_A A_ is the wheel, which is moulded in sand in the usual way. The part _B B_ of the mould, which forms the rim or tread of the wheel, consists of a heavy cast-iron ring. The melted iron is poured into this mould and comes in contact with _B B_. This has the effect of chilling the hot iron, as has been explained. In cooling, the wheel contracts; and for that reason the part between the rim _C_ and the hub _D_ is made of a curved form, as shown in the section, so that if one part should cool more rapidly than another these parts can yield sufficiently to permit contraction without straining any portion of the wheels injuriously. For the same reason the ribs on the back of the wheels, as shown in Figure 51, are also curved. As an additional safeguard to the unequal contraction in cooling, the wheels are taken out of the mould while they are red-hot, and placed in ovens where they are allowed to remain several days so as to cool very slowly. Figure 52, on p. 145, represents a section of the tread and flange of a chilled wheel, showing the peculiar crystalline appearance of the chilled iron. [Illustration: Fig. 51.--Cast-iron Car Wheels.] In making cast-iron wheels the quality of the iron used is of the utmost importance. The difficulty in making good wheels lies in the fact that most iron which is ductile and tough will not chill, whereas hard white iron, which has the chilling property in a very high degree, is brittle, and wheels which are made of it are liable to break. There are some kinds of cast-iron produced in this country which have the two qualities combined, in a very remarkable degree; that is, they are ductile and tough, and will also chill. Wheel-founders also mix different qualities of irons to produce wheels with the required strength, and which will resist wear; that is, they use a certain amount of hard white iron which will chill, with that which is ductile and soft. By changing the proportions, any required amount of chill can be produced. The danger is that iron which has little strength or ductility will be fortified with hard chilling iron, and a very weak wheel will thus be the result. Thousands of such wheels have been bought and used because they are cheap, and many lamentable accidents are undoubtedly due to this cause. To guard against this, car-wheels should always be subjected to rigid tests and inspection. In Europe wheels are made of wrought-iron, with tires which were also made of the same material before the discovery of the improved processes of manufacturing steel, but since then they have been made of the latter material. Owing to the breakage of a great many cast-iron wheels of poor quality, steel-tired wheels are now coming into very general use on American roads under passenger-cars and engines. A great variety of such wheels is now made. The "centres" or parts inside the tires of some of them are cast-iron, and others are wrought-iron constructed in various ways.