Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/252

 248 this sac (p, p.) to contain a pericardial fluid, the place of which is alternately changed. from the pericardium to the siphuncle, we shall find in this shifting fluid an hydraulic balance or adjusting power, causing the shell to sink when the pericardia fluid is forced into the siphuncle, and to become buoyant, whenever this fluid returns to the pericardium. On this hypothesis also the chambers would be continually filled with air alone, the elasticity of which would readily admit of the alternate expansion and contraction of the siphuncle, in the act of admitting or rejecting the pericardia fluid.

The principle to which we thus refer the rising and sinking of the living Nautilus, is the same which regulates the ascent and descent of the Water Balloon: the application of external pressure through a membrane that covers the column of water in a tall glass, forces a portion of this water into the cavity, or single air-chamber of the balloon, which immediately begins to sink; on the removal of this pressure, the elasticity of the compressed air causing it to return to its former volume, again expels the Water, and the balloon begins to rise.

I shall conclude this attempt to illustrate the structure and economy of fossil Nautili by those of the living species, by showing in what manner the chambers of the pearly Nautilus, supposing them to be permanently filled only with air, and the action of the siphuncle, supposing it to be the receptacle only of a fluid secretion, interchanging its place alternately from the siphuncle to the pericardium, would be subsidiary to the movements of the animal, both at the surface, and bottom of the sea.

First, The animal was seen and captured by Mr. Bennett,