Another element, of which American engineers have freely availed themselves, is curvature. More power is required to draw a train of cars around a curved track than upon a straight line. In England the radius of curvature is limited to half a mile, or thereabouts. The English railway-carriage is placed on three axles, all of which are fixed to the body of the vehicle; the passage of curves, of even a large diameter, is thus attended by considerable wear and strain; but in America, the cars, which are much longer than those upon English roads, are placed upon a pintle or pin at each end, which pin is borne upon the centre of a four-wheeled truck,--by which arrangement the wheels may conform to the line of the rails, while the body of the car is unaffected. This simple contrivance permits the use of curves which would otherwise be entirely impracticable. Thus we find curves of one thousand feet radius upon our roads, over which the trains are run at very considerable speed; while in one remarkable instance (on the Virginia Central Railroad, before named) we find the extreme minimum of 234 feet. Such a track does not admit of high speeds, and its very use implies the existence of natural obstacles which prevent the acquirement of great velocities. In fine, the use which the engineer makes of grades and curves, when the physical nature of the country and the nature and amount of the traffic expected are known, may be taken as a pretty sure index of his real professional standing, and sometimes as an index of the moral man; as when, for example, he steepens his grades to suit the contractor's ideas of mechanics,--in other words, to save work. Not less in the construction of bridges and viaducts, than in the preparation of the road-bed proper, does the American engineering faculty display itself. Timber, of the best quality, may be found in almost every part of the country, and nowhere in the world has the design and building of wooden bridges been carried to such perfection and such extent as in the United States. We speak here of structures built by such engineers as Haupt, Adams, and Latrobe, --and not of those works, wretched alike in design and execution, which so often become the cause of what are called terrible catastrophes and lamentable accidents, but which are, in reality, the just criticisms of natural mechanical laws upon the ignorance of pretended engineers.