Note 90 — Chapter VII · RAILWAY BRIDGES AND VIADUCTS.

Mr. Brunel about this time introduced a great improvement in the manufacture of wrought-iron bolts for bridges, when these, as is usually the case are screwed at the ends. A screw cannot be made on a bolt without the metal being cut into to the extent of the depth of the thread, and the strength thereby considerably reduced. The section, taken at the bottom of the thread, is much smaller than that of the bolt; and, moreover, owing to the abrupt change at the commencement of the thread, the strength is not so great as that due to the reduced sectional area. The improvement consisted in swelling out the iron of the end of the bolt where the thread was to be made, so that the diameter, at the deepest part of the thread, should be fully equal to that of the bolt. The saving of metal by this improvement, especially in the case of long bolts, is very considerable.