The arrangement of Trevithick's engine was exceedingly ingenious. It exhibited a beautiful simplicity of parts; the machinery was arranged in a highly effective form, uniting strength with solidity and portability, and enabling the power of steam to be employed with very great rapidity, economy, and force. Watt's principal objection to using high-pressure steam consisted in the danger to which the boiler was exposed of being burst by internal pressure. In Trevithick's engine, this was avoided by using a cylindrical wrought-iron boiler, being the form capable of presenting the greatest resistance to the expansive force of steam. Boilers of this kind were not, however, new. Oliver Evans, of Delaware, had made use of them in his high-pressure engines prior to the date of Trevithick's patent; and, as Evans did not claim the cylindrical boiler, it is probable that the invention was in use before his time. Nevertheless, Trevithick had the merit of introducing the round boilers into Cornwall, where they are still known as "Trevithick boilers." The saving in fuel effected by their use was such that in 1812 the Messrs. Williams, of Scorrier, made Trevithick a present of £300, in acknowledgment of the benefits arising to their mines from that source alone. Trevithick's steam-carriage was the most compact and handsome vehicle of the kind that had yet been invented, and, indeed, as regards arrangement, it has scarcely to this day been surpassed. It consisted of a carriage capable of accommodating some half-dozen passengers, underneath which was the engine and machinery inclosed, about the size of an orchestra drum, the whole being supported on four wheels--two in front, by which it was guided, and two behind, by which it was driven. The engine had but one cylinder. The piston-rod outside the cylinder was double, and drove a cross-piece, working in guides, on the opposite side of the cranked axle to the cylinder, the crank of the axle revolving between the double parts of the piston-rod. Toothed wheels were attached to this axle, which worked into other toothed wheels fixed on the axle of the driving-wheels. The steam-cocks were opened and shut by a connection with the crank-axle; and the force-pump, with which the boiler was supplied with water, was also worked from it, as were the bellows to blow the fire and thereby keep up the combustion in the furnace. The specification clearly alludes to the use of the engine on railroads as follows: "It is also to be noticed that we do occasionally, or in certain cases, make the external periphery of the wheels uneven by projecting heads of nails or bolts, or cross grooves or fittings to railroads where required, and that in cases of hard pull we cause a lever, belt, or claw to project through the rim of one or both of the said wheels, so as to take hold of the ground, but that, in general, the ordinary structure or figure of the external surface of those wheels will be found to answer the intended purpose."