14 



NATURAL HISTORY OF THE PRIMEVAL WORLD. 



for iiioreasing its strength, founded on a principle tliat 

 we see everywhere practically applied in works of 

 human art and science— the principle, namely, by which 

 the strength and rigidity of a thin plate of metal are 

 considerably enlianced by corrugating, or fluting, the 

 surface. 



A common pencil-case, if made of corrugated or 

 Ihited metal, is stronger than if the same quantity of 

 metal were disposed in a simple tube. 



Similarly, culinary moulds of tin or copper are 

 strengthened by arranging folds, or flutings, around 

 their margin, or upon their convex surfaces. 



The application, now so general, of thin plates of 

 corrugated iron or zinc to the purpose of building self- 

 supporting roofs, in wiiich the corrugations answer tlie 

 purpose and supply the place of beams or rafters, is 

 actually founded on the very same principle that 

 Btrenglhens the vaulted shells of the Ammonites. 



For in all these cases the ribs, or elevated portions, 

 bestow upon the layers of shell or plates of metal all 

 the extra strength which results from the convex form 

 of an arch, without materially increasing the burthen ; 

 while the intermediate depressed parts between these 

 arches are at once suspended and supported by the 

 tenacity and substantialness of the material. 



The general principle of dividing and subdividing the 

 ribs, in order to multiply sup[X)rts as the vault enlarges, 

 is conducted nearly upon the same plan, and for the 

 same purpose, as the divisions or subdivisions of the 

 ribs beneath the (jroin ivork, in the flat vaulted roofs of 

 the later Gothic architecture. 



But many species oi Ammonites are further strength- 

 ened by the elevation of parts of the ribs into little 

 dome-shaped tubercles, or bosses, thus superadding the 

 strength of a dome to that of the simple arch, at each 

 point where those bosses are uiserted. Tin's contrivance 

 has also been imitated by the Gothic architects, who 

 have applied the bosses to the intersections of the ribs 

 in their highly ornamented roofs. 



Bearing in mind, then, all these extraordinary 

 instances of a surpassing skill and foresight, may we 

 not say, with Dr. Buckland, that we reverently recognize 

 tlie exercise of discretion and economy in the midst of 

 abundance; distributing internal supports but sparingly 

 to parts which, from their external form, were already 

 strong, and dispensing them abundantly beneath those 

 jiirts only which without them would have been weak. 



" We find," says the eminent geologist we have 

 referred to, " an infinity of variations in the form and 

 sculpture of the external shell, and a not less beautiful 

 variety in the methods of internal fortification, all 

 adapted, with arcliiteotural advantage, to produce a 

 combination of ornament anil utility. The ribs also 

 are variously multiplied, as the increasing space 

 demands increased support ; and are variously adorned 

 and armed with domes and bosses, wherever there is 

 need of more than ordinary strength." 



The utility of the transverse plates and air-chambers 

 of the Ammonites will be readily comprehended. The 

 firmer are intended to increase the strength of the 

 external shell by multiplying the subjacent points of 

 resistance to outward (ircssure. At no great depth the 

 weight of the sea will force a cork into a bottle filled 



with air, or crush a hollow cylinder or sphere of thin 

 copper. As the air-chambers of the Ammonites were 

 subject to a similar burthen when at the bottom of the 

 sea, they could only be preserved from destruction by 

 some peculiar provision. 



The reader will now inquire, how or in wdiat manner 

 the little ocean-roamer ascended or descended in the 

 deeps at will ? 



This valuable power was obtained by the agency of 

 the siphuncle, or, as we would call it, locomotor tube, 

 which, as in the case of the Nautilus, was virtually a 

 pipe, ejecting or admitting the sea-water according as 

 the animal wished to rise or sink. When it desired to 

 retire to the depths it took on board, so to speak, the 

 necessary quantity of sea-water; of which it didy 

 relieved itself when it wished to ascend. 



The manner in which the shell of the Ammonite 

 was adapted to the twofold purpose of acting as a float, 

 and protecting the body of its inhabitant, displays the 

 wisdom and power of the Creator no less signally than 

 the structure of the elephant's trunk. As the animal 

 inci'eased in size, and ailvanced along the outer chamber 



Ammonite restoi 



of its shell, the spaces which it left behind were suc- 

 cessively converted into air-chambers, thus increasing 

 simultaneously the potency and efficiency of the float. 

 This float, regulated by the siphuncle, and penetrating 

 the entire series of the chambers, formed an hydraulic 

 instrument of extraordinary delicacy. 



To sum up : — 



To creatures that occasionally floated, a thick heavy 

 shell would have been wholly inapplicable. On the 

 other hand, a thin shell, inclosing air, would have been 

 subject to a pressure that must have proved fatal to 

 it. A series of provisions, therefore, was designed by 

 the Creator to secure the necessary strength while 

 preserving the necessary lightness. 



First, the shell was made up of a tube coiled round 

 itself and externally convex. 



Secondly, it was fortified by a series of ribs and 

 vaultings disposed in the form of arches and domes on 

 the convex surface of this tube, and still further aug- 

 menting its strength. 



