"I have a cistern of cold water, with a proper condenser in it, connected between the bottom of the boiler-case and the force-pump to the bottom of the boiler, therefore I can partially condense by cold water sides, or by cold air sides just as I please, by rising or sinking the water in the cistern.
"The boiler is made very strong to try different temperatures, and an additional length to the water-pump makes all very suitable for a great number of experiments, and if there is any good in the thing I will bring it out.
"I shall have indicators at different places to prove what advantages can be gained. I hope to have the pleasure of your company during those experiments, which I think will throw more light on this subject than ever has yet been done. Some trials since I last wrote to you make me very confident that much good will arise from these experiments, but to what extent is uncertain.
"I remain, Sir,
"Your most obedient servant,
"Richard. Trevithick.
Trevithick did not use letters to illustrate his sketch, knowing that Davies Gilbert would comprehend it; but the reader of to-day may not find it so easy, therefore the writer has added them with a slight detail description, he having been Trevithick's daily companion when those drawings and experiments were made. a, top of boiler; b, water line; c, centre of wheel; d, cast-iron wheel and chain; e, chimney, 13 in. in diameter; f, fire-tube, 2 ft. diameter; g, outer boiler-case, 3 ft. diameter, 15 ft. long; h, water space of 6 in.; i, boiler steam-case, 3 ft. 4 in. diameter; j, small holes through which steam and water are forced into the boiler; k, force-pump, 10 in. diameter, 2 ft. 9 in. stroke; l, steam-cylinder, 14 in. diameter, 6-ft. stroke; m, piston-rod; n, fire-door; o, fire-bars; p, pump for testing the power of the engine.
There is a natural tendency in men of genius to unwittingly return, under new forms, to old ideas. The ideas are similar, though in combination with new forms and new acquirements; even the outline of this 1828 boiler, with the exception of its outer steam-casing, is very like that in a letter to Davies Gilbert fourteen years before,[171] of which Trevithick had kept no copy. When in the foregoing letter he wrote, "There is a steam-case round the outside with a 1½-inch space; this keeps the boiler hot and partially condenses the steam before it is again forced into the boiler," he had forgotten that twenty-seven years before, when constructing his first high-pressure steam-engines, he thus specified his invention:—"The steam which escapes in this engine is made to circulate in the case round the boiler, where it prevents the external atmosphere from affecting the temperature of the included water, and affords by its partial condensation a supply for the boiler itself."[172]
Not one of his numerous patent specifications has been found among his papers, neither do his letters refer to them; probably he never read them after the first necessary examinations.
"Hayle, November 5th, 1829.
""Mr. Gilbert,