Two years afterwards Trevithick constructed the first successful railway locomotive, which was used on the Merthyr Tydvil Railway in the year 1804. This engine had an eight-inch cylinder, of four feet six inches stroke, placed horizontally as at present, and working on a cranked axle; while, in order to secure a continuous rotatory motion, a fly-wheel was placed on the end of the axle. When we add to this, that the fly-wheel was furnished with a break, that the boiler had a safety-valve or a fusible plug beyond the reach of the engineer, and that the patent includes the production of a more equable rotatory motion--"by causing the piston rods of _two_ cylinders to work on the said axis by means of cranks at a quarter of a turn asunder"--it is scarcely too much to say that nothing material was added to the design of the locomotive until the invention of the tubular boiler in 1829.[43] On the occasion of its first trial, on the 21st of February, 1804, this engine drew carriages containing ten tons of bar iron for a distance of nine miles, at the rate of five miles an hour. The specification of the patent for Trevithick's steam-carriage mentions a plan for causing the wheels, _in certain cases_, to take a stronger hold of the ground by means of sundry rough projections, but it also adds that, _in general, the ordinary structure or figure of the external surface of these wheels will be found to answer the intended purpose_, which appears to have been the case in the above-mentioned engine.[44] After making a few experiments with his engine, Trevithick forsook the locomotive for other projects of his versatile genius, and this great invention was left to be perfected and carried into general use by George Stephenson.