_Answer._ Their construction is similar to that of a car axle-box, the standard form of which is represented in figs. 207, 208 and 209. Fig. 207 is a section lengthwise of the axle, fig. 208 a section crosswise of the axle, and fig. 209 a sectional plan. _A_ is the journal of the axle, which is inclosed by a cast iron box, _K K_, which is open in front and at the back. The front has a cover, _H_, which is either fastened by a spring, as shown in the illustrations, or is bolted to the box. The axle enters the box from the back, _I_, and has either a wood or leather packing, _J J_, called a _dust-guard_, to keep the dust from getting in and the oil from leaking out of the box. _D_ is a brass journal-bearing, which rests against a cast iron bearing piece or _key_, _E_, which is put in so that by removing it through the opening _F F_, the brass bearing can be raised up high enough to clear the collar, _G_, on the end of the axle and thus be removed in the same way. The lower portion, _L_, of the box under the axle is usually filled with cotton or woolen waste saturated with oil. This constantly presses against the axle, and thus keeps it oiled. QUESTION 354. _How are the tender trucks constructed?_ _Answer._ They are made of various patterns, some of which have wooden frames and in other cases the frames are made of iron. The truck illustrated in our engraving, figs. 204 and 205, is made of iron and is very similar to the engine truck, excepting, as already stated, the frames are outside instead of inside of the wheels. QUESTION 355. _How is the tender supported on the trucks?_ _Answer._ It rests on the centre of the front truck and on a bearing, _n_, fig. 204, on the frames on each side of the back truck. This arrangement gives three bearing points, the advantages of which have already been explained. A truck which supports the load which it carries in the centre is said to be _centre-bearing_, and if the load is carried on each side, _side-bearing_. QUESTION 356. _How are the brakes attached to the tender?_ _Answer._ They are attached usually to the back truck alone, but in some cases to both trucks, and are operated by a wheel, _N_, fig. 204, and a shaft which extends below the tender frame and on which a chain attached to the brakes is wound. QUESTION 357. _How much water and coal does an ordinary tender carry?_ _Answer._ A tender for a thirty-ton engine carries from 1,500 to 2,000 gallons of water and three to four tons of coal, and weighs from 40,000 to 45,000 pounds loaded. An engine of this kind and its tender therefore weigh about 100,000 pounds, which, being even figures, can easily be remembered. PART XVIII. FRICTION AND LUBRICATION. QUESTION 358. _What is meant by friction?_