each square inch of surface exposed to it, and that if the piston in the cylinder imagined by Papin, had a diameter of only one square foot, its superficial magnitude would be about 114 square inches. The pressure of the atmosphere upon it, therefore, would be 114 times fifteen pounds, or 1710 pounds. Papin first proposed to produce the vacuum under the piston by means of common air pumps, worked by a water-wheel; and by such means he conceived that the power of a river, stream, or waterfall might be conveyed by pipes to a distance. While he was in England, in 1687, he laid his contrivance before the Royal Society of London, but was met by objections and difficulties, the nature of which he does not explain.

It is, however, apparent, from what has been already explained, that such a method of proceeding would amount to a mere transfer of power, and would not, properly speaking, be itself a moving force: the moving power would, in reality, be the force of the water by which the water-wheel would be driven; and the air-pumps, tubes, together with the piston and cylinder, would be merely means of conveying the power of the water-wheel to the objects to be moved, or the machinery to be driven. Papin states, that, long before this, he had attempted to expel the air from his cylinder by means of gunpowder; but, notwithstanding all the precautions which he could take, there always remained a considerable quantity; so much, indeed, as to deprive the vacuum of more than half its proper force. At length he adopted an expedient for the production of a vacuum which forms a most important step in the progressive invention of the steam engine, and which gives to Papin's name a high place in the history of that machine. This method is explained in the following paragraph of a work published by Papin in 1695, at Cassel, entitled Recueil de diverses Pièces touchant quelques nouvelles Machines," p. 53.

"I have endeavoured," says he, "to attain this end (viz. the production of a vacuum in the cylinder) in another way. As water has the property of elasticity, when converted into steam by heat, and afterwards of being so completely re

appearance of this elasticity, I have thought that it would not be difficult to work machines in which, by means of a moderate heat and at a small cost, water might produce that perfect vacuum which has vainly been sought by means of gunpowder."

This remarkable passage is given in the work just cited, as an extract from the " Leipsic Acts," of August, 1690.

Let us pause here to explain more fully this important discovery.

(29.) We have explained that, in its conversion into vapour, by the application of heat, water, besides acquiring the property of elasticity, undergoes a vast enlargement of bulk, filling, under ordinary circumstances, about 1700 times more space than it occupied in the liquid form. This fact was known generally, though not with numerical accuracy, by Papin, having been the foundation of the machines previously invented and published by De Caus and Lord Worcester; the happy idea of reversing the process occurred to him. If water in its conversion into steam swelled into many hundred times its original bulk, it would necessarily follow, that steam, being reconverted into water, would shrink into its primitive dimensions. Papin therefore saw, that if he could by any means expel the air from his cylinder under the piston, and replace it by the pure vapour of water, he could cause that vapour to be reconverted into a comparatively minute quantity of water by depriving it of the heat which sustained it in the state of steam, and that by accomplishing this, the space in the cylinder under the piston would become a vacuum; that by such means, the pressure of the atmosphere above the piston would take full effect, and would urge the piston down; that by introducing more steam under the piston, it might be again raised by the elastic force of the steam, the destruction of which by cold water would again produce the descent of the piston with the same mechanical force; and that in this way the alternate ascent and descent of the piston might be continued indefinitely.

In accordance with these ideas, Papin constructed a model consisting of a small cylinder, in which was placed a solid pis

ton; and in the bottom of the cylinder under the piston was contained in a small quantity of water. The piston being in immediate contact with this water, so as to exclude the atmopheric air, on applying fire to the bottom of the cylinder, steam was produced, the elastic force of which raised the piston to the top of the cylinder; the fire being then removed, and the cylinder being cooled by the surrounding air, the steam was condensed and reconverted into water, leaving a vacuum in the cylinder into which the piston was pressed by the force of the atmosphere. The fire being applied and subsequently removed, another ascent and descent were accomplished; and in the same manner the alternate motion of the piston might be continued. Papin described no other form of machine by which this property could be rendered available in practice; but he states generally, that the same end may be attained by various forms of machines easy to be imagined.*

THOMAS SAVERY, 1698.

(30.) The discovery of the method of making a vacuum by the condensation of steam was reproduced, before 1698, by Captain Thomas Savery, to whom a patent was granted in that year for a steam engine to be applied to the raising of water, &c. Savery proposed to combine the machine described. by the Marquis of Worcester with an apparatus for raising water by suction into a vacuum produced by the condensation of steam.

Savery appears to have been ignorant of the publication of Papin, and stated that his discovery of the condensing principle arose from the following circumstance :

Having drunk a flask of Florence at a tavern, and flung the empty flask on the fire, he called for a basin of water to wash his hands. A small quantity which remained in the flask began to boil, and steam issued from its mouth. It occurred to him to try what effect would be produced by inverting the flask and plunging its mouth in the cold water. Putting on a thick glove to defend his hand from the heat, he seized the

* Recueil de diverses Pièces touchant quelques nouvelles Machines,

flask, and the moment he plunged its mouth in the water the liquid immediately rushed up into the flask and filled it.

Savery stated that this circumstance immediately suggested to him the possibility of giving effect to the atmospheric pressure by creating a vacuum in this manner. He thought that if, instead of exhausting the barrel of a pump by the usual laborious method of a piston and sucker, it was exhausted by first filling it with steam, and then condensing the same steam, the atmospheric pressure would force the water from the well into the pump-barrel, and into any vessel connected with it, provided that vessel were not more than about thirty-four feet above the elevation of the water in the well. He perceived also, that, having lifted the water to this height, he might use the elastic force of steam in the manner described by the Marquis of Worcester to raise the same water to a still greater elevation, and that the same steam which accomplished this mechanical effect would serve, by its subsequent condensation, to reproduce the vacuum, and draw up more water. It was on this principle that Savery constructed the first engine in which steam was ever brought into practical operation.

SAVERY'S ENGINE.

WORKING APPA

ACCIDENTAL DISCO

BOILERS AND THEIR APPENDAGES. RATUS.MODE OF OPERATION. DEFECTS OF THE ENGINE.NEWCOMEN AND CAWLEY.-ATMOSPHERIC ENGINE. VERY OF CONDENSATION BY INJECTION. HUMPHREY POTTER MAKES THE ENGINE WORK ITSELF. ADVANTAGES OF THE ATMOSPHERIC ENGINE OVER THAT OF SAVERY. IT CONTAINED NO NEW PRINCIPLE. ITS PRACTICAL SUPERIORITY.

(31.) THE steam engine contrived by Savery, like every other which has since been constructed, consists of two parts, essentially distinct. The first is that which is employed to

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