1334
THEORY AND PRACTICE OF ENGINEERING.
Book IT.
The intermediate ribs, between the iron pillars, rest also on iron shoes, fitted on to theheads of upright pieces, which rest on the centre of the longitudinal plate, which is intwo thicknesses, 12 inches by 12 inches, and has a bearing of 22 feet. The principalswhich form the truss are 12 inches by 9 inches, and they rise 7 feet 8 inches: thus theentire weight of these intermediate ribs is thrown by means of this simple truss on tothe main pillars. The roofs over the galleries are covered with slate, the other part with cop-per, laid on diagonal boarding supported by longitudinal rafters notched on the great ribs.
For roofs of considerable span an arrangement of horizontal and vertical iron ties hasbeen successfully employed ; and timber, as in the present example, when framed with care
v i *; i i
Fig. 2139.
Fig. 2140. ROOFS WITH HORIZONTAL TIES.
answers the same purpose, and is not so costly; each horizontal timber becomes a strut anda tie, and should be placed perpendicularly to perform the same office.
Roofs with iron ties are now commonly preferred for manufactories: at the Butterley
Fig. 2141.
BUTTERLBY IRON WORKS.