This point gained, the objectors advanced a step, but again came to a stand, and said, “If you can move a train on a level, that is all, —­you can’t go up hill.”  But trial proved that easy inclines (called grades) could be surmounted,—­say, rising ten feet for each mile in length.

The objectors take another step, but again put down their heavy square-toed foot, and say, “There! aren’t you satisfied? you can go over grades of twenty feet per mile, but no more,—­so don’t try.”  And here English engineers stop,—­twenty feet being considered a pretty stiff grade.  Meanwhile, the American engineers Whistler and Latrobe, the one dealing with the Berkshire mountains in Massachusetts, the other with the Alleghanies in Virginia, find that not only are grades of ten and of twenty feet admissible, but, where Nature requires it, inclines of forty, sixty, eighty, and even one hundred feet per mile,—­it being only remembered, the while, that just as the steepness of the grade is augmented, the power must be increased.  This discovery, when properly used, is of immense advantage; but in the hands of those who do not understand the nice relation which exists between the mechanical and the financial elements of the question, as governed by the speed and weight of trains, and by the funds at the company’s disposal, is very liable to be a great injury to the prospects of a road, or even its ruin.

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.