It was urged at one time, that the best road would have the grades undulating from one end to the other,--so that the momentum acquired in one descent would carry the train almost over the succeeding ascent; and that very little steam-power would be needed. This idea would have place, at least to a certain extent, if the whole momentum was allowed to accumulate during the descent; but even supposing there would be no danger from acquiring so great a speed, a mechanical difficulty was brought to light at once, namely, that the resistance of the atmosphere to the motion of the train increased nearly, if not quite, as the square of the speed; so that after the train on the descent acquired a certain speed, a regular motion was obtained by the balance of momentum and resistance, --whence a fall great enough to produce this regular speed would be advantageous, but no more. On the other hand, the extra power required to draw the train up the grades much overbalances the gain by gravity in going down. Here, then, we have the two extremes: first, spending more money than the expected traffic will warrant, to cut down hills and fill up valleys; and second, introducing grades so steep that the amount of traffic does not authorize the use of engines heavy enough to work them. The direction of the traffic, to a certain extent, determines the rate and direction of the inclines. Thus, the Reading Railroad, from Philadelphia up the Schuylkill to Reading, and thence to Pottsville, is employed entirely in the transport of coal from the Lehigh coal-fields to tide-water in Philadelphia; and it is a very economically operated road, considering the large amount of ascent encountered, because the load goes down hill, and the weight of the train is limited only by the number of empty cars that the engine can take back. This adoption of steep inclines may be considered as an American idea entirely, and to it many of our large roads owe their success. The Western Railroad of Massachusetts ascends from Springfield to Pittsfield, for a part of the way, at 83 feet per mile. The New York and Erie Railroad has grades of 60 feet per mile. The Baltimore and Ohio climbs the Alleghanies on inclines of 116 feet per mile. The Virginia Central Road crosses the Blue Ridge by grades of 250 and 295 feet per mile; and the ridge through which the Kingwood Tunnel is bored, upon the Baltimore and Ohio Railroad, was surmounted temporarily by grades of 500 feet per mile, up which each single car was drawn by a powerful locomotive.