We have already described the high pressure en gine invented by Lett pold. Without knowing what had been done by Leupold, Mr. Watt conceived the same idea, but his views were never put in practice, and it was reserved to Mr. Trevithick of Camborne in Cornwall, to bring the high pressure engine into general use. In conjunction with Mr. Vivian of the same place, he took out a patent in 1802, for a high pressure engine. His principal object was to form an engine so compact and portable that it could be ap plied to the moving of carriages on rail roads. This great object he completely attained, and it was first applied as the moving power of carriages on a railway at Merthyr Tydvil in 1805. Since that time it has been employed in various collieries near Leeds, Wigan, and Newcastle upon Tyne.
This engine is represented in Plate DIX. Fig. 4., where AA is a cylindrical boiler of cast iron from three to four feet in diameter, and from nine to twelve feet long. The fire is made in a double wrought iron tube of the form of a syphon (one of the legs is shown at D) lying horizontally within the large cylinder AA. The two ends of the double tube are attached to the plate d. One of the ends is occupied with the fire door and ash pit, and the bars on which the fire is made, while its other end contracts into the iron chimney or flue T, having a door Z below for removing the soot. The boiler is filled with water above the surface of the double tube D, as shown in the figure. The steam cylinder A is almost wholly inclosed within the boiler, so as to be kept at the same heat as the water. The piston rod II attached to the piston G is fixed to the middle of a cross bar I at right an gles to the length of the boiler. At the extremity of the cross bar arc two connecting rods L, the lower ends of which are jointed to two cranks which drive the axis of the fly-wheel M. When a vertical reci procating motion is required, it is attained at once from the cross bar I. The fourway cock for admitting the steam into the cylinder, and already represented in Leupold's engine, is shown at i, f, g, k. The steam from the boiler passes directly through the passage g, and brings steam to the cock, so that it can be admitted either above or below the piston, ac cording to the position of the cock, f being the pas sage that carries it above the piston, and k that which carries it below the piston. The fourth passage i al lows the steam to fly off into the flue T after it has exerted its expansive force upon the piston. The me
thod of opening and shutting the cocks is similar to that used in other engines. The safety valve n is kept down by a lever p v with a weight p. The cold water is carried to the boiler by a pipe r enclosing the waste-pipe IF which is kept hot by the discharged steam, and which, therefore, gives out its heat to the cold water.
In the engines which Mr. Trevithick erected in Cornwall, he introduced the cylindrical tube boiler which is now generally used in that part of England. It is shown in Plate DIX. Fig. 5, where as represents the part containing the water. The fire place is at the end of the tube A, and the heated air, after passing through A, returns through the flue B, which passes horizontally beneath the boiler to the end at which the fire is situated. Here it divides into two branches which pass along c c, into the flues DD, by which it is conveyed along the sides of the boiler, and thence escapes to the chimney.* ll'oolf's Engine.
The engines introduced by Mr. Woolf were founded on an erroneous law of expansion, which, having been given by so respectable an engineer as the direct result of experiment, misled many writers into the belief of its correctness. But though the law which he assumed was erroneous, yet the engines which he introduced had considerable merit. They were nothing more indeed, than the application of high pressure steam to the double cylinder engine of Hornblower, in which Mr. Watt's condensing apparatus was em ployed in consequence of the expiry of his patent.
When Mr. Woolf went into Cornwall he erected some engines of this kind, and by having them made and fitted together with much more accuracy than had hitherto been the practice in that neighbourhood, _he obtained from them a much better performance than had yet been obtained from Mr. Watt's engines. One of these at Iluel Abraham mine was found, during a trial of twenty-four hours, to lift seventy million of pounds one foot high by the combustion of one bushel of coal (Mr. Henwood in Dr. Brewster's Journal, No. xix, p. 36.) Their effect was very far beyond that of any of Mr. Watt's engines which were then at work in the neighbourhood, as appears from a state ment of their performance which was published pe riodically ; but this arose not from any superiority in the principle of Woolf's engine, but solely from the great attention which he paid to the joints, and the fitting together of the parts.