into a machine-moulded toothed ring bolted to the top of the truck;
this ring is 11 ft. 4-7/8 in. in diameter, and contains 172 teeth 2½
in pitch. The slewing pinion is driven by intermediate gearing from
the bottom of the vertical shaft mentioned above. For the turning
motion two distinct sets of rollers are provided; these are carried by
cross-girders placed between the side frames; one set runs against a
cast-iron roller path bolted round the bottom of the post, and the
other on the large horizontal roller path seen in the engraving. The
latter is 14 ft. in diameter; it is built up of two deep curved
channel irons with top and bottom plates forming a circular box
girder, on the top of which a heavy flat rail is riveted, and the
whole turned up in the lathe. The racking and traveling motions are
driven from the top end of the vertical shaft; the racking gear
consists of wire ropes attached to each side of the traveling carriage
and coiled round a large barrel, the outer rope being brought over a
pulley at the end of the jib. The rails for the carriage rest on
rolled joints bolted to the underside of jib. This arrangement
involves the use of an overhung traveling carriage, but enables the
jib to be of a stiff box section, the side stiffness being further
secured by wind ties.

The traveling motion is worked by a second vertical shaft, which
passes down the center of the post, and by means of a cross shaft is
geared to the front axle, from which four of the ground wheels are
driven.

The post is octagonal, built up of plates ¾ in. thick; at the bottom
end it is secured to the girders of the truck, and at the top is
shrunk on to a large gudgeon 12 in. in diameter, which enters a
casting fixed in the back end of the jib; on the top of the gudgeon