He ascertained that the resistances to traction were mainly three; the first being upon the axles of the carriages, the second, or rolling resistance, being between the circumference of the wheel and the surface of the rail, and the third being the resistance of gravity. The amount of friction and gravity he could accurately ascertain; but the rolling resistance was a matter of greater difficulty, being subject to much variation. He satisfied himself, however, that it was so great when the surface presented to the wheel was of a rough character, that the idea of working steam carriages economically on common roads was dismissed by him as entirely impracticable. Taking it as 10 lbs to a ton weight on a level railway, it became obvious to him that so small a rise as 1 in 100 would diminish the useful effort of a locomotive by upwards of 50 per cent. This was demonstrated by repeated experiments, and the important fact, thus rooted in his mind, was never lost sight of in the course of his future railway career. It was owing in a great measure to these painstaking experiments that he early became convinced of the vital importance, in an economical point of view, of reducing the country through which a railway was intended to pass as nearly as possible to a level. Where, as in the first coal railways of Northumberland and Durham, the load was nearly all one way,—that is, from the colliery to the shipping-place,—it was an advantage to have an inclination in that direction. The strain on the powers of the locomotive was thus diminished, and it was easy for it to haul the empty waggons back to the colliery up even a pretty steep incline. But when the loads were both ways, he deemed it of great importance that the railroad should be constructed as nearly as possible on a level. These views, thus early entertained, originated in Stephenson’s mind the peculiar character of railroad works as distinguished from other roads; for, in railways, he early contended that large sums would be wisely expended in perforating barriers of hills with long tunnels, and in raising the lower levels with the excess cut down from the adjacent high ground. In proportion as these views forced themselves upon his mind and were corroborated by his daily experience, he became more and more convinced of the hopelessness of applying steam locomotion to common roads; for every argument in favour of a level railway was, in his view, an argument against the rough and hilly course of a common road.