In 1802 MeBRILTILEVITHICK & VTVIAN (two engineers resi at Cambourne, in Cornwall,) obtain a patent for a high-pressure engine. The first applica tion of high-pressure steam to an engine, was, as previously stated, described by Leopold; the next proposal was vaguely made by Mr. Watt, in the fourth par ticular of his specification of 1769, but lie danger incident to the employment of an agent so formidable, was considered, so it would seem, too perilous an obstacle in the way of its introduction ; and up to this period, practical men had been contented with the bare conviction of its adaptability, without incur ling the daring risk of its actual op crhe distinction between a high and a low-pressure engine lies in the former being wrought solely by the expansive force of steam upon the piston both in its ascent and descent, whilst, in the latter, the elasticity of the steam is employed in lieu of atmospheric pressure. •• (See page 719) The advan tages of a high-pressure engine are chiefly the simplicity and cheapness of its construction, the small space which it occupies, and the enormously greater force applied to the piston—the pressure at which it is worked extending fre quently to sixty or eighty pounds per square inch. To these advantages, how ever, are opposed the liability of the boiler to bursting, and the impairment of the machinery by negligence or insufficient strength of the material) The object for which Messrs. Trevithick and Vivian designed their engine, was that it might be applied to carriages for locomotion, for which the atmo spheric engines, by their bulk, the cumbrous machinery of the condensing apparatus, and other obvious considerations, were quite unfitted. Forsaking, therefore, the now well-beaten path which their predecessors had followed, and fortified in .their attempt by the facilities which superiority of construction in the details of mechanism at this time afforded (facilities that the earlier experi mentalists certainly did not possess, and obtained only through the attention that had been excited, by the successful improvements made in other engines,) they at length contrived a machine, the introduction of which, by its excellence and efficacy, formed a new era in the history of steam power.
A, Fig. 1, represents the boiler, made of a round figure to bear the expansive action of strong steam. The boiler is fixed in a case D, luted inside with fire clay, the lower part of which constitutes the fire-place B, and the upper cavity affords a space round the boiler in which the flame, or heated vapour, circulate. till it comes to the chimney E. The case D and the chimney are fixed upon a platform F, the case being supported upon four legs ; C represents the cylinder, enclosed for the most part in the boiler, having its nozle, steam-pipe, and bottom cast all in one piece (in order to resist the strong steam), and also with the dockets in which the iron uprights of the external frame are firmly fixed. G represents a cock for conducting the steam, as may be more clearly seen by observing Pig. 2, which is a plan of the top of the cylinder. b, IV. 2, represents the passage from the boiler to the cock G; this passage has a throttle-valve or shut, adjust able by a handle, so as to wire-draw the steam, and suffer the supply to be quicker or slower. The position of the cock is such that the communication from the boiler through b, by a channel in the cock, is made good to d, which denotes the upper space of the cylinder above the piston, at the same time that the steam pipe a (more fully represented in lig. 1,) is made to afford a passage from the lower space in the cylinder, beneath the piston, to the channel C, through which the steam may escape into the outer air, or be directed or applied to heating fluids, or other useful purposes. It will be obvious, that if the cock be turned
one quarter of a turn in either direction, it will make a communication from the boiler passage b, to the lower part of the cylinder, by or through a, at the same time that the passage r, from the upper part of the cylinder, will communicate with C, the passage for conveying off the steam. P Q is a piston rod, moving between guides, and driving the crank R S, by means of the rod Q B, the axis of which crank carries the fly T, and is the first mover to be applied to drive the machinery at S. X Y is a double snail, which, in its rotation presses down the small wheel 0, and raises the weight N, by a motion in the joint M of the lever 0 N, from which downwards proceeds an arm M L, and consequently the extremity L is at the same time urged outwards. This action draws the hori zontal bar L I, and carries the lever handle H I, which moves upon the axis of the cock G, through one-fourth of a circle. It must be understood that H I is fore-shortened (the extremity I being more remote from the observer than the extremity H), and also there is a clack or ratchet-wheel on the part H, which gathers up during the time that L is passing outwards, and does not then move the cock G, but that, when the part X of the snail opposite 0, that is to say, when the piston is about the top of its stroke, then the wheel 0 suddenly falls into the concavity of the snail, and the extremity of L, by its return, at once pushes I H through the quarter circle, carries with it the cock G, and turns the steam upon the top of the piston, and also affords a passage for the steam to escape from beneath the piston. Every stroke, whether up or down, produces this effect by the half-turn of the snail, and reverses the steam ways as before described; or the cock may be turned by various well-known methods, such as the plug, with pins or clumps striking on a lever in the usual way, and the effect will be the same, whether the quarter turns be made backward or for ward, or be a direct circular motion, as is produced by the machinery here repre sented : but the wear of the cock will be more uniform and regular if the turns be all made in the same way. Other illustrations of this simple and beautiful machine appear in the article RAILWAYS.
A patent was obtained by Mr. Arrnua WooLvs, in 1804, for improvements in steam-engines, founded upon an assumed discovery by hint, respecting the expansive properties of steam. It had been asserted, before his time, that steam, acting with the expansive force of four pounds per square inch, was capable of expanding itself to four times the volume it then occupied, and be still equal to the pressure of the atmosphere. Mr. Woolfe's experiments led him to infer that, in like manner, steam of the force of five pounds the square inch, could expand itself to five times its volume, and so on correspondingly in numerical relation up to twenty, forty, and fifty pounds the square inch. Subsequent investigation, however, demonstrated the error of his experiments, and the en tire fallacy of the proposition deduced from them. In a former page are given tables of the expansive force of steam at different temperatures, and the gene ral correctness of the statements which there appear has been since established, by the results of an inquiry instituted by the French Academy of Sciences.