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 1-1/2-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] [Footnote 171: See Trevithick's letter, 7th May, 1815, vol. i., p. 364.] [Footnote 172: See patent specification of 1802, vol. i., p. 128.] 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. "MR. GILBERT, "HAYLE FOUNDRY, _November 5th, 1829_.