_Answer._ The periphery or tread of each wheel is made conical, but of the same size as the other, and with the small diameter of the cone outside, as shown in fig. 158. On a straight track if the position of the wheels on the rails is such that their two flanges are equally distant from the rails, as shown, then obviously at the points of contact with the rails the wheels are of the same diameter. That is, _a_ is equal to _b_. But in running on a curved track, if the wheels are of the same diameter, as has been shown, they will roll in a straight line and consequently towards the outside of the curve. The flange _c_--supposing it to be at the outside of the curve--will therefore roll towards the rail, and consequently the outside wheel will rest on the rail at a point nearer the flange, as shown in fig. 159, where the diameter _a_ is larger, and the inside one further from the flange where the diameter _b_ is smaller than at _a_ and _b_ in fig. 158; and consequently the action of the wheels is the same as though their peripheries were made of the form shown in fig. 157. QUESTION 256. _How are the axles of locomotives made to assume a position radial to the curves in the track?_ _Answer._ This is only done approximately, as the mechanical difficulties in the way of doing it perfectly are so great as to render it impracticable. By attaching the truck to the locomotive by a flexible connection or centre-pin, _s_, as shown in fig. 160 (which represents a plan of the wheels of an ordinary locomotive), it is plain that the truck axles _e f_ and _g h_, instead of remaining parallel to the driving-axles _a b_ and _c d_, will, by turning around the centre-pin, _s_, adjust themselves to the curve so as to approximate as closely to radii as is possible for two axles which are held parallel to each other. Of course the further apart they are the greater will be their divergence from the position of radii, and whether the tread of the wheels be cylindrical or conical the further apart their axles are the greater will be the divergence of the paths in which they would naturally roll from that of any curve on which they must roll. Thus, if the axles were twice as far apart as they are represented in fig. 157, and in the position shown in the dotted lines _l l′_ and _o o′_, the wheels, if they are conical, would then naturally roll in curves drawn from the centres _p_ and _q_. If the wheels are cylindrical, they would roll in straight lines. In either case the divergence of their paths _l s_ and _l r_ from the curve of the track is greater than _a h_ and _a′ i_, the paths in which they would roll if their axles were nearer together. This divergence increases with the distance between the axles, and therefore the lateral slip of the wheels must be in the same proportion. [Illustration: _Fig. 160._ _Scale ³⁄₁₆ Inch -- 1 Foot._] QUESTION 257. _Is the resistance to rolling diminished by placing the truck axles nearer together?_