PART III. HISTORY AND DESCRIPTION OF THE STEAM-ENGINE. Ir is not uninteresting to trace the progress of a great invention, from the first rude attempts till it attains a somewhat perfect form-to observe how often men have been on the brink of the discovery, and yet allowed it to escape them, and to mark the successive changes it undergoes; and there is, perhaps, no better or easier way to understand the later and more complex forms it assumes, than tracing it from the first simple conception, and at each stage contrasting it with its previous condition. We shall, therefore, prefix to the description of the modern engine a brief sketch of the progress of the invention from the earliest records. It will smooth the way very much in the investigation of the subject, to examine shortly the various modes of applying steam as a power, and divide the invention into separate stages as much as possible. This will enable us more easily to come to a judgment on the comparative value of each step. 171. There are two modes of using steam for the production of force or power. 1. Directly, when the force or pressure of the steam itself produces the power. 2. Indirectly, when the steam is used to form a vacuum, and thereby to give effect to some power acting into the where the vacuum is made. space I.-DIRECT ACTION OF STEAM. 172. The direct force of steam has been proposed to be applied to the production of motion in three principal modes. First, in point of simplicity, though not in order of chronological development, by the impetus which it has on issuing from a vessel in which water is boiled. Every one has noticed that the steam from a boiler is projected with a considerable degree of force. On the principle of the windmill, this current of steam in motion will communicate motion to the vanes of a wheel properly adjusted to receive it, and thus turn the wheel. This very simple plan for steam power was proposed by Branca in 1629, and by Kircher some years after. This 173. Second. The pressure of confined steam. is the next mode, in respect to obviousness and simplicity. If water be boiled, and the steam be prevented escaping, it will accumulate, and press with great force on the bodies which confine it, pushing aside, with more or less power, those which present least resistance-and thus raising liquids in contact with it, bursting the vessel in which it is confined, or moving a piston along a cylinder— according to the mode in which it is applied. The power of confined steam has been made use of in two principal modes: 1. To raise water, to which it is directly applied, as in the plans of Porta, De Caus, Savery, and Papin's second scheme.-2. to press a piston through a cylinder, and thus cause a rectilineal motion, which may be adapted by appropriate machinery to any purpose; as done by Leopold and Watt. 174. Third. Steam may be employed as a source of power, on the principle of the re-action of a fluid issuing forcibly from a tube, and if projected sideways, thereby tending to push back the tube in a direction opposite to that in which the fluid issues, as in Barker's mill. If the tube be made to turn on a pivot, and a continued stream issue sideways from its extremity, the tube will be turned continuously round, and thus a rotatory motion will be procured. This is the principle of the first steam machine, Hero's Eolipile, described 130 years B.C., and it was revived by Kempel in 1794, and brought lately into operation by Avery in America, and by Ruthven in this country. II. INDIRECT ACTION OF STEAM. 175. Steam is condensible into water; its elastic force is thereby reduced almost to an insensible amount, a nearly perfect vacuum being produced. If steam be driven into a vessel containing air, and with an aperture to permit the air, or air and steam, to pass out, the air will soon be expelled, and the space filled solely with steam. This aperture being now shut, and also that by which the steam entered, and the steam being cooled, it condenses, and the external air, or any other force acting towards the vacant space, not being now resisted, yields a source of force or power which may be easily applied so as to produce motion. Such are the modes in which steam may be used in the production of motion. 176. To produce the steam-engines of Savery, Newcomen, and Watt, the three which have come into use, some one or more of the preceding general properties of steam are called into action. But besides these, there are a number of points in the mechanical adaptation of the parts, which are essential to the construction of these engines. The following statement embraces the leading conditions of every kind, connected both with the steam and with the mechanism necessary to the invention of these engines, and which therefore have a value in relation to the steam-engine, and entitle the discoverer to a place among those who contributed to the development of steam power. Upon analysing the steam-engine, we find the following leading and distinct ideas entering into its construction and operation: Properties of Steam. 1. That a steady and continuously-acting power is procured by the confinement and regulated escape of the steam of boiling water. 2. That when steam is condensed, a vacuum is produced, into which the adjoining bodies will tend to rush. 3. That steam is most rapidly condensed by projecting water into it. 4. That the vapour of water has a considerable force or pressure, even at temperatures much below the boiling point, as 100° to 140°. 5. That a vacuum may be produced by steam, without cooling the vessel in which it is contained, if a communication be made between it and another vessel in which a continual vacuum is kept up. 6. That if the external pressure (the compressing force) on a quantity of confined steam be less than the force of the steam, it will tend to expand, and cause motion in the bodies confining it. 7. That the steam of common water contains air which does not condense on being cooled. Mechanical Contrivances. 1. That a piston may be pushed along a cylinder by the force of an aerial body beneath it, as steam, or the gases formed by the ignition of gunpowder. 2. That if a vacuum be produced in a cylinder below a piston, the atmospheric pressure will cause the piston to descend. 3. That an alternate rectilineal motion is easily transmitted to a distance by a lever (beam) working on a pivot.* 177. These modes of applying steam, these principles of its action, and these mechanical contrivances being kept in view, we find the following successive stages, and distinct ideas, in the development of the use of steam as a moving power ;-at the side are shown the names of the individuals who contributed each step. It is only by such an analysis that we can judge of the comparative value of the services of the successive labourers in this great work. 1. The publication of the fact that steam may be made to yield a force or power. 2. The publication of the fact that steam be made to yield a steady continuously-acting power.† may HERO, 130 B. C. HERO, 130. B. C. WORCESTER, 1663. *I have here given only those leading mechanical contrivances used in NEWCOMEN'S Engine. Those in WATT's Engine are so numerous, that they would require considerable space, and would render the subject complex: they will be alluded to separately afterwards. It will be at once perceived that this, a steady continuously-acting |