[Illustration: Fig. 52.--Section of the Tread and Flange of a Car Wheel.] What is known as the Allen paper wheel is used a great deal in this country, especially under sleeping-cars. A section and front view of one of these wheels is shown by Figure 53. It consists of a cast-iron hub, _A_, which is bored out to fit the axle. An annular disk, _B B_, is made of layers of paper-board glued together and then subjected to an enormous pressure. The disk is then bored out to fit the hub, and its circumference is turned off, and the tire _C C_ is fitted to it. Two wrought-iron plates, _P P_, are then placed on either side of it, and the disk, plates, tire, and hub are all bolted together. The paper, it will be seen, bears the weight which rests on the hub of the axle and the hub of the wheel. [Illustration: Fig. 53.--Allen Paper Car Wheel.] Steel tires have the advantage that when they become worn their treads and flanges may be turned off anew, whereas chilled cast-iron wheels are so hard that it is almost impossible to cut them with any turning tool. For this reason machines have been constructed for grinding the tread with a rapidly revolving emery-wheel. In these the cast-iron wheel is made to turn slowly, whereas the emery-wheel revolves very rapidly. The emery-wheel is then brought close to the cast-iron wheel, so that as they revolve the projections on the latter are cut away, and the tread is thus reduced to a true circular form. These machines are much used for "truing-up" wheels which have been made flat by sliding, owing to the brakes being set too hard. It would require a separate article to give even a brief description of the different kinds of cars which are now used. The following list could be increased considerably if all the different varieties were included. Baggage-car, Boarding-car, Box-car, Buffet-car, Caboose or conductor's car, Cattle- or stock-car, Coal-car, Derrick-car, Drawing-room car, Drop-bottom car, Dump-car, Express-car, Flat or platform car, Gondola-car, Hand-car, Hay-car, Hopper-bottom car, Horse-car, Hotel-car, Inspection-car, Lodging-car, Mail-car, Milk-car, Oil-car, Ore-car, Palace-car, Passenger-car, Post-office car, Push-car, Postal-car, Refrigerator-car, Restaurant-car, Sleeping-car, Sweeping-car, Tank-car, Tip-car, Tool or wrecking car, Three-wheeled hand-car. The following table gives the size, weight, and price of cars at the present time. The length given is the length over the bodies not including the platforms.