Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/249

 Rh much avail as ribs to increase the aggregate amount of strength. (See Pl. 32, Fig. 1. a. to b.)

Thirdly, the arch is rendered still stronger by the position of the edges of the internal Transverse plates, nearly at right angles to the sides of the external shell, (See Pl. 32, Fig. 1, b. to c.) The course of the edges of these transverse plates beneath the ribs of the outer shell is so directed, that they act as cross braces, or spanners, to fortify the sides of the shell against the inward pressure of deep water. This contrivance is analogous to that adopted in fortifying a ship for voyages in the Arctic Seas, against the pressure of ice-bergs, by the introduction of an extraordinary number of transverse beams and bulk heads.

We may next notice a fourth contrivance by which the apparatus that gives the shell its power of floating, is progressively maintained in due proportion to the increasing weight and bulk of the body of the animal, and of the external chamber in which it resides; this is effected by successive additions of new transverse Plates across the bottom of the outer chamber, thus converting into air chambers that part, which had become too small for the body of the Sepia. This operation, repeated at intervals in due proportion