Watt and his friends.

It is usual to provide, in the cabinets of experimental apparatus for the instruction of the students of universities, [Pg084] small working models of the most useful machines. In the collection for the illustration of the lectures delivered to the Natural Philosophy class in the University of Glasgow was a working model of Newcomen's atmospheric engine, applied to a pump for raising water; which, however, had never been found to work satisfactorily. The Professor of Experimental Philosophy of that day, Dr. John Anderson (the founder of the celebrated Andersonian Institution), sent this model in 1763 to Watt's workshop, to be repaired. Its defects soon disappeared, and it was made to work to the satisfaction of the professor and students.

This simple discharge of his duty, however, did not satisfy the artisan; and his wonted activity of mind rendered this model a subject of profound meditation, and led him into a course of practical inquiry respecting it, which formed the commencement of a most brilliant career of mechanical discovery. The improvement—we might almost say the creation—of the steam engine, by this great man, must not therefore be regarded, as so often happens with mechanical discoveries, as the result of fortuitous observation, or even of a felicitous momentary inspiration. Watt, on the other hand, conducted his investigation by a course of deep thought, and of experiments marked by the last refinement of delicacy and address. If he had received a more extended and liberal education, one would have thought that he had adopted for his guide the celebrated maxim of Bacon:—

"To write, speak, meditate, or act, when we are not provided with facts to direct our thoughts, is to navigate a coast full of dangers without a pilot, and to launch into the immensity of the ocean without either rudder or compass."

The model which he had repaired, had a cylinder of only two inches diameter, and six inches stroke. After he had put it in complete order, he found, that although the boiler was much larger in proportion to the cylinder than those of real engines, yet, that it was incapable of supplying the cylinder with steam in sufficient quantity to keep it at work. To enable it to continue to move, he found it necessary to lessen the quantity of water raised by its pump, so as to [Pg085] reduce the load on its piston very much below the proper standard according to the common rules for large engines.

He ascribed the great inferiority in the performance of the model, compared with the performance of the large engines, to the small size of the cylinder, and to its material. The cylinder of the model was brass, while those of large engines were of cast iron; and brass being a better conductor of heat than iron, he concluded that more heat in proportion was lost from this cause in the model, than in the larger engines. He observed that the small cylinder was so heated when the steam was admitted into it, that it could not be touched by the hand; but, nevertheless, that this heat contributed nothing to its performance, inasmuch as before the piston descended, the cylinder required to be cooled.

Still he found that the force with which the piston descended was considerably less than that which the atmospheric pressure ought to supply, supposing a tolerably perfect vacuum to be produced under the piston. This led him to suspect that the water injected into the cylinder was not perfectly effectual in condensing the steam. The experiments which he had previously made on the increased temperature at which water boils under pressures greater than that of the atmosphere, led him by analogy to the conclusion that it would boil at lower temperatures if it were submitted to a pressure less than the atmosphere, and he was aware that Dr. Cullen and others had then recently discovered that in vacuo, water would boil at so low a temperature as 100°. These notions suggested the probability that the water injected into the cylinder being heated by the condensed steam, might produce vapour of a low temperature [Pg086] and reduced pressure under the piston, which would account for the deficiency he observed in the power of the engine.

No means occurred to him by which he could ascertain, by direct experiment, the temperatures at which water would boil under pressures less than that of the atmosphere. He sought, however, to determine it by the following method. Having ascertained, by repeating and multiplying the experiments which he had tried in 1762, on high-pressure steam, he obtained a table of the temperatures at which water boils at various pressures greater than that of the atmosphere. These results he laid down in a series forming a curve, of which the abscissa represented the temperatures, and the ordinates the pressures. He then continued this curve, backwards as it were, and obtained, by analogy, an approximation to the boiling temperatures, corresponding to pressures less than that of the atmosphere. In other words, having obtained by his experiments a notion, however imperfect, of the law or rule observed by the temperatures at which water boils at different pressures greater than that of the atmosphere, he calculated by the same law or rule what the pressures would be at different pressures less than that of the atmosphere.

Applying these results to the interior of the cylinder of the atmospheric engine, he obtained an approximation to the pressure of the vapour which would be produced from the warm water formed by the cold water injected into the cylinder, and the steam condensed by it; and he accordingly found that vapour, having a pressure seriously injurious to the power of the engine would be produced in the cylinder, unless considerably more water of injection was thrown in than was customary.

