was horizontal; and therefore the weight on the supports is the same in both cases, thus showing that the load _A B_ can move about the centre of suspension when supported as shown in fig. 185 as freely as it can if arranged as shown in fig. 184. It therefore follows that in the distribution of the weight of each side of the locomotive on the wheels and on the track, it may be regarded the same as though it was supported at one point, which is the fulcrum of the equalizing-lever. QUESTION 304. _What advantage results from supporting the weight of the back part of the locomotive on two points?_ _Answer_. If the back part of the locomotive rests on only two points and the front end on the centre of the truck, then the whole weight of the engine will be sustained on three points. Now it is a well known fact that any tripod, like that on which an engineer’s level is mounted, or a three-legged stool, will adjust itself to any surface, however uneven, and stand firmly in any position; whereas if there are more than three points of support, if they are all of the same length the surface on which they rest must be a plane, otherwise some of them will not touch. All railroad tracks have inequalities of surface, and therefore it is of the utmost importance that a locomotive should be able to adjust itself on its points of support to any unevenness of the track on which it must run. This is possible only when the weight rests on three points of support. [Illustration: _Fig. 186._ _Fig. 187._ _Fig. 188._ _Scale, ³⁄₈ inch = 1 foot._] QUESTION 305. _How is the truck constructed?_