The commonest axles now used are of a conical form, with a box of plate iron fitted to them. This box is made by welding the two edges of the iron together in a broad projecting seam, which helps to secure it to the nave. The inside of the box is sunk into hollows for the purpose of holding the lubricating grease. At the upper end of the arm the axle is left square, and against this a large iron washer is usually shrunk on hot. Against this washer the box works. To secure the wheel against coming off a small iron collar is placed on the reduced outer end of the arm, and a linch-pin is driven through the arm beyond it. An improvement on this kind of axle is when the collar at the upper end or shoulder is made solid by welding, and a screw nut with a linch-pin through it is substituted for the collar and linch-pin. These nuts are commonly made six-sided, with a mortise or slot for the linch-pin through each side, in order to afford greater facility for adjustment. In all other particulars this axle is the same as the last, except that it is occasionally case-hardened to prevent wear and friction. In travelling, these axles require to be fresh greased every two or three days, and the trouble thus caused is very considerable, besides the risk of omission, in which case the axle is likely to be entirely spoiled. The commonest kind of oil axle is called the “mail,” because the peculiar mode of fastening was first used in the mail coaches. The arm is not conical, but cylindrical, in the improved kind. At the shoulder of this axle a solid disc collar is welded on for the box to work against. Behind this shoulder collar revolves a circular flange-plate of wrought iron, pierced with three holes corresponding with holes in the wheel from front to back, through which long screw-bolts are driven, and their nuts screwed sufficiently tight against the circular flange-plate to allow easy motion. The wheel, when in motion, thus works round the shoulder collar, while the flange-plate secures it against coming off. This is not neat or accurate, but it is simple and secure, and no nut or linch-pin is required to the axle in front, while the front of the nave can be entirely covered in. When screwed up for work, a washer of thick leather is placed between the shoulder collar and the box, and another between the shoulder collar and the circular disc, which extends over the whole surface of the back of the nave. The box of this axle is of cast iron. The front is closed with a plate of metal, between which and the end of the axle-arm a space is left of about 1 inch as a reservoir for oil, which is poured in through a tube passing through the nave of the wheel and closed by a screw pin. At the back of the box there is a circular reservoir for oil, ¾ inch in depth and ½ inch wide. When the wheel is in motion the revolving of the box keeps the lubricating material in circulation between the two reservoirs; any portion getting below the arm at the shoulder