_Answer._ Because it is necessary to have one pair of wheels guide the other. In an ordinary wagon the front axle is guided by the pole or shafts. Nearly every one knows the difficulty of moving a wagon when the pole or shafts are removed, especially if it be pushed from behind. The movement of the front axle is then uncontrolled, and it is impossible to direct the motion of the vehicle. The same thing would occur with a locomotive if a single pair of wheels were used, and attached in the same way as the front axle of a wagon. Thus if a single pair of wheels were connected to a locomotive by a centre-pin, _a_, fig. 149, so that the axle would be free to move around this pin, then if one of the wheels should strike an obstruction, say a stone, _s_, fig. 150, there would be nothing to prevent the axle from being thrown into the position shown in fig. 150, and the wheels would be quite sure to leave the track. When two pairs of wheels are used and both axles attached to the same frame, which is connected to the engine by a centre-pin, _s_, fig. 151, between the two axles, then the wheels in moving round the centre-pin must move around the centre _s_ in arcs of circles, _m n_, _m n_, described from the centre _s_. These arcs, it will be observed, cross the rails. Now if the wheels should move in that direction, the flange of one of them would come in contact with the rail and prevent it from moving any farther. It is therefore evident that wheels arranged in that way can only move about the centre-pin as far as the curvature of the track will permit. Trucks are sometimes used with only one pair of wheels, but the centre-pin is then placed some distance behind the centre of the axle, or in the same relation to it that the centre _s_ is to the axle _a a′_ in fig. 151. It is evident that if the frame for such a truck turns around the centre-pin, the wheels must move across the track in the same way as represented by the arcs _m n_, in fig. 151. The construction and operation of trucks with a single pair of wheels will be more fully explained hereafter. [Illustration: _Fig. 152._ _Fig. 153._ _Scale ³⁄₁₆ Inch = 1 Foot._ _Fig. 154._ _Fig. 155._ _Fig. 156._] QUESTION 254. _Why will a locomotive run around curves easier if the front axles are attached to a truck frame which is connected to the locomotive by a flexible connection?_