It was apparent that the actual quantity of steam usefully employed in the cylinder at each stroke, was only the quantity which filled the cylinder; and therefore, in order to ascertain the quantity of steam lost by the imperfections of the machine, it was necessary to compare the actual quantity of steam transmitted by the boiler to the cylinder at each stroke, with the quantity which would just fill the cylinder. The difference would of course be wasted. But to determine [Pg087] the actual quantity of steam supplied by the boiler to the cylinder, there was no other means than by observing the quantity of water evaporated in the boiler. That being observed, it was necessary to know the quantity of steam which that water formed; and it was therefore necessary to determine the quantity or volume of steam which a given volume of water produced.

Let us suppose the piston at the top of the cylinder, and the space in the cylinder below it, filled with steam so as to balance the pressure of the atmosphere above the piston. Under such circumstances the steam, as will presently be explained, must have the temperature of boiling water. But that the steam should have, and should maintain, this temperature, it was evidently necessary that the inner surface of the cylinder in contact with it should have the same temperature: for if it had a lower temperature, it would take heat from the steam, and reduce the temperature of the latter. Now the cylinder being a mass of metal, has a quality in virtue of which heat passes freely through its dimensions, so that its inner surface could not be maintained at a temperature more elevated than that of its dimensions extending from the inner surface to the outer surface. Therefore, to maintain the steam contained in the cylinder at the proper temperature, it was essential that the whole of the solid metal composing the cylinder should be itself at that temperature.

Things being in this state, it was required that a vacuum should be produced under the piston to give effect to the atmospheric pressure above it, by relieving it from the pressure below. This, indeed, would appear to have been attained by introducing as much cold water within the cylinder as would be sufficient to reconvert the steam contained in it into water; but Watt found, in his experiments on the atmospheric model, that the piston would not descend with the proper force, unless a vastly greater quantity of water were introduced into the cylinder than the quantity which he had ascertained to be [Pg088] necessary for the reconversion of the steam into water. The cause of this he perceived and fully explained.

If we suppose as much, and no more, cold water introduced into the cylinder as would reconvert the steam contained in it into water, then we should have in the bottom of the cylinder a quantity of warm water with a vacuum above it: but the entire mass of metal composing the cylinder itself, which was previously at the temperature of boiling water, would still be at the same temperature. The warm water, resting in contact with this metal in the bottom of the cylinder, would be immediately heated by it, and would rise in its temperature, while the metal of the cylinder itself would be somewhat lowered in temperature by the heat which it would thus impart to the warm water contained in it. Under these circumstances, as we shall presently explain, steam would be produced from the water, which would fill the cylinder; and although such steam would not have a mechanical pressure equal in amount to the atmosphere, and therefore would not altogether prevent the piston from descending if it had no load to move, yet it would deprive the engine of so great a portion of its legitimate power as to render it altogether inefficient. But this defect would be removed by throwing into the cylinder a sufficient quantity of cold water, not only to destroy the steam contained in it, but also to cool the entire mass of metal composing the cylinder itself, until it would be reduced to such a temperature that the vapour proceeding from the water contained in it would have so small a pressure that it would not seriously or injuriously obstruct the descent of the piston.

The piston being made to descend with such force as to render the machine practically efficient, it would then be necessary again to make it ascend; and to accomplish this, Watt found that the boiler should supply a quantity of steam many times greater than was necessary to fill the cylinder. Mature reflection on the circumstances which have been just explained, enabled him to discover how this undue quantity of steam was rendered necessary.

Let it be recollected, that when the piston has reached the bottom of the cylinder, the whole mass of the cylinder, and [Pg089] the piston itself, are reduced to so low a temperature that the vapour of water, having the same temperature, has no pressure sufficiently great to obstruct the action of the machine. When, in order to make the piston ascend, steam is introduced from the boiler into the cylinder under the piston, this steam encounters, in the first instance, the cold surfaces of the metal forming the bottom of the cylinder and the bottom of the piston. The first effect of this is to convert the steam which comes from the boiler into water, an effect which is produced by that steam imparting its heat to the metal with which it comes into contact. This destruction of steam continues until the metal exposed to contact with it has been heated up to the temperature of boiling water. Then, and not till then, the steam below the piston will have a pressure equal to that of the atmosphere above it, and the piston will begin to ascend. As it ascends, however, the sides of the cylinder which it exposes to the contact of the steam are cold, and partially destroy the steam. Steam, therefore, must be supplied from the boiler to replace the steam thus destroyed; nor can the piston reach the top of the cylinder until such a quantity of steam shall have flowed from the boiler into the cylinder, as shall be sufficient not only to fill the cylinder under the piston, but likewise, by its condensation, to raise the whole mass of the cylinder and piston to the temperature of boiling water.

