Gran' lents. —111 respect of the cost of constructing and operating a rail road the admissible gradients play an important role, since not only the possibility of adapting the road-bed more or less closely to the natural sur face without the necessity of resorting to artificial structures (bridges, tun nels, etc.) depends upon them, but also the weight and capacity of the locomotives that will be required, and, consequently, the weight and speed of the trains. There is a grade-limit beyond which a locomotive cannot be employed with useful effect, inasmuch as the power that must be ex pended to move the engine itself—and therefore not available for haul ag-e—is so great on very steep grades that there is no surplus for useful traction. As the power of the locomotive is brought into action by the grip or friction of its driving-wheels upon the rails, the maximum pulling power will be obtained when the entire weight of the machine is supported on the driving-wheels, thus giving them the greatest amount of adhesion. Even where this condition is most closely approximated the effect of steep gradients on the tractive power of the locomotive is so serious that they must be carefully avoided, so that the profile of the road-bed shall conform as nearly to a level as the surface-nature of the territory through which it is laid will permit.
To exhibit this detrimental influence of gradients on the tractive power of the locomotive the following example may serve: Assuming the case of an engine weighing (with tender) forty tons, and capable, when exerting its maximum power, of drawing forty-two loaded cars weighing ?our hun dred and twenty tons at the rate of twenty miles an hour on a level, the loads it would be able to draw at the same speed on various ,gradients are exhibited in the following table—namely: To avoid such serious loss of effective power the earliest railroads were built with the road-bed nearly level, to accomplish which often involved enormous expense. This was rendered imperative on account of the very deficient power of the locomotives. As locomotive-builders learned to in crease the power of the machine it became possible to operate roads with steeper gradients, and the railway-train to-day easily scales the heights of the Alps of Europe, and of the Alleghanies, the Rocky Mountains, and even the more formidable Andes, of America. Where the precipitous na ture of the ground renders the construction of a railroad upon the surface impossible, or, on account of the cost of its operation, inexpedient, resort must be had to other systems of construction or to tunnels.
CHIVeS. —The curves that should be permitted on the projected line re quire careful consideration, and will enter largely into the question of selecting the route, inasmuch as they not onlv cause a loss of locomotive power, but also throw injurious strains upon the locomotive and cars and increase greatly the wear upon the rails. The shorter the radius of the curves, the more pronounced will be their injurious effects. On striking a curve there will be a certain g-rinding action of one of the front wheels and of the diagonally-opposite hind wheel against the rail, which causes rapid wear and tear of wheels, journals, and rails. Furthermore, as the
outside rail on a curve is longer than the inside one, the outside wheel must travel farther in rounding it; but, as each pair of wheels is rigidly attached to its axle, the outer wheel must slip or slide on the rail, which induces additional friction and wear of wheel and rail. These difficulties may be overcome partially by an arrangement of the journal-boxes so as to permit of a certain amount of " end-plav " to the axle and by making the tread of the wheels slightly conical, or by slightly increasing the gauge of the road in the curve, or by the device of a " loose wheel," by which one wheel may revolve independently of the other.
Elemtion of Rail at Curves.—To counterbalance the effect of the cen trifugal force generated in swing,ing around curves—which is to press the flange of the wheel against the outer rail—the outer rail is given an eleva tion above the inner one, the amount of elevation being determined by the sharpness of the curve. Inasmuch, however, as the forces here called into play will vary in intensity according- to the speed, the weight of the train, and other circumstances, the corrective applied must be such as will be most likely to snit the average condition of practice. The curve, therefore, in spite of the artifices that are employed to reduce its bad effects, remains an evil which must be avoided as far as possible in 'practice. It may be noticed at this point that, generally, the two evils of frequent curves and steep gradients are found associated in the sante road.
Xumber of Tracks.—Where the projected road promises to acquire fu ture importance, or where it passes through a thickly-settled district, the road-bed should be constructed sufficiently wide and substantial to admit of a double track. The road-bed mar be widened and additional tracks laid when the traffic justifies the outlay.
Track-zeidtk: Standard Gauge.—Respecting the width of the track., the so-called " standard gauge" (4 feet Syj inches of width between the rails) is now in very common use in both Europe and America upon all the main railroad lines. This particular width of gauge was adopted by the English as a standard, for no assignable reason other than that it was the common gauge of road-vehicles in England, and has been copied by railroad-builders elsewhere. The advantag,e of uniformity in gauge in permitting of the interchange of traffic between the different lines of rail road is obvious, and accounts for the very general adoption of the standard gauge. There is still, however, considerable divergence from uniformity. Russia adopts 5 feet as the standard; Spain and Portug-al, 5Y feet; Ireland, feet. In the United States there was until within the past ten years much irregularity in gauge in various sections, but all the main lines, and most of the secondary lines which diverged from them, have since changed their gauges to conform to the standard.