Curiosities of Science. 



its mouth, the operculum measures the progressive widening of 

 the spiral chamber by the progressive stages of its growth. 



******* 



The animal, as he advances in the construction of his shell, 

 increases continually his operculum, so as to adjust it to his 

 mouth. He increases it, however, not by additions made at the 

 same time all round its margin, but by additions made only on 

 one side of it at once. One edge of the operculum thus re- 

 mains unaltered as it is advanced into each new position, and 

 placed in a newly-formed section of the chamber similar to the 

 last but greater than it. 



That the same edge which fitted a portion of the first less 

 section should be capable of adjustment so as to fit a portion 

 of the next similar but greater section, supposes a geometri- 

 cal provision in the curved form of the chamber of great com- 

 plication and difficulty. But God hath bestowed upon this 

 humble architect the practical skill of the learned geometri- 

 cian ; and he makes this provision with admirable precision in 

 that curvature of the logarithmic spiral which he gives to the 

 section of the shell. This curvature obtaining, he has only 

 to turn his operculum slightly round in its own place, as he 

 advances it into each newly-formed portion of his chamber, to 

 adapt one margin of it to a new and larger surface and a dif- 

 ferent curvature, leaving the space to be filled up by increasing 

 the operculum wholly on the outer margin. 



******* 



Why the Mollusks, who inhabit turbinated and discoid shells, 

 should, in the progressive increase of their spiral dwellings, af- 

 fect the peculiar law of the logarithmic spiral, is easily to be 

 understood. Providence has subjected the instinct which 

 shapes out each to a rigid uniformity of operation. Professor 

 Mostly : Philos. Trans. 1838. 



HYDRAULIC THEORY OF SHELLS. 



How beautifully is the wisdom of God developed in shaping 

 out and moulding shells ! and especially in the particular value 

 of the constant angle which the spiral of each species of shell 

 affects, a value connected by a necessary relation with the 

 economy of the material of each, and with its stability and 

 the conditions of its buoyancy. Thus the shell of the Nautilus 

 PompiLius has, hydrostatically, an A-statical surface. If placed 

 with any portion of its surface upon the water, it will imme- 

 diately turn over towards its smaller end, and rest only on its 

 mouth. Those conversant with the theory of floating bodies 

 will recognise in this an interesting property. Ibid. 



