In 1690 Denis Papin suggested that the condensation of steam should be employed to make a vacuum under a piston previously raised by the expansion of the steam. Papin's was the earliest cylinder and piston steam engine, and his plan of using steam was that which afterwards took practical shape in the atmospheric engine of Newcomen. But his scheme was made unworkable by the fact that he proposed to use but one vessel as both boiler and cylinder. A small quantity of water was placed at the bottom of a cylinder and heat was applied. When the piston had risen the fire was removed, the steam was allowed to cool, and the piston did work in its down-stroke under the pressure of the atmosphere. After hearing of Savery's engine in 1705 Papin turned his atten tion to improving it, and devised a modified form, with a floating diaphragm or piston on the top of the water to keep the water and steam from direct contact with one another. Papin's engine may be described as a non-condensing single-acting steam pump, with steam cylinder and pump cylinder in one.
While Papin was thus going back from his first notion of a piston engine to Sav ery's cruder type, a new inventor had appeared who made the piston engine a practical success by separating the boiler from the cylinder and by using (as Savery had done) artificial means to condense the steam. This was Thomas Newcomen, who in 1705, with his assistant, John Cawley, gave the steam engine the form shown in fig. 2. Steam admitted from the boiler to the cyl inder by the opening of a valve allowed the piston to be raised by a heavy counterpoise on the other side of the beam. Then the steam valve was shut and a jet of cold water entered the cylinder and condensed the steam. The piston was consequently forced down by the pressure of the atmosphere and did work on the pump. The next entry of steam expelled the condensed water from the cylinder through an escape valve. The piston was kept tight by a layer of water on its upper surface. Condensation was at first effected by cooling the outside of the cylinder, but an accidental leakage of the packing water past the piston showed the advantage of condensing by a jet of injection water, and this plan took the place of surface condensation. The engine used steam whose pressure was little if at all greater than that of the atmosphere.
About 1711 Newcomen's engine began to be introduced for pumping mines. It is doubtful whether the action was originally automatic, or depended on the periodical turning of taps by an attendant. The common story is that in 1713 a boy named Hum phrey Potter, whose duty it was to open and shut the valves of an engine he attended, made the engine self-acting by causing the beam itself to open and close the valves by suitable cords and catches. This device was simplified in 1718 by Henry Beighton, who suspended from the beam a rod called the plug-tree, which worked the valves by means of tappets. By 1725 the engine was in common use in collieries, and it held its place without material change for about three-quarters of a century in all. Near the close of its career Newcomen's engine was much improved in its mechanical details by John Smeaton, who built many large engines of this type about the year 177o, just after the great step which was to make Newcomen's engine obsolete had been taken by James Watt.
Compared with Savery's engine, Newcomen's had (as a pump ing engine) the great advantage that the intensity of pressure in the pumps was not in any way limited by the pressure of the steam. It shared with Savery's, in a scarcely less degree, the de fect already pointed out, that steam was wasted by the alternate heating and cooling of the vessel into which it was led. Though obviously capable of more extended uses, it was in fact almost exclusively employed to raise water—in some instances for the purpose of turning water-wheels to drive other machinery. Even contemporary writers complain of its great consumption of fuel.
In i 763 James Watt, an instrument maker in Glasgow, while engaged by the University in repairing a model of Newcomen's engine, was struck with the waste of steam to which the alternate chilling and heating of the cylinder gave rise. He saw that the remedy, in his own words, would lie in keeping the cylinder as hot as the steam that entered it. With this view he added to the engine a new organ—namely the "separate condenser"—an empty vessel separate from the cylinder, into which the steam should be allowed to escape from the cylinder, to be condensed there by the application of cold water either outside or as a jet. To preserve the vacuum in his condenser he added a pump called the air pump, whose function was to extract from it the condensed steam and water of condensation, as well as any air which might come in by leakage or by solution in the steam or the injection water. Then, as the cylinder was no longer used as a condenser, he was able to keep it hot by clothing it with non-conducting bodies, and in particular by the use of what is called a steam-jacket—a layer of hot steam between the cylinder and an external casing. Further and still with the same object, he covered in the top of the cylin der, taking the piston-rod out through a steam-tight gland or stuffing-box, and allowed steam instead of air to
upon the piston's upper surface. After much experiment Watt patented his improvements in 1769; they are described in a specification from which the following extracts are taken : "My method of lessening the consumption of steam, and conse quently fuel, in fire-engines, consists of the following principles:— "First, That vessel in which the powers of steam are to be employed to work the engine, which is called the cylinder in common fire engines, and which I call the steam vessel, must, during the whole time the engine is at work, be kept as hot as the steam that enters it ; first by inclosing it in a case of wood, or any other materials that transmit heat slowly ; secondly, by surrounding it with steam or other heated bodies ; and, thirdly, by suffering neither water nor any other substance colder than the steam to enter or touch it during that time.