_Answer._ The insides of the legs, _a_, _a′_, _b_, _b′_, are protected with _shoes_ or _wedges_, _h_, _h_, which are held stationary, and the box slides against the faces of the shoes, thus wearing the shoe or wedge but not the frame. QUESTION 282. _Why are the shoes usually made wedge-shaped?_ _Answer._ They are made in that way so that when they become worn, by moving one or both of them up in the jaws, the space between them is narrowed and the lost motion is taken up. They are moved by the screws _i_, _i_. If the boxes should become loose from wear, it would cause the engine to thump at each revolution of the wheels or stroke of the piston. QUESTION 283. _How are the springs for the driving-wheels made?_ _Answer._ They are made of steel plates which are placed one on top of the other. These plates are of different lengths, as shown at _B_, _B′_, in fig. 169, and are from 3 to 4 in. wide and ⁵⁄₁₆ to ⁷⁄₁₆ thick. The length of the springs measured from the centre of one hanger to the centre of the other is usually about three feet. [Illustration: _Fig. 170._] [Illustration: _Fig. 171._] QUESTION 284. _What determines the amount which a spring will bend under a given load?_ _Answer._ The number of plates, their thickness, length and breadth, and of course the material of which they are made. This can be explained if we suppose we have a spring plate of a uniform thickness, _h_, and a triangular form, of which fig. 170 is a side view and 171 a plan, and that it is clamped fast at its base, _b_. It is a well known mechanical law that any material of this form and under these conditions will have a uniform strength through its whole length to support any load, _P_, suspended at its end, and also that it will bend or deflect in the form of an arc of a circle. [Illustration: _Fig. 172._ _Fig. 173._ _Fig. 174._ _Fig. 175._ Scale ³⁄₄ in. = 1 foot.] QUESTION 285. _How are locomotive springs usually made?_ _Answer._ In locomotives the arrangement of springs is always such that they are either supported in the middle and moved at the two ends, or such that they are supported at the two ends and loaded in the middle; for our consideration it is indifferent which of the two kinds of springs is taken for the present illustration. That shown in plan and elevation in figs. 172 and 173, which is formed of a wide plate placed diagonally, and which in reality consists of two such triangular pieces as were represented in fig. 171 united at their bases _m m_, and loaded at two opposite corners, _e_ and _f_, would answer the requirements mentioned if the great breadth, _m m_, were not an obstacle. This breadth is obviated by cutting the spring into several strips, _a a_, _b b_, _c c_, _d d_, ... _i_, fig. 173, of equal width, and placing these not side by side, but one over the other, as shown in figs. 174 and 175.