eces, for convenience of
stowage, and connected together, when required, by screws. Its inner
end expands into a head, in which are placed four steel sockets, at
equal distances from each other, which receive the points. Two of the
sockets opposite to each other are secured permanently. The two others
are movable. A tapering plate or wedge, the sides of which are
cylindrical, runs through a slit in the head; an aperture in the inner
ends of the movable socket embraces the cylinder, so that when the
plate is moved forward or backward, the sockets are projected or
withdrawn. The tapering of the plate has a certain known proportion to
its length, so that if it is moved in either direction a given
distance, a proportional movement is imparted to the sockets, and to
the points which they contain. It is easy to see how, in this manner,
a movement of .10 in. may increase or diminish the distance between
the points .01 in. Therefore there would be no difficulty in
estimating, to a considerable degree of accuracy, a difference of .001
in. between the points. In general, however, the distance on the plate
required to move the points .01 in. is about .06 in. only.

A square sliding rod is connected with the tapering plate, and runs
through the whole length of the tube, projecting some inches beyond
the outer end. This rod has as many parts as there are joints in the
staff, and, like them, connects by screws. Each section of the rod
works in its proper joint, through a square socket at each end, and is
prevented from falling out by pins. When screwing the joints together,
if the ends of the rod are pressed up to each other they become
connected by the same motion.

The staff is graduated to inches and quarters, so that the distance of
the points from the muzzle of the gun may always be known. A centre
line, starting from the centre of the upper socket, is marked upon the
staff throughout its length.

The handle in use at present is of brass, made to fit over the outer
end of the staff, and to connect with the sliding rod by a screw,
having a large milled head at the outer extremity of the handle. It
may be used on either joint, as required. A slit through the handle
permits a part of the staff near the end to be seen beneath. A scale
on one side of the slit is graduated with the distance that the rod
moves, to throw the points .01 in. apart.

That part of the handle containing the slit and scale is separated
from the other par