In all of the locomotives which have been illustrated, excepting the "South Carolina," the axles were held by the frames so that the former were always parallel to each other. In going around curves, therefore, there was somewhat the same difficulty that there would be in turning a corner with an ordinary wagon if both its axles were held parallel, and the front one could not turn on the kingbolt. The plan of the wheels and running gear of the "South Carolina" shows the position that they assumed on a curved track (Fig. 5). It will be seen that, by reason of their connection to the boiler by kingbolts, _K K′_, the two pairs of wheels could adjust themselves to the curvature of the rails. This principle was afterward applied to cars, and nearly all the rolling-stock in this country is now constructed on this plan, which was proposed by Mr. Allen in a report dated May 16, 1831, made to the South Carolina Canal & Railroad Company; and an engine constructed on this principle was completed the same year. In the latter part of the year 1831 the late John B. Jervis invented what he called "a new plan of frame, with a bearing-carriage for a locomotive engine," for the use of the Mohawk & Hudson Railroad. Jervis's engine is shown by Figure 9. In a letter published in the _American Railroad Journal_ of July 27, 1833, he described the objects aimed at in the use of the truck as follows: The leading objects I had in view, in the general arrangement of the plan of the engine, did not contemplate any improvement in the power over those heretofore constructed by Stephenson & Company,[11] but to make an engine that would be better adapted to railroads of less strength than are common in England; that would travel with more ease to itself and to the rail on curved roads; that would be less affected by inequalities of the rail, than is attained by the arrangement in the most approved engines. [Illustration: Fig. 9.--John B. Jervis's Locomotive, 1831, and Plan of its Running Gear.] In Jervis's locomotive the main driving-axle, _A_, shown in the plan of the wheels and running gear, was rigidly attached to the engine-frame, _a b c d_, and only one truck, or "bearing-carriage," _e f g h_, consisting of the two pairs of small wheels attached to a frame, was used. This was connected to the main engine-frame by a kingbolt, _K_, as in Allen's engine. The position of its wheels on a curve, and the capacity of the truck, or "bearing-carriage," to adapt itself to the sinuosities of the track are shown in the plan. The effectiveness of the single truck for locomotives, in accomplishing what Mr. Jervis intended it for, was at once recognized, and its almost general adoption on American locomotives followed.