3^0 FLUTED FORM OF RIBS. 
surface of the external shell, there arise me- 
chanical advantages for increasing its strength, 
founded on a principle that is practically applied 
in \yorks of human art. The principle I allude 
to, is that by which the strength and stiffness 
of a thin metallic plate are much increased by 
corrugating, or applying JIutiugs to its surface. 
A common pencil-case, if made of corrugated or 
fluted metal, is stronger than if the same quan- 
tity of metal were disposed in a simple tube. 
Cidinary moulds of tin and copper are in the 
same way strengthened, by folds or flutings 
around their margin, or on their convex surfaces. 
The recent application of thin plates of corru- 
gated iron to the purpose of making self-sup- 
porting roofs, in which the corrugations of the 
iron supply the place, and combine the power of 
beams and rafters, is founded on the same prin- 
ciple that strengthens the vaulted shells of Am- 
monites. In all these cases, the ribs or elevated 
portions, add to the plates of shell, or metal, 
the strength resulting from the convex form of 
an arch, without materially increasing their 
weight ; whilst the intermediate depressed parts 
between these arches, are suspended and sup- 
ported by the tenacity and strength of the 
material. (See PI. 37, Figs. I— 10.*) 
* The figures engraved at PI. 37, afford examples of various 
contrivances to give strength and beauty to the external shell. 
The first and simplest mode, is that represented in 11. 35 and 
