Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/262

 258 by the tenacity and strength of the material. (See Pl. 37, Figs. 1—10. )

The general principle of dividing and subdividing the ribs, in order to multiply supports as the vault enlarges, is conducted nearly on the same plan, and for the same purpose, as the divisions and subdivisions of the ribs beneath the groin work, in the flat vaulted roofs of the florid Gothic Architecture.

Another source of strength is introduced in many species of Ammonites by the elevation of parts of the ribs into little dome-shaped tubercles, or bosses, thus super adding the strength of a dome to that of the simple arch, at each point Where these bosses are inserted.