ceases to be condensed, and mixes with the air which filled the cylinder. The steam and heated air, having a greater force than the atmospheric pressure, will open a valve placed at the end x of a small tube in the bottom of the cylinder, and which opens outwards. From this (which is called the blowing valve*) the steam and air rush in a constant stream, until all the air has been expelled, and the cylinder is filled with the pure vapour of water. This process is called blowing the engine preparatory to starting it. When it is about to be started, the engine-man closes the regulator R, and thereby suspends the supply of steam from the boiler. At the same time he opens the condensing valve Ht; and thereby throws up a jet of cold water into the cylinder. This immediately condenses the steam contained in the cylinder, and produces the vacuum. (The atmosphere cannot enter the blowing valve, because it opens outwards, so that no air can enter to vitiate the vacuum.) The atmospheric pressure above the piston now takes effect, and forces it down in the cylinder. The descent being completed, the engine-man closes the condensing valve H, and opens the regulator R. By this means he stops the play of the jet within the cylinder, and admits the steam from the boiler. The first effect of the steam is to expel the condensing water and condensed steam which are collected in the bottom of the cylinder, through the tube Y, containing a valve which opens outwards (called the eduction valve), which leads to the hot cistern L, into which this water is therefore discharged. When the steam admitted through R ceases to be condensed, it balances the atmospheric pressure above the piston, and thus permits it to be drawn to the top of the cylinder by the weight of the rod D. This ascent of the piston is also assisted by the circumstance of the steam being somewhat stronger than the atmosphere. When the piston has reached the top, the regulating valve R is closed, and the condensing valve н opened, and another descent produced, as before, and so the process is continued. * Also called the snifting valve, from the peculiar noise made by the air and steam escaping from it. + Also called the injection valve. The manipulation necessary in working this engine was, therefore, the alternate opening and closing of two valves; the regulating and condensing valves. When the piston reached the top of the cylinder, the former was to be closed, and the latter opened; and, on reaching the bottom, the former was to be opened, and the latter closed. (42.) The duty of working the engine requiring no great amount of labour, or skill, was usually entrusted to boys, called, cock boys. It happened that one of the most important improvements which has ever been made in the working of steam engines was due to the ingenuity of one of these boys. It is said that a lad, named Humphrey Potter, was employed to work the cocks of an atmospheric engine, and being tempted to escape from the monotonous drudgery to which his duty confined him, his ingenuity was sharpened so as to prompt him to devise some means by which he might indulge his disposition to play without exposing himself to the consequences of suspending the performance of the engine. On observing the alternate ascending and descending motion of the beam above him, and considering it in reference to the labour of his own hands, in alternately raising and lowering the levers which governed the cocks, he perceived a relation which served as a clue to a simple contrivance, by which the steam engine, for the first time, became an automaton. When the beam arrived at the top of its play, it was necessary to open the steam valve by raising a lever, and to close the injection valve by raising another. This he saw could be accomplished by attaching strings of proper length to these levers, and tying them to some part of the beam. These levers required to be moved in the opposite direction when the beam attained the lowest point of its play. This he saw could be accomplished by strings, either connected with the outer arm of the beam, or conducted over rods or pulleys. In short, he contrived means of so connecting the levers which governed the two cocks by strings with the beam, that the beam opened and closed these cocks with the most perfect regularity and certainty as it moved upwards and downwards. Besides rendering the machine independent of manual su perintendence, this process conferred upon it much greater regularity of performance than any manual superintendence could ensure. This contrivance of Potter was very soon improved by the substitution of a bar, called a plug frame, which was suspended from the arm of the beam, and which carried upon it pins, by which the arms of the levers governing the cocks were struck as the plug-frame ascended and descended, so as to be opened and closed at the proper times. The engine thus improved required no other attendance except to feed the boiler occasionally by the cock т, and to attend the furnace. (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 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 airpump; 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 con |