330 
ACTION AT THE BOTTOM. 
Thirdly, It remains to consider the effect of 
the air, supposing it to be retained continually 
within the chambers, at the bottom of the sea. 
Here, if the position of the moving animal be 
beneath the mouth of the shell, like that of a 
snail as it crawls along the ground, the air within 
the chambers would maintain the shell, buoyant, 
and floating over the body of the animal in a 
thus diminished, without increasing the bulk of the shell, into 
whose cavities the fluid is withdrawn, the specific gravity of the 
whole mass is suddenly increased, and the animal begins to sink. 
The air within each chamber remains under compression, as 
long as the siphuncle continues distended by the pericardial 
fluid; and returning, by its elasticity, to its former state, as soon 
as the pressure of the arms and body is withdrawn from the 
pericardium, forces the fluid back again into the cavity of this 
organ ; and thus the shell, diminished as to its specific gravity, 
has a tendency to rise. 
The place of the pericardial fluid, therefore, will be always in 
the pericardium, excepting when it is forced into and retained in 
the siphunele, by muscular pressure, during the contraction of 
the arms and body closed up within the shell. When these are 
expanded, either on the surface, or at the bottom of the sea, 
the water will have free access to the branchiae, and the move- 
ments of the heart will proceed freely in the distended pericar- 
dium; which will be emptied of its fluid at those times only, when 
the body is closed, and the access of water to the branchiae con- 
sequently impeded. 
The following experiments shew that the weight of fluid requi- 
site to be added to the shell of a Nautilus, in order to make it 
sink, is about half an ounce. 
I took two perfect shells of a Nautilus Pompilius, each weigh- 
ing about six ounces and a half in air, and measuring about seven 
inches across their largest diameter; and having stopped the 
Siphuncle with wax, I found that each shell, when placed in fresh 
water, required the weight of a few grains more than an ounce to 