Such were the circumstances which forced themselves upon the attention of Watt, in the course of repairing, and subsequently trying, the model of the atmospheric engine, at Glasgow. Being informed generally of the uses of the engine in the drainage of mines, and of the vast expense attending its operation, by reason of the quantity of fuel which it consumed, he saw how important any improvement would be by which the extensive sources of waste which had thus presented themselves could be removed. He saw also, that all that portion of steam which was expended, not in filling the cylinder under the piston, but in heating the great mass of metal composing the cylinder and piston, from a low temperature to that of boiling water, upon each stroke of the piston, was so much heat lost, and that the proportion of the fuel expended in evaporating the steam thus wasted would be saved, if by any [Pg090] expedient he could make the piston descend without cooling the cylinder. But in order to estimate the full amount of this waste, and to discover the most effectual means of preventing it, it was necessary to investigate the quantity of heat necessary for the evaporation of a given quantity of water; also, the quantity of steam which a given quantity of water would produce, as well as other circumstances connected with the temperature and pressure of steam. He, therefore, applied himself to make experiments with a view to elucidate these questions; and succeeded in obtaining results which led to the discovery of some of the most important of those physical phenomena, on the due application of which, the efficacy of the steam engine, which he afterwards invented, depended, and which also form striking facts in the general physics of heat.

[Pg091] Having once ascertained this point, he was able, by observing the quantity of water evaporated in the boiler of the atmospheric model, to compute the volume of steam which was supplied to the cylinder. It was evident, that for every cubic inch of water evaporated in the boiler, eighteen hundred cubic inches of steam were supplied to the cylinder. Having accurately observed the evaporation of the boiler for a short time, and the number of strokes made by the piston in the same time, he found that the quantity of water evaporated in the boiler would supply about four times as much steam as the cylinder would require. He consequently inferred, that about three-fourths of the steam produced was wasted.

The next question to which he directed his experiments, was to ascertain the quantity of cold water necessary to be injected into the cylinder, in order to condense the steam contained in it. To ascertain this, he attached a pipe to a boiler, by which he was enabled to conduct the steam from the boiler into a glass jar containing cold water at fifty-two degrees of temperature. The steam, as it passed from the boiler through the pipe, was condensed by the cold water, and continued to be so condensed, until, by the heat which it imparted to the water, the latter began to boil, and would then condense no more steam. On comparing the water in the glass jar, when boiling, with the water originally contained in it at fifty-two [Pg092] degrees, the quantity was found to be increased in the proportion of six to seven, very nearly; from which he inferred, that to reduce one ounce of steam to water, it was necessary to mix about six ounces of cold water with it.

He was further led to the conclusion, that steam contains a vast quantity of heat, by the following experiment. He heated, in a close digester, a quantity of water several degrees above the common boiling point. When thus heated, by opening a stop-cock, he allowed the compressed steam to escape into a cold vessel; in three or four seconds, he found that the heat of the water in the digester was reduced from a very high temperature to the common boiling point; yet, that all the steam which escaped from it, and which carried off with it the superabundant heat, formed only a few drops of water when condensed; from which he inferred, that this small quantity of water, in the form of steam, contained as much heat as was sufficient to raise all the water in the digester from the boiling point to the temperature at which it was before the steam was allowed to escape.

Having thus ascertained the exact quantity of cold water which ought to be injected into the cylinder in order to condense the steam which filled the cylinder, he found, on comparing the quantity necessary to be injected in order to enable the piston to descend, that this quantity was about four times as great as that which was necessary to condense the steam. This led him to the conclusion, that about four times as much heat was destroyed in the cylinder as needed to be destroyed, if the object were the mere condensation of the steam. This result fully corroborated the other conclusion, deduced, from the proportion which he found between the quantity of steam supplied by the boiler and the actual contents of the cylinder.

He, therefore, eagerly sought his friend Dr. Black, to whom he communicated these results. Then, for the first time, he [Pg093] was informed, by Black, of the theory of LATENT HEAT, which had recently been discovered by him, and of which these very phenomena formed the basis.

Some passages in the works of Dr. Robison produced an erroneous impression, that a large share of the merit of the discoveries of Watt which have been just explained was due to Dr. Black, to whose instructions on the subject of latent heat Watt was represented to have owed the knowledge of those facts which led to his principal inventions and improvements. We shall here give, in the words of Watt himself, his explanation of the circumstances which led to this error. This explanation is given in a letter addressed by Watt to Dr. Brewster, in May 1814, and prefixed to the third volume of Brewster's edition of Robison's Mechanical Philosophy:—