_Answer_. It consists, as has already been stated, of two pairs of wheels.[76] These are attached to a frame, _b′ b′_, plates I, II and III. The axles have boxes called _truck-boxes_, and brass bearings similar to those used on the driving-axles. These boxes work in jaws, also similar to those on the main engine frame, excepting that they have no attachment to prevent them from being worn by the motion of the boxes up and down in the jaws. Fig. 186 is a horizontal section, fig. 187 a plan, and fig. 188 a transverse section[77] of a truck. The frame _C D E F_, fig. 187, shown also at _h′ h′_, figs. 186 and 188, is of rectangular form and is forged in one piece. The legs _f f_ which form the jaws for the boxes, are bolted to the frame as shown in fig. 186. To the lower end of these legs a brace, _g g g_, is bolted, which thus unites them together. On each side one spring, _S F S_, is placed under the frame and in the reverse or inverted position to that of the driving-springs. A pair of equalizing levers, _G G G_, is placed on each side of the truck, one of them on the inside of the frame and the other on the outside, as shown in the plan. The ends of these equalizers rest on the top of the truck-boxes, and the springs are attached to the levers at _i i_ by the hangers, _j j_. The truck-frame rests on the top of the spring-strap, _F_, which is made of the form of an arc of a circle, or “_rounded_,” as it is termed by workmen, so that it can move freely about the point of support. It is evident that this arrangement of spring and equalizer operates in the same way as that employed for the driving-wheels in distributing the weight on each of the wheels, and that the truck-frame is supported on two points, _k_, _k_, figs. 186 and 187. The weight of the front end of the engine rests on a cast iron _centre-plate_, _H H_. This centre-plate rests on four bars, _l l_, _l l_, and _m m, m m_, two of which are bolted to the frame transversely and the other two longitudinally, as shown in the plan. These bars are elevated in the centre as shown in figs. 186 and 188. The transverse bars are trussed with two corresponding bars, _n n_, fig. 188, below. These _truss-bars_ as they are called, are bolted to the upper bars with bolts, _o_, _o_, but are separated from the top-bars by distance pieces, _P_, _P_, figs. 186 and 188. The centre-plate _H H_, called the _lower centre-plate_, has an annular groove in it, which receives a corresponding projection on the casting _K K_, called the _upper centre-plate_, which is bolted to the bed-plate of the cylinders, as shown in plate II. The upper centre-plate has a pin, _y_, called a _centre-pin_, fig. 186 and plate II, attached to it, which passes through the lower centre-plate, and has a key underneath the latter plate. This key is intended to prevent the engine from “jumping” off of the truck on a rough track or in case of accident. The annular groove and the projection which fits into it