1st. The common lifting jack is used for field carriages only. The arm, which may be adjusted, within certain limits, to any required height, is a lever of the first kind, and is applied accordingly. The body of the jack is the support on which it works.
2nd. The tooth and pinion jack is of greater power than the common jack, and is used for heavy carriages. It is applied vertically under the carriage, which is raised by turning the winch of the jack.
3rd. The screw jack is an elevating screw of large dimensions, and is of greater power than either of the other two. Like the tooth and pinion jack, it is applied vertically under the body, which is raised, like the breech of a gun, by turning up the screw. The foot of it is furnished with three spikes, to prevent its slipping when the screw is turned.
If a wheel is to be extricated from a rut, into which it has sunk so far that the lifting jack cannot be applied in the ordinary manner, the jack is placed as close to the wheel as possible, on the outside of it. A piece of rope, passed under the nave, is made fast to the extremity of the arm, if the common jack is used, or to the fork, if either of the other kinds be employed. The jack is then worked as usual, and stones or other hard substances are thrown under the wheel, to prevent its sinking, as often as it may be necessary to take fresh purchases.
The tooth and pinion and screw jacks may be employed to move bodies horizontally a few inches, provided a good abutment or support can be found for the foot of the stock. By the application of two jacks, in opposite directions, bodies may be brought together to be spliced, riveted, &c.
Article 4.
ROLLERS.
Rollers are solid cylinders of wood, used in mounting guns upon their carriages, in shifting them from carriage to carriage, and in moving them through passages too narrow to admit of the use of the ordinary means of transport. Their dimensions vary, according to the nature of the service for which they are intended.