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ously impracticable, which at various times had suggested themselves to his very fertile and warm imagination. No contemporary record exists to illustrate or verify his description of the contrivance which we presume to call a steam-engine, or to inform us where, and in what manner, it was carried into effect; though it is evident, from his account, that he had actually constructed and worked a machine that raised water by steam. His description of the method is short and obscure; but inclines us to think, contrary to what many have supposed, that the force of his engine was derived solely from the elasticity of steam; and that the condensation of steam by cold was no part of his contrivance. This last, we believe, was the invention of Captain Savary, who, in 1696, published an account of his machine, in a small tract entitled the Miner's Friend, having erected several engines previous to that period. In these engines the alternate condensation and pressure of the steam took place in the same vessel into which the water was first raised, from a lower reservoir, by the pressure of the atmosphere, and then expelled into a higher one by the elastic force of strong steam.

"Steam, it must be observed, was thus employed merely to produce a vacuum, and to supply the strength that was applied, for a like effect, to the sucker or piston of an ordinary pump; and it was a great step to have discovered a method of bringing the air to act in this manner, by the application of heat to water, without the assistance of mechanical force.

"The next essential improvement was made by Newcomen, for which he obtained a patent in 1705. It consisted in separating the parts of the engine in which the steam was to act from those in which the water was to be raised; the weight of the atmosphere being employed only for the purpose of pressure, and the steam for that of first displacing the air, and then forming a

vacuum by condensation. Newcomen was thus enabled to dispense with the use of steam of great and dangerous elasticity, to work with moderate heats, and to remove at least some part of the causes of wasteful and ineffectual condensation. To him we are indebted for the introduction of the steam cylinder and piston, and for their connexion with the pump by means of the main lever with its rods and chains; to which we might add several other subordinate contrivances, which do great credit to his ingenuity.

"Still, however, the machine required the constant attendance of a man to open and shut the cocks at the proper intervals, for the alternate admission of steam and cold water: and although traditional report attributes the invention of the mechanism by which the engine was made to perform this work itself, to the ingenuity of an idle boy, we know that the contrivance was first perfected by Mr Henry Beighton 1717, who also improved the construction of several other parts of the engine. From this time to the year 1764, there seems to have been no material improvement in the structure of the engine, which still continued to be known by the appellation of Newcomen's, or the atmospheric engine. The boilers, however, had been removed from under the cylinder in some of the larger engines, and the cylinder had been fixed down to a solid basis. Still the steam was condensed in the cylinder; the hot water was expelled by the steam; the piston was pressed down by the weight of the atmosphere, and kept tight by being covered with water. It was moreover considered as necessary that the injection cistern should be placed on high, in order that the water might enter with great force. It had been found by experience, that the engine could not be loaded, with advantage, with more than seven pounds on each square inch of the piston; and the inferiority of that power to the known pressure

of the atmosphere, was, without due consideration, imputed wholly to friction. The bulk of water, when converted into steam, was very erroneously computed; the quantity of fuel necessary to evaporate a given quantity of water was not even guessed at; whether the heat of steam is accurately measured by its temperature was unknown; and no good experiment had been made to determine the quantity of injection water necessary for a cylinder of given dimensions. In a word, no man of science in this country had considered the subject since Desaguliers; and his writings, in many respects, tended more to mislead than instruct.

"Such was the state of matters, when, fortunately for science and for the arts, Mr Watt, then a mathematical instrument-maker at Glasgow, undertook the repair of the model of a steam engine belonging to the University. In the course of his trials with it, he found the quantity of fuel and injection water it required, much greater in proportion than they were said to be in large engines; and it soon occurred to him, that this must be owing to the cylinder of this small model exposing a greater surface, in proportion to its contents, than larger cylinders did. This he endeavoured to remedy, by making his cylinders and pistons of substances which conducted heat slowly. He employed wood prepared on purpose, and resorted to other expedients, without producing the desired effect in any remarkable degree. He found also, that all attempts to produce a greater degree of exhaustion, or a more perfect vacuum, occasioned a disproportionate expenditure of steam. In reflecting upon the causes of these phenomena, the recent discovery, that water boiled in an exhausted receiver at low degrees of heat (certainly not exceeding 100 degrees of Fahrenheit, but probably, when the vacuum was perfect, much lower), occurred to him; and he immediately concluded, that, to obtain any considerable degree of exhaustion,

the cylinder and its contents must be cooled down to 100 degrees at least; in which case, the reproduction of steam in the same cylinder must be accompanied with a great expense of heat, and consequently of fuel. He next endeavoured to ascertain the temperature at which water boils when placed under various pressures; and not having any apparatus at hand, by which he could make his experiments under pressures less than that of the atmosphere, he began with trying the temperature of water boiling under greater pressures; and by laying down a curve, of which the abscissæ represented the temperatures, and the ordinates the pressures, he found the law by which the two are connected, whether the pressure be increased or diminished.

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Observing also, that there was a great error in Desaguliers's calculation of the bulk of water when converted into steam, and that the experiment on which he founded his conclusion was in itself fallacious, he thought it essential to determine this point with more accuracy. By a very simple experiment with a Florence flask, which our limits will not allow us to detail, he ascertained, that water, when converted into steam under the ordinary pressure of the atmosphere, occupies about eighteen hundred times its original space.

"These points being determined, he constructed a boiler in such a manner, as to show by inspection, with tolerable accuracy, the quantity of water evaporated in any given time; and he also ascertained, by experiment, the quantity of coals necessary to evaporate a given quantity of water.

"He now applied his boiler to the working model above-mentioned; when it appeared, that the quantity of steam expended at every stroke exceeded many times what was sufficient to fill the cylinder; and deducing from thence the quantity of water required to form as much steam as would supply each stroke of the engine,

he proceeded to examine how much cold water was used for injection, and what heat it gained; which, to his very great surprise, he found to be many times the number of degrees which could have been communicated to it by a quantity of boiling water equal to that of which the steam was composed. Suspecting, however, that there might be some fallacy in these deductions, he made a direct experiment to ascertain the degree of heat communicated by steam to water; when it clearly appeared, that one part of water, in the form of steam, at 212o, had communicated about 140 degrees of heat to six parts of water. The fact, thus confirmed, was so contrary to all his previous conceptions, that he at first saw no means of explaining it. Dr Black indeed had, some time before, made his discovery of latent heat; but Mr Watt's mind being otherwise engaged, he had not attended sufficiently to it, to make himself much acquainted with the doctrine: but upon communicating his observations to the Doctor, he received from him a full explanation of his theory; and this induced him to make further experiments, by which he ascertained the latent heat of steam to be above 900 degrees.

"The causes of the defects of Newcomen's engines were now evident. It appeared that the steam could not be condensed so as to form an approximation to a vacuum, unless the cylinder, and the water it contained, were cooled down to less than 100°; and that, at greater degrees of heat, the water in the cylinder must produce steam, which would in part resist the pressure of the atmosphere. On the other hand, when greater degrees of exhaustion were attempted, the quantities of injection water required to be increased in a very great ratio ; and this was followed by a proportionate destruction of steam on refilling the cylinder.

"Mr Watt now perceived, that to make an engine in which the destruction of steam should be the least

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