_Answer._ They are made of cast iron, usually in one piece. Figs. 166 and 167 represent sections of two forms of wheels used for cars. Those used for locomotive trucks are similar to these, excepting that they are usually a little smaller in diameter. They are made with a disc or plate which unites the tire to the hub, and in some cases they have ribs cast in the inside, as shown in the two figures. Some are made with single and others with double plates, as shown in the engravings, and still others with spokes similar to the driving-wheels. The tread of the wheel is hardened by a process called _chilling_. This is done by pouring the melted cast iron into a mould of the form of the tread of the wheel. The mould is also made of cast iron, but being cold cools the melted iron very suddenly, and thus hardens it somewhat as steel is hardened when it is heated and plunged into cold water.[71] [71] It should be mentioned here that it is only certain kinds of cast iron which will be hardened in this way, or will “_chill_,” as it is called. The cause to which this chilling property is due is not known. QUESTION 274. _How are the boxes, journals and journal-bearings of the truck-wheels made?_ _Answer._ They are very similar to those for the driving-wheels, their chief difference being that those for the truck-wheels are smaller than those for the driving-wheels. QUESTION 275. _How are the frames for locomotives constructed?_ _Answer._ The frames, _H H H_, plates I, II and III, are made of bars of wrought iron from 3 to 4 inches thick and about the same in width. They are usually made in two parts, the one at the back part of the engine, to which the driving-boxes and axles are attached, and the other at the front end, to which the cylinders are bolted. The back part, or main frame, as it is called, is represented in figs. 168 and 169, and consists of a top bar, _H H_, to which pieces, _a_, _a′_, _b_, _b′_, called _frame-legs_, are welded. Two of these form what is called a _jaw_, which receives the axle-box, as shown in fig. 169. To the bottom of each jaw a _clamp_, _c_, is bolted to hold the two legs together. The two legs, _a_ and _b′_, are united by a brace, _d d_, welded to the bottom of the legs. A brace, _m_, unites the back end of the frame with the leg _b_, and is welded to each. The front part of each frame consists of a single bar, _e_, which is bolted to the back end, as represented in figs. 168 and 169, which show the construction clearer than any description would. These front bars extend forward to the front end of the engine, and a heavy timber, called a _bumper-timber_, _E′ E′_, plates I, II and III, extends across from one to the other and is bolted to each of them. This timber is intended to receive the shock or blow when the locomotive runs against any object, such as a car. The _cow-catcher_ or _pilot_, _S_, is fastened to this timber. [Illustration: _Fig. 168._ _Fig. 169._ Scale ³⁄₈ in. = 1 foot.]