[Illustration: Fig. 19.] Here is one very important item which must not be overlooked in the wheelwright department, and that is, the size of the axle-box. The axle-box is a lining of cast iron, on which the axle-arm takes its bearing. Two forms of these are given in Figs. 18 and 19. [Illustration: Fig. 20.] Fig. 20 shows an improved form of stock. It will be seen it is to be applied to straight wheels, and requires no further description beyond noting that the stock is not weakened so much as in the ordinary way by mortising, an iron band, A, circumscribing the nave and forming a hold for the spokes. Wheels should be made with a sufficient number of spokes to properly divide the space at the felloes, and afford sufficient support to prevent sinking in between the spokes, and at the same time avoid too many to weaken the stock. The less the number of spokes, the stronger the hub and the weaker the felloe. Judgment should be used in dividing the difference, so as to make each part of the wheel strong in proportion. CHAPTER VII. AXLES. An axle, or an axletree, for a locomotive wheel vehicle, is that portion of wood or metal, or both combined, which serves as the axis or centre for the wheels to turn round on. The name axle-_tree_ at once indicates the substance originally employed for it, viz. wood. Axletrees are of two kinds; those which are fixed firmly in the wheels and revolve in gudgeons beneath the wheels, and those in which the wheel moves independently of the axle. The former, as being the rudest, was probably the first kind used. The earliest fixed axletrees were simply pieces of hard timber, with the ends rounded down into a conical form, that form being the easiest to fit to the wheel. Subsequently they were plated with iron to resist wear. In the earliest iron axles the conical form was still preserved, for the obvious reason of easy adjustment to the wheel. These iron axles were not made in a solid piece, but were merely short ends bedded in and bolted to a wooden centre. Examples of these axles may still be seen in heavy carts and waggons.