The usual length of a one-tram plate is three feet; the is one and a half inch high ; the sole, or bed, three and a half or four inches broad, and three-fourths of an inch thick ; but these dimensions are varied, according to eunistances : the most approved weight has been 42 lbs. for each plate. The ends from which the plugs project, and on which the tenons and notches are made, should be a quarter of an inch thicker than the other parts of the plate.
By this method, the wheels of wagons cannot be obstructed by the heads of the nails rising above the surface, and the blocks are not disturbed by fixing the plates.
Turn-outs are made by means of a movable or switch-rail, at the angle where the turn-out track branches from the main one. This rail is two or three feet, more or less, in length, and one end may be moved over that angle, and laid so as to form a part of the main track, or the turn-out track. The switch-rail is usually moved by the hand, so as to form a part of that track on i which the wagon is to move.
The principal consideration, in regard to the carriages, relates to their bearings on the axle and the rim of the wheel. The rule given by Wood, as to the bear ing on the axle, is, that, in order to pro duce the least friction, the breadth of the bearing should be equal to the diameter of the axle at the place of bearing. This diameter must be determined by the weight to be carried ; and the breadth of the bearing will accordingly vary with it. The objection to the plate-rail is, that the breadth of the hearing of the rim of the wheel upon such a rail causes an unne cessarily additional friction ; and the re sistance to the wheel is increased in con sequence of the greater liability of such a rail to collect dust and other impedi ments upon its surface. The edge-rail is preferable in these respects ; but, at first, these rails were liable to one difficulty, in consequence of their wearinggrooves in the rim of the wheel, so that the fric tion is continually increasing, and the wheel so becomes unfit for use. To remedy this defect, the rims are case hardened, or chilled by rolling them, when hot, against a cold iron cylinder. Wheels so case-hardened are found to be subject to very little wear.
It has been found, in practice, that, for the ordinary inclinations of railroads, N feet per mile, the wheels may be so constructed as to move a train of wagons by their mere adhesion to the rails. The
inclination which can be so overcome must evidently depend on the kind of surfaces of the rim of the wheel and the rail, the weight bearing upon the wheels, the weight to be moved, and the resist ance from the friction of the train of wagons ; so that no precise rule can be given that shall be applicable to roads and wheels of different materials and construction.
Curves on Railways.—With a view to insure the public safety, the British legislature has generally required that no curve shall be allowed upon a main line with a less radius than one mile : the exceptions to this are where one railway passes into another ; and at the termini, or the entrance of depots or stations. In such situations the trains must slacken their speed, and therefore a sharp curve is attended with less danger. It has ap peared, however, that these restrictions upon the radii of curves have been more stringent than safety requires. In a course of experiments made by Dr. Lardner, it has been established that curves of a mile radius produce no sen sible increase of resistance at the usual speed of railway trains, and therefore curves of considerably less radius may be traversed at that speed. without danger. There is no legislative restric tion on the subject in our country, where the radius of the curve is sometimes very short.
Resistance of Air to Railway Trains.— Until very recently, it has been consid ered by engineers that the resistance to railway trains was almost entirely, due to friction and mechanical effects, and that that part of the resistance which de pends on the atmosphere formed so in considerable a portion of the whole that it might be disregarded in practice. The result of a course of experiments, made within the last two years, by Dr. Lard ner, have, however, indicated a serious' amount of resistance due to the air. If Dr. Lardner's conclusion shall be further confirmed, the great expense attending the maintenance of very high speed on railways, and the improbability of at taining in the ordinary work of a line the extraordinary velocity which some per sons now contemplate, will be apparent.