gradually works its way out and is wasted. The oil in the back reservoir does not waste by leakage so rapidly as that in the front; but when the leather washer becomes saturated with water the oil is liable, by reason of its lightness, to float on the water in or about the washer, and thus get wasted. This axle requires frequent examination when very much in use; but as it is neat in appearance, and under ordinary circumstances tolerably safe in working, and is not very expensive, it is much used. Both axle-box and axle-arm are case-hardened. The other kind of axle used by carriage-builders is that known as “Collinge’s Patent.” The original intention of the inventor was to make it a cylindrical arm, with the box running round it against a coned shoulder, and secured by a coned nut in front; but, as it was found in practice that a leather washer was necessary at the shoulder to prevent jarring, this part of the plan was abandoned. The commonest form of this axle now in use consists of a cylindrical arm with a broad shoulder collar. The box is of cast iron, and the back of it is similar to that of the mail axle before described. The front of it has a rebate cut in the box to receive a small conical collar and the screw of an oil cap. The arm of the axle is turned down in the lathe to two-thirds of the total thickness from the point where the rebate of the box begins. A flat side is filed on this reduced portion, and along it is made to slide a small collar of gun metal, with a conical face in the interior to fit against the coned interior of the rebate in the box. Against this collar, technically called the “collet,” a nut of gun metal is screwed, and against that again a second nut of smaller size, with a reversed thread, is tightly fixed. These two nuts, thus screwed in different directions, become as firm as though they were part of the axle itself, and no action of the wheel can loosen them, because the collet, which does not turn, removes all friction from them. But, as a further security, the end of the axle-arm projects beyond the farthest nut, and is drilled to receive a spring linch-pin. Over all a hollow cap of gun metal is screwed into the end of the box. This contains a supply of oil for lubricating purposes. When the wheel is in motion the oil is pumped upwards from the cap and passes along the arm to the back reservoir, constantly revolving round the cap with the wheel. If the cap be too full of oil—that is, if the summit of the column of oil in the cap be at a horizontal level above the leakage point at the shoulder—it will pump away rapidly, and be wasted till it comes to the level of the leak, where it will be economically used. It is essential to the perfection of an oil action that the oil should not be permanently above the level of the leak, but that small portions should be continually washing up into that position by the action of the wheel in turning.