In order to insure their greater durability and freedom from friction these axles and their boxes are always case-hardened, _i.e._ their rubbing surfaces are converted into steel to a trifling depth by the process of cementation with animal charcoal for about two hours, when they are plunged into water. The boxes are ground on to the arms with oil and emery, either end being applied alternately, until a true fit between the two is accomplished. The mode in which oil acts as a lessener of friction is by its being composed of an infinite number of movable globules, over which the fixed surfaces of the arm and box roll without causing that friction and wearing away which would be the result of the two iron surfaces worked together without any lubricant. This saving in the wear and tear of the axle-arm is accomplished by the destruction of the oil. From this we deduce that the greater the mass of oil or grease used the longer will the axle run, and in order to facilitate this as much as possible there should be so much space left between the bearing surfaces of the arm and the box as will allow of a film of oil to be between them. A highly polished surface is desirable in an axle and box, as the bearing is more perfect and true. A rough surface is a surface of sharp angles, which will pierce through the oil and cause friction by contact. To guard against the axle running dry, the arm is reduced in thickness at the centre for about an inch to allow a lodgment for the oil, and in the process of working this constitutes a circular pump, which draws up the oil from the front cap and distributes it over the area of the arms. But this, of course, will soon run dry, so that the best remedy to prevent the oil being exhausted and the sticking of the axle-arm in the box is careful attention. A danger arising from careless fitting is the introduction of grit into the box. This grit is composed of small grains of silex, which is very much harder than iron or steel; the consequence is that it cuts and scores the bearing surfaces in all directions, and keys them firmly together, so that it is sometimes necessary to break the box to pieces in order to get it off the arm. A patent was taken out to remedy these defects by casting three longitudinal triangular grooves in each box. The advantages gained by this are, that if grit gets in it finds its way to the bottom of the grooves and does not interfere with the action of the wheel, and, moreover, the grooves keep up a constant surface of oil in contact with the arm, instead of trusting to the mere capillary attraction. This does not interfere with the bearing surface in any marked degree. In order that the axle shall be perfect the following considerations are necessary:— That there be sufficient bearing surface for the arm to rest on. That the box be of a convenient shape for insertion in the wheel.