The central boss, or hub, as it is in some places called, of a wheel, through which the axletree passes, and which receives the ends of the spokes in deep mortices made therein. Although the naves made of wood are usually of great solidity, these parts are so subject to strains and concussions as not to be so durable as the mass of material might lead one to suppose. In consequence of this defect, a patent was taken out some years ago for making this part of cast iron, which has been extensively, and, we doubt not, advantageously adopted.