372 DOCK ENGINEERING.
the entire length can be available open space, and the only benefit con-
ferred is that of exercising some restrioted choice as to its disposition.
Doors are of two varieties—rolling (or sliding) and folding.
Rolling or sliding doors consist of frames of timber or iron, with a facing
of the same material. Movement is made with wheels, which run either on
a ground rail or upon a rail above the door. The grooves in a ground rail
are liable to become choked with dirt and grain, and need frequent cleaning.
Figs. 353 and 354.—Iron Shed Door—Elevation and Vertical Section.
They hold water, which in winter freezes and causes inconvenience. The
use of lower wheels further necessitates an upper guide rail for the top of
the door. Usually two rows of slide rails are provided, the doors being
arranged in pairs to overlap slightly. Fastenings are made in the usual
way by drop-bolts, hasps, &c. Fig. 351 is an elevation of a wooden, and
fig. 353 of an iron, door constructed in this manner at Liverpool.