Steam Engine

In any historical sketch of the steam-engine, stress must be laid on the work of James Watt. But a process of evolution had been going on before his time which prepared a crude and primi tive device for the immense improvements it received at his hands. The labours of Watt stand in natural sequence to those of Newcomen, and Newcomer's to those of Papin and Savery. Sav ery's engine, in its turn, was the reduction to practical form of an ancient scientific toy. Along another line of development, the modern steam turbine, which we owe to the genius of Charles Parsons, can be traced back to an early prototype.

In the Pneumatica of Hero of Alexandria (c. 13o B.c.) there is described the aeolipile, which may be called a primitive steam reaction turbine. It consists of a hollow globe pivoted so that it can turn on a pair of central trunnions, and supplied with steam through one of them, which is hollow. The steam escapes from the globe to the outside air through two bent pipes facing tan gentially in opposite directions at the ends of a diameter perpen dicular to the axis. The globe revolves by reaction from the es caping steam. Hero's volume also mentions (Greenwood's trans lation of Hero's Pneumatica) another device which may be de scribed as the prototype of the pressure engine. A hollow altar containing air is heated by kindling a fire on it ; the air expands and by its pressure drives some of the water in a vessel below into a hanging bucket, which then descends, opening the doors of a shrine. When the fire is extinguished the air contracts, the bucket empties, and the doors close.

In a treatise on pneumatics ( 16o ) by Giovanni Battista della Porta there is shown a somewhat similar apparatus, but with steam for working substance. Its pressure forces up water from a separate vessel. He also points out that the condensation of the steam may be used to produce a vacuum and thereby suck up water from a lower vessel. His suggestions go far to anticipate the engine which, a century later, in the hands of Savery, became the first commercially successful steam engine.

Meanwhile Edward Somerset, second Marquis of Worcester, described in his Century of Inventions (1663) a method of rais ing water by the agency of steam. His description is obscure, and no drawings of the device are extant. It appears to have consisted of a pair of displacement chambers, from each of which alter nately water was forced, probably by admitting steam from an independent boiler, while the other vessel was allowed to refill.

Savery,

1698.—The earliest steam-engine to take a practical form and find employment in industry was that of Thomas Savery, who, in 1698, obtained a patent for a water-raising engine, shown in fig. 1. Steam is admitted to one of the oval vessels A, dis placing water, which it drives up through the check-valve B. When the vessel A is emptied of water the supply of steam is stopped, and the steam already there is condensed by allowing a jet of cold water from a cistern above to stream over the outer surface of the vessel. This produces a vacuum and causes water to be sucked up through the pipe C and the valve D. Meanwhile steam has been displacing water from the other vessel, and is ready to be condensed there. The valves B and D open only up wards. The supplementary boiler and furnace E are for feeding water to the main boiler; E is filled while cold and a fire is lighted under it; it then forces a supply of feeding-water into the main boiler F. The gauge cocks G, G are an interesting detail. An other form of Savery's engine had only one displacement-cham ber and worked intermittently. In the use of artificial means to condense the steam, and in the application of the vacuum so formed to raise water by suction from a level lower than that of the engine, as well as in practical features, Savery's engine marked an inventive advance. It found considerable employment in pumping mines and in raising water to supply houses and towns, and even to drive water-wheels. A serious difficulty which pre vented its general use in mines was the fact that the height through which it would lift water was limited by the pressure the boiler and vessels could bear. Pressures as high as 8 or io at mospheres were employed—and that, too, without a safety-valve —but Savery found it no easy matter to deal with high-pressure steam. Apart from this drawback, the waste of fuel was enormous, from the condensation of steam which took place on the surface of the water and on the sides of the displacement-chamber at each operation; the consumption of coal was, in proportion to the work done, some twenty times greater than in a modern engine. Before Savery's engine was displaced by its successor, New comen's, it was improved by J. T. Desaguliers who applied to the safety valve (invented by Papin), and substituted condensa tion by a jet of cold water within the vessel for the surface con densation used by Savery. To Savery is ascribed the first use of the term "horse-power" as a measure of performance.