Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/250

 246 to the successive stages of growth of, the outer shell, maintains its efficacy as a float, enlarging gradually and periodically until the animal has arrived at full maturity.

A fifth consideration is had of mechanical advantage, in disposing the Distance at which these successive transverse Plates are set from one another. (See Pl. 31. Fig. 1. and Pl. 32, Fig. 1, 2.) Had these distances increased in the same proportion as the area of the air chambers, the external shell would have been without due support beneath those sides of the largest chambers, where the pressure is greatest: for this a remedy is provided in the simple contrivance of placing the transverse plates proportionally nearer to one another, as the chambers, from becoming larger, require an increased degree of support.

Sixthly, The last contrivance, which I shall here notice, is that which regulates the ascent and descent of the animal by the mechanism of the Siphuncle. The use of this organ has never yet been satisfactorily made out; even Mr. Owen's most important Memoir leaves its manner of operation uncertain; but the appearances which it occasionally presents in a fossil state, (see Pl. 32, Fig. 2, 3. and Pl. 33,) supply