334 TUBULAR BOILER, SUPERHEATING STEAM, 



Trevithick did not use letters to illustrate his sketch, 

 knowing that Da vies 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 descrip- 

 tion, 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 ; 

 c?, cast-iron wheel and chain; e, chimney, 13 in. in 

 diameter ; /, 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 

 throiigh which steam and water are forced into the 

 boiler; k, force-pump, 10 in. diameter, 2 ft. 9 in. stroke ; 

 /, 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, 1 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 4-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 

 atinosphere from affecting the temperature of the in- 



1 Sec Trevithick's letk-r, 7th May, 1815, vol. i., p. 364. 



