When the body is suspended from =C= springs by leather braces, great care should be used in the selection of the material for these latter, and for this purpose the best and strongest leather is required. The use of brake retarders to the hind wheels has now for some years superseded the old-fashioned drag shoes. It is evident that the action on two wheels must be better than on one only. The brake can be applied or removed without stopping the carriage, which is necessary if a drag shoe be used. This is rather an important consideration in undulating country, for it would be a great inconvenience to have to get down and put on the drag shoe when descending a hill, and when at the bottom to stop and get down again to remove it, in order to proceed along level ground or up the next hill, and so keep on like this all day. The lever brake was the original form, as still seen on drags, &c., but in many parts it is superseded by the foot or treadle brake, more especially in Scotland. This kind of brake is also the one used by the London Omnibus Company. The blocks which press upon the wheels have been made of various substances—cast iron, wrought iron, brass, wood, india-rubber, and leather. The wood is the best for the hold on the iron tyre and absence of noise and smell, but it wears out fast. India-rubber, especially for light carriages, seems to be the most satisfactory. We have given, generally, the operation of framing together the under or carriage parts of the vehicle. But as some very important considerations regulate the shape, construction, and formation of most of these parts, they must be discussed separately. For this purpose they will be considered under the following headings:— Wheels. Axles. Springs. Wheel-plates and fore-carriages. Ironwork generally. CHAPTER VI. WHEELS. A wheel for a locomotive vehicle is a circular roller, either cylindrical or conical, the width or thickness of which is considerably less than its diameter. It may be either solid or constructed of various pieces, in which latter case it is called a framed wheel. It may also be made of wood or metal, or a combination of both.