are intended to receive the strain which otherwise would bear against the centre-pin and would be liable to break or bend it. [76] In some rare cases three pairs of wheels are employed for locomotive trucks. Six-wheeled trucks are very commonly used under passenger cars. [77] The right half is a section through the centre of the axle, or of _G g_, of fig. 186, and the left half a section through the centre of the truck, or on _g g_, of fig. 186. From this description it will be seen that while the truck-frame rests on two points, _k_ and _k_, the weight of the engine is supported by the centre-plate of the truck. As the back part substantially rests on the centres of the two equalizers, it will be seen that this distribution of the weight fulfills the conditions of the tripod, or as it has been called, the “_three-legged principle_.” [Illustration: _Fig. 189._ _Fig. 190._ _Fig. 191._ Scale ³⁄₈ in. = 1 foot.] QUESTION 306. _How are trucks arranged so as to give them lateral motion?_ _Answer._ When this is done, the lower centre-plate is usually suspended in some way from the truck-frame on links or hangers, so that it can swing laterally. One method of doing this is shown in figs. 189, 190 and 191. Fig. 190 is a front view, fig. 191 a plan, and fig. 189 a transverse section of such an arrangement. The centre-plate _H H_ has cast with it an extension, _B B_, the ends of which are suspended on links, _L_, _L_, called _suspension-links_, the upper ends of which are attached to bars, _m_, _m_, which are set edgeways and extend across the truck-frames. It is evident that with this arrangement the lower centre casting can swing crosswise of the track on the links _L_, _L_, and that the front end of the engine will thus have a lateral motion independent of the truck. PART XIII. ADHESION AND TRACTION. QUESTION 307. _What is meant by the “adhesion” of a locomotive?_ _Answer._ It is the resistance which prevents or opposes the slipping of the driving-wheels on the rails, and is due to the friction of the former on the latter. QUESTION 308. _On what does the amount of this friction depend?_ _Answer._ Like all friction it depends upon the weight or pressure of the surfaces in contact, and consequently upon the load which rests on each wheel. It also depends upon the condition of the rails, and probably to some extent upon the material of which they and the tires on the wheels are made. [Illustration: _Fig. 192._ Scale ¹⁄₄ in. = 1 foot.] QUESTION 309. _How much force is required to make the driving-wheels of a locomotive slip on an ordinary railroad track?_