Through the front transom, and near the front ends of the ties, an iron axle-tree is passed, working in iron boxes fitting in the ties. On the projecting ends of this axle-tree the rollers or manœuvring wheels are fixed (the axes of which are _eccentric_ with the axis of the axle-tree) the extreme ends of the axle, being octagonal in shape, to fit the wrench of the iron handspike. [Sidenote: 137]] These _eccentrics_ are so arranged that when the centres of the wheels are at their lowest points, the surfaces of the wheels bear on the rails of the chassis and raise the gun-carriage tie from it; and when the centres are at the highest points, the surfaces of the wheels do not touch the rails, and the ties are in contact with them.[2] A similar arrangement is made for the rear part of the carriage, except that the axle does not extend all the way through, but the wheel on each side has a projecting piece of axle which works into a box placed near the end of the tie. [2] A couple of notches or indentations are made on the ends of the eccentric axles. When these notches are in a vertical line, the wheels rest on the _rails_; but when they are in a horizontal or inclined line, the ties rest on _them_. The wheel is thrown into or out of gear, that is, made to bear on the rail of the chassis or relieved from it, by turning the axle with a wrench placed on the octagonal end. In the direction of the radii of the wheels, but inclined outwards, mortises are placed for the reception of the end of the iron handspikes, by acting on which while inserted the wheels are turned, and the carriage moved back and forth on the chassis. Ordinarily, when the wheels are thrown into gear, the carriage being back, it will run into battery of itself. The elevating arrangement consist of an elevating screw, working into a screw-bed, which slides in a vertical box, and carries on the top of it a movable _pawl_ to fit into the notches cut in the breech of the gun, in order to give considerable elevations. For the purpose of transferring the pawl from one notch to the next, it has a slit in it, through which the elevating bar is passed, and the gun supported by making use of the edge of the elevating-box as a fulcrum. This arrangement is over the rear transom. [Sidenote: 138]] 59. Describe the chassis of the columbiad-carriage. It consists, like those used with other barbette-carriages, of two rails connected by three transoms; but the tops of the rails are shod with iron plates, and the rear hurters are the large heads of heavy bolts which pass entirely through the rails. The front hurters are fixed to the front transom by a heavy plate and bolt.