(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 piston; [Pg047] 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 atmospheric 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.[8] THOMAS SAVERY, 1698.