The first glance shows a seeming resemblance in outline to Trevithick's patent drawing of 1832, having one cylinder above the other; but a closer examination proves the application of the principles of his patents of 1815 and 1831, embracing screw-propeller, direct-acting engines, tubular boilers, high-pressure steam used expansively, and condensation by cold surface preventing the necessity of using salt water in the boilers.
This engine, in outline, has a strong likeness to Trevithick's engines, going back even to his first patent of 1802,[210] followed by the direct-action high-pressure steam yacht of 1806,[211] and again in 1808[212] by the iron steamer with direct-action long-stroke cylinders, with highly expansive steam and surface condensers, to which, in 1815,[213] was added the patent compound expansive steam pole and piston engine and screw-propeller, embodying during the first fifteen years of the present century, both in principle and in detail, the most approved form of marine steam-engine with fewness and simplicity of form of moving parts; but compare it with the Watt patent engine, and its difference is obvious; no beam or parallel motion, no injection-water necessitating the air-pump, no low-pressure steam. The late Mr. William Wilson, of Perran Foundry, son of Boulton and Watt's financial agent in Cornwall, informed Mr. Henwood that he was with Mr. Watt when some one stated that Mr. Trevithick was working his engine with steam of 40 lbs. on the inch; when Mr. Watt replied, "I could work my engine with steam of 100 lbs. to the inch, but I [would not] be the engineman."[214]
Progressive experience, with increasing demand for economy and speed, have caused the principles and the details of Trevithick's steam-engines to be matters of national importance seventy years after their discovery, for as far back as that he used highly-expansive steam,[215] and on the question of a separate cylinder for expansion as used in the modern steamboat combined engines, he wrote, "I think one cylinder partly filled with steam would do equally as well as two cylinders; that one at Worcester shuts off the steam at the first third of the stroke, and works very uniformly with a considerable saving of coal."[216] Those modern marine engines use about the same steam pressure and expand about in the same proportion. With the direct action from the piston-rod to the crank-shaft, the multitubular boiler and screw-propeller, and the surface condenser perfected in 1831 and 1832, at which time his construction of a marine steam-engine would have been just what it now is forty years later. Those latter patents also embrace the principle of superheating steam, practically shown many years before,[217] and still used by marine engineers of modern times.
In tracing the wisdom of his designs just before the close of an eventful life, reference may be made to the trial of a common road locomotive in 1871:—"Experimental trip of the Indian Government steam train engine, 'Ranee,' from Ipswich to Edinburgh.—The results of the trial with the 'Chenah,' though satisfactory so far as the engines proper were concerned, were vitiated by the failure of the boiler; on the completion of the second engine, the 'Ranee,' the field boiler and variable blast-pipe were used; the boiler is about 4 feet diameter at the bottom and 8 feet high."[218]
The form and dimensions of the exterior of the Ranee tubular boiler are very similar to Trevithick's patent drawing and specification of 1832; even the variable blast-pipe was used by him in 1802.[219]
The last years of Trevithick's eventful life were chequered with hopes and disappointments when, in the early part of 1830, he wrote to his friend Gerard:—
"This morning I called here for the purpose of forwarding my information to the committee of the House. I called on Mr. Thompson to inform him what Mr. Gilbert said respecting it. His answer was, that the direct method would be by forwarding a petition in the way proposed when at the lobby. In consequence, I have forwarded the petition to Sir Matthew Ridley. Yesterday I took the coach to Highgate, by way of Camden Town, and of course had to walk up Highgate Hill. I found I was able to walk up that hill with as much ease and speed as any of my coach companions. However strange this maggot may appear in my chest and brain, it is no more than true. I wish among all you long-life-preserving doctors you could find out the cause of this defect, so as to remedy this troublesome companion of mine."
His health was breaking down, and his petition for a gift from the public purse, so hopefully commenced two years before, was doomed, after another year's bandying from pillar to post, to be forgotten and unanswered.
"Eastbourne, December 26th, 1831.
"Dear Trevithick,