206 DEFENCE AGAINST PRESSURE. 



self-supporting roof, pursues the same course that is 

 adopted to give strength to the shells of ammonites. 



A further mode of answering the same end, is by 

 introducing at the different parts of these ribs bosses, 

 far more efficient for the purpose than those which 

 architects have added to their Gothic roofs. 



Flutings and Bosses. 



Thus far we have observed instances of admirable 

 arrangement; another must not be passed over. The 

 engraving exhibits a longitudinal section of an ammon- 

 ite, bisecting the transverse plates in the central line : 

 here their curvature is most simple ; but on each side 

 of it the curvature becomes more complicated, until, as 



