(43.)

However the merit of the discovery of the physical principles on which the mechanical application of steam depends may be awarded, it must be admitted that the engine contrived by Newcomen and his associates, considered as a practical machine, was immeasurably superior to that which preceded it; superior, indeed, to such a degree, that while the one was incapable of any permanently useful application, the other soon became a machine of extensive utility in the drainage of mines; and, even at the present time, the atmospheric engine is not unfrequently used in preference to the modern steam engine, in districts where fuel is abundant and cheap; the expense of constructing and maintaining it being considerably less than that of an improved steam engine. The low pressure of the steam used in working it, rendered it perfectly safe. While Savery's engine, to work with effect, required that the steam confined in the vessels should have a bursting pressure amounting to about thirty pounds per square inch, the pressure of steam in the boiler and cylinder of the atmospheric engine required only a pressure about one pound per square inch. The high pressure also of the steam used in Savery's engine, was necessarily accompanied, as we shall presently explain, by a greatly increased temperature. The effect of this was, to weaken and gradually destroy the vessels, especially those which, like the steam vessels V and V′ ( fig. 12.), were alternately heated and cooled.

Besides these defects, the power of Savery's engines was [Pg073] also very restricted, both as to the quantity of water raised and as to the height to which it was elevated. On the other hand, the atmospheric engine was limited in its power only by the dimensions of its piston. Another considerable advantage which the atmospheric engine possessed over that of Savery, was the facility with which it was capable of driving machinery by means of the working-beam. The merit, however, of Newcomen's engine, regarded as an invention, and apart from merely practical considerations, must be ascribed principally to its mechanism and combinations. We find in it no new principle, and scarcely even a novel application of a principle. The agency of the atmospheric pressure acting against a vacuum, or partial vacuum, had been long known: the method of producing a vacuum by the condensation of steam had been suggested by Papin, and carried into practical effect by Savery. The mechanical power obtained from the direct pressure of the elastic force of steam, used in the atmospheric engine to balance the atmosphere during the ascent of the piston, was suggested by De Caus and Lord Worcester. The boiler, gauge pipes, and the regulator, were all borrowed from the engine of Savery. The idea of using the atmospheric pressure against a vacuum or partial vacuum, to work a piston in a cylinder, had been suggested by Otto Guericke, an ingenious German philosopher, who invented the air-pump; and this, combined with the production of a vacuum by the condensation of steam, was subsequently suggested by Papin. The use of a working-beam could not have been unknown. Nevertheless, the judicious combination of these scattered principles must be acknowledged to deserve considerable credit. In fact, the mechanism contrived by Newcomen rendered a machine which was before altogether inefficient, highly efficient: and, as observed by Tredgold, such a result, considered in a practical sense, should be more highly valued than the fortuitous discovery of a physical principle. The method of condensing the steam by the sudden injection of water, and of expelling the air and water from the cylinder by the injection of steam, are two contrivances not before in use, which are quite essential to the [Pg074] effective operation of the engine. These processes, which are still necessary to the operation of the improved steam engine, appear to be wholly due to the inventors of the atmospheric engine.

ATMOSPHERIC ENGINE.