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.

“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 pos

sible, and the vacuum the most perfect, it was necessary that the cylinder should condense no steam on filling it, and that, when condensed, the water, forming the steam, should be cooled down to 100 degrees, or lower. In reflecting on this desideratum, he was not long in finding that the cylinder must be preserved always as hot as the steam that enters it; and that, by opening a communication between this hot cylinder when filled with steam, and another vessel exhausted of air, the steam, being an elastic fluid, would rush into it, until an equilibrium was established between the two vessels; and that if cold water, in sufficient quantity, were injected into the second vessel, the steam it contained would be reduced to water, and no more steam would enter until the whole was condensed.

"But a difficulty arose-How was this condensed steam and water to be got out of the second vessel without letting in air? Two methods presented themselves. One was, to join to this second vessel (which, after him, we shall call the condenser) a pipe, which should extend downwards more than 34 feet perpendicular, so that the column of water contained in it, exceeding the weight of the atmosphere, would run out by its own gravity, and leave the condenser in a state of exhaustion, except in so far as the air, which might enter with the steam and injection water, should tend to render the exhaustion less perfect: this air he proposed to extract by means of a pump. The second method which occurred, was to extract both air and water by means of a pump or pumps; which would possess the advantage over the other, of being applicable in all situations. This latter contrivance was therefore preferred; and is known by the common name of the Air-pump. There still remained some defects unremedied in Newcomen's cylinder. The piston in that engine was kept tight by water; much of which passing by the sides, injured the va