SHELLS OF THE AMMONITES. 205 



The water, in this instance, must have been forced 

 through the cork. 



Other cases are mentioned in which a copper air- 

 tube was crushed quite flat under a pressure of about 

 three hundred fathoms ; and a claret bottle, filled with 

 air and well corked, was burst before it had descended 

 four hundred fathoms. Let, then, these instances be 

 considered, and additional interest will be given to the 

 question : How can shells be made so buoyant as to 

 float on the waters, and yet so strong as to resist effectu- 

 ally this amazing pressure ? 



If the entire shell of an ammonite be examined, it 

 will be found to be one continuous arch — the strength 

 of which is proverbial — an arch coiled spirally round 

 itself, so that the base of the outer whorls rests on the 

 crown of the inner whorls, and thus the back is pecu- 

 liarly calculated to resist pressure. See page 187. 



A scientific workman knows how to give strength, to 

 an arch ; and the shell of the ammonite gains strength 

 also from the arrangement of the ribs by which it is 

 beautifully adorned. Many illustrations of this might 

 be given ; for the artisan who makes around the mar- 

 gins, or on the convex surfaces, of tin and copper moulds 

 for domestic use folds or flutings, or gives them to 

 a silver pencil-case, or to the iron intended to form a 



