purlin connections and diagonal bracing , being sufficient to render the construction rigid and indepen dent of the side walls ( see Figure 43 ) . There are nine wrought - iron ribs , 75 feet span , each 2 feet deep , placed 15 feet apart , and the level of the crown of the arch is 30 feet above… … diagonally under the roof covering from the springing of each alternate rib to the crown of the arch three bays distant , crossing the intermediate ribs at the purlin connections .
