They can be used with advantage, only on a perfectly level plane surface.
When a cylindrical body is moved on rollers, they must be perfectly horizontal, or it will roll off them; and even when the rollers are horizontal, steadying handspikes should, if possible, be applied, to guard still further against such an accident.
The rollers must be placed at right angles to the direction in which they are intended to move, projecting equally on each side of the axis of the gun, or other body, which they support.
A gun laid upon rollers may be moved, either by hauling upon it with ropes, or by means of levers. In the last case, two holes crossing each other at right angles, are pierced near each end of the several rollers, and in them are inserted the points of iron levers, by means of which the rollers are turned in the required direction.
In order that rollers may be used with the greatest effect, and that there may be as little necessity as possible for shifting them during the course of an operation, it must be borne in mind,—
1st. That a roller moves in a direction perpendicular to its own axis.
2nd. That it moves over half the space only which is traversed by the body it supports.
Article 5.
CRAB CAPSTAN